Molecules 2019, 24, 269
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Aluminum complexes reported by North [18] and Kleij [19] are extremely active in that they are able
to catalyse the cycloaddition of CO under ambient temperatures and pressures. Although these
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reaction conditions are impressive, the multistep synthesis of the catalysts tend to have poor E-factors,
leading to increased use of resources and energy, consequently increasing the carbon footprint for
the process and overall environmental impact. Alternatively, metal halide salt catalysts tend to be
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much cheaper, but often use high temperatures and pressures of > 100 C and 20–60 bar, often along
with the necessitated addition of a hydroxyl containing co-catalyst to aid stabilization of the alkoxide
intermediate and drive the reaction to high conversions. Interestingly, there have been a number of
natural organic compounds reported as effective co-catalysts, such as cellulose [20], β-cyclodextrin [21],
lecithin [22] and betaines [23]. Various amino acids have been reported as very effective co-catalysts,
with histidine being one of the most active [24]. One of the most effective metal halide salts reported
to date is potassium iodide (KI). Typically, this is produced from potassium hydroxide and iodide,
where potassium is either mined from underground ore deposits, salt lakes or brines, and the iodide
can be sourced from either caliche ore, brines or seaweeds (along with potassium salts), each often
having direct environmental consequences [25–27]. Further to this, extraction, isolation, purification
and transportation of such species require huge amounts of energy (the average mine consumes 25
MW/h per tonne of material processed [28]), having an indirect environmental impact.
This paper details an investigation of the use of two different seaweeds as one-component, natural
catalysts for the formation of cyclic carbonates due to the high quantities of metal halides found in the
plant along with amino acids in the form of proteins. Direct use of the plant with minimal preparation
has a significant number of benefits; the plant naturally sequesters inorganic minerals out of the sea
removing the need for refining and purifying metal salts or halogens, seaweed is sustainable and
extremely abundant, it is accessible and cheap throughout the world, along with the benefit of having
reduced operator and environmental risk with regards to handling and manipulation. Kelp was chosen
due to the well reported high concentrations of iodide and salts incorporated within the plant [29–31]
and dulse was chosen as a comparison. Both contain amino acids, proteins and polysaccharides (such
as cellulose, alginate, fucoidan and carrageenan) that could be beneficial to the synthesis [32]. Aside
from being edible, both seaweeds already have industrial non-food applications with established
harvesting and processing routes. They are commonly used as organic fertilizers and soil conditioners
(
both today and historically) [33], for the production of seaweed gums for use in the pharmaceutical,
medical and cosmetics industries (extraction of alginates from Laminaria digitata, along with agar and
carrageenan from Palmaria palmata) [34] as well as use in animal feed due to their protein content (up
to 50% crude protein for red seaweeds and approximately 14% for the more easily harvested brown
seaweeds) [35].
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. Results and Discussion
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.1. Conversion to Styrene Carbonate Using Model Catalyst and Co-Catalyst
Initially, a screen of different metal halides catalysts with different amino acid co-catalysts was
performed as a baseline for comparison with the seaweed catalysts and also to identify any synergistic
effect between the metal and amino acid co-catalyst. Full details of the procedures used are detailed
in Experimental Section 4.3. Basic amino acids histidine and lysine were chosen for their reported
ability to activate CO by formation of a carbamate salt [36], whereas glycine was chosen as a non-basic
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polar comparison, Table 1. Overall, the less electronegative potassium (K) salts were found to be more
active than the sodium (Na) and calcium (Ca) equivalents. A small quantity of 1-phenyl-1,2-ethanediol
by-product was observed, which was found to increase when incorporating polar amino acids histidine
and lysine as co-catalysts, most likely due to residual water contamination, (SI, Table S1a–e). However,
the addition of amino acid co-catalysts containing a basic group (histidine and lysine) significantly
increased conversions to styrene carbonate for all metal halide catalysts containing a bromide and
chloride counter ion; histidine was found to give the greatest synergistic effect of all, in accordance