Zeolite adsorbents and catalysts for the recovery and production of biochemicals
Abstract: Fossil based energy resources are dominating the world’s primary energy consumption for the last century. However, with decreasing crude oil reservoirs and the role they play in global warming by emitting greenhouse gases, the focus has been turned towards improved utilization of renewable resources and the need for new, sustainable fuels and chemicals is more urgent than ever. Biomass is a carbon neutral resource that can be used to produce biofuels and other useful chemicals. One such chemical is 1-butanol (or simply butanol), which has great potential as a gasoline substitute because of its favorable fuel properties. Butanol can be produced from acetone, butanol and ethanol (ABE) fermentation using e.g. Clostridium acetobutylicum. However, the concentration of butanol in fermentation in the resulting broth is limited to ca. 20 g/L due to its toxicity for microorganisms. Butyric acid is a precursor to butanol, which is produced prior to butanol in ABE fermentation. Butyric acid is an important industrial chemical, which can be further converted into a number of commercial compounds e.g. acetate butyrate, butyl acetate and butanol. Arginine is a semi-essential amino acid that has vast applications in the field of pharmaceutical and food industry. In addition, arginine can replace inorganic nitrogen as nitrogen source in fertilizers. It can be produced via fermentation of sugars using engineered microorganism like E. Coli, but like butanol its concentration is restricted to approximately 12 g/L. Due to low concentration of these useful chemicals in the resulting fermentation broths recovery of these chemicals remain challenging with today’s options and therefore novel recovery process should be developed.In this study, zeolite adsorbents were used to recover butanol, butyric acid and arginine from model and real fermentation broths. Zeolite MFI adsorbent efficiently adsorbed butanol from model solutions with a saturation loading of 0.11 g/g- zeolite. On the other hand, adsorption of butyric acid was found to be strongly pH dependent, with high adsorption below and little adsorption above the pKa value of the acid. A structured adsorbent in the form of steel monolith coated with a silicalite-1 film was also used and performance was evaluated by performing breakthrough experiments at room temperature using model ABE fermentation broths and the results were compared with those obtained using traditional adsorbent sin the form of beads. Desorption studies showed that a high quality butanol product with purity up to 95.2% for butanol-water system and 88.5% for the ABE system can be recovered with the structured silicalite-1 adsorbent. Further, zeolite X adsorbents in the form of powder and extrudates was used to recover arginine from a real fermentation broth and also from aqueous model solutions. To the best of our knowledge, this is the first time recovery of arginine from real fermentation broths using any type of adsorbent is reported. Arginine loading of 0.15 g/g was observed at pH 11 using zeolite X powder. The selectivity for arginine over ammonia and alanine from the fermentation broth at pH 11 was 1.9 and 8.3, respectively, for powder and 1.0 and 4.1, respectively, for extrudates. Synthesis gas (CO + H2) can be produced e.g.by gasification of lignocellulose biomass. This synthesis gas can be used to produce methanol, which subsequently may be converted into gasoline using zeolite ZSM-5 catalyst. However, during Methanol to Gasoline (MTG) process, undesirable carbon residue (coke) is formed that gradually reduces the activity of catalyst. It was hypothesized that intracrystalline defects in the zeolite formed during conventional synthesis may accelerate the deactivation rate by coke formation. In this work, a novel ZSM-5 zeolite catalyst essentially free of intracrystalline defects was synthesized and evaluated in the MTG reaction,. The novel catalyst showed significantly higher resistance towards deactivation by coke formation as compared to a reference catalyst containing defects.
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