Anionic Exchange Membrane (AEM) Stability: Synthesis Optimization of Multication Side Chains

Michael Lee, Chris Cornelius Author

04/01/2021 Added

12 Plays

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Study quantifying key parameters in the synthesis of precursor sidechains that will functionalize polymer membranes dealing with ion transport.

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[00:00:01.030]Hello and welcome.
[00:00:02.010]Today, I'll be going over various precursors
[00:00:04.250]that will enhance my anionic exchange membrane's stability.
[00:00:06.750]Specifically, I will be addressing key parameters
[00:00:09.670]in optimizing synthesis of some multication side chains.
[00:00:14.435]Globally, there has been demands
[00:00:16.300]for higher alternative sources of energy,
[00:00:19.030]as well as freshwater production from seawater.
[00:00:21.300]For certain energy sectors,
[00:00:22.500]fuel cells have been thought to be a potential solution,
[00:00:24.870]while electrodialysis could aid in treating
[00:00:26.634]the brine water product stream
[00:00:28.500]that forms after re-filtering seawater.
[00:00:30.800]When considering the vast array of energy production,
[00:00:33.200]current fuel cell development is a promising alternative
[00:00:36.940]to combustion gas and diesel engines.
[00:00:38.690]By replacing the combustion engine,
[00:00:40.350]fuel cells will fall into the transportation
[00:00:42.390]and portable energy sectors.
[00:00:43.610]At this scale,
[00:00:44.443]energy demands will be met at a more individual basis.
[00:00:47.060]Once built, fuel cells' energy production cycles
[00:00:51.080]are more efficient than combustion processes
[00:00:53.420]due to less restrictive factors.
[00:00:55.190]A pivotal component in fuel cells
[00:00:57.080]and electrodialysis technology are electrolytes.
[00:00:59.700]My focus has been on the
[00:01:01.070]anionic-exchange-membrane-type ionomers,
[00:01:03.730]which are an important part of both
[00:01:05.010]previously discussed technologies and numerous others.
[00:01:09.830]To give you a better picture of the processes
[00:01:11.890]ionomer or polymer electrolytes facilitate,
[00:01:14.250]I will give a brief overview.
[00:01:16.020]An ionomer's main objective
[00:01:17.210]is to selectively transport ions
[00:01:18.940]through facilitated diffusion.
[00:01:20.540]For example, in fuel cells there are hydroxide ions
[00:01:22.725]and electrons that are generated through
[00:01:24.780]oxidation-reduction reactions.
[00:01:26.476]These ions are needed to be selectively transported
[00:01:29.107]to generate an electrical current.
[00:01:31.100]The overall chemical equation for these
[00:01:33.507]starts with inputting hydrogen and oxygen,
[00:01:36.525]and outputting electrical energy
[00:01:38.376]and other byproducts,
[00:01:40.350]if a humid environment is present.
[00:01:43.000]For alkaline fuel cells, there are many benefits.
[00:01:45.750]Located within the fuel cell,
[00:01:47.420]the ionomer consists of a durable backbone
[00:01:49.520]for mechanical strength and several functional groups.
[00:01:51.867]Specifically, functional groups for these membranes
[00:01:55.495]are normally made of positive ions,
[00:01:57.740]including quaternary ammonia, phosphonium
[00:01:59.496]or sulfonium with flavylium
[00:02:01.920]thought to be the most stable.
[00:02:03.547]Impacting factors on ion transport here include
[00:02:06.390]electrostatics, diffusion, hydrogen bonding,
[00:02:09.170]other phenomenon.
[00:02:10.440]Importantly, the ability to transport ions is desirable
[00:02:12.660]in many applications besides fuel cells,
[00:02:14.640]such as redox flow batteries and electrolysis.
[00:02:18.380]One established procedure I plan to follow and optimize is
[00:02:21.120]the Menshutkin reaction, which involves
[00:02:22.847]the alkylation of a tertiary amine,
[00:02:24.740]in this case, a tetramethyl-1-6-hexanediamine,
[00:02:27.335]which will react with the halocarbon, 1-bromohexane.
[00:02:30.620]According to the literature, this SN2 reaction takes place
[00:02:32.883]over the course of 12 hours at six degrees Celsius.
[00:02:35.800]To maximize the rate-limiting step, a 10:1 molar ratio
[00:02:39.237]with respect to hexanediamine will be performed
[00:02:41.930]in an aproduct chloroform solvent under an argon atmosphere,
[00:02:45.430]which minimizes water concentration.
[00:02:47.900]After evaporating the solvent and performing
[00:02:50.490]recrystallization using anhydrous methanol,
[00:02:52.880]the product of this SN2 reaction will be recovered.
[00:02:57.000]I will be using proton NMR in a (indistinct) formation.
[00:02:59.424]The slides that will follow will be important to track
[00:03:03.240]tertiary amine peaks around 2.25
[00:03:05.840]and quaternary ammonia peaks around 3.41,
[00:03:09.147]which are the clear indicators that
[00:03:11.160]the reaction has completed.
[00:03:15.400]Further increasing fractional free volume
[00:03:17.780]and counter ion transport, et cetera,
[00:03:19.620]decane, undecane, (indistinct) and dodecane bromide,
[00:03:23.204]with an additional trimethylammonium group,
[00:03:26.610]will be synthesized in a similar manner to
[00:03:29.670]the dodecane bromide.
[00:03:33.070]Looking deeper in the problematic reactions,
[00:03:35.280]a couple of elimination reactions have been recorded.
[00:03:37.775]One instance is an E1-type reaction where
[00:03:40.360]the alpha hydrogen and the neighboring methyl group
[00:03:42.790]can be stolen, resulting in the formation of
[00:03:44.790]an alkene, water, and a tertiary amine.
[00:03:47.441]Additionally, an E2-type Hofmann reaction, which involves
[00:03:50.610]a hydroxyl ion attacking the beta hydrogen.
[00:03:53.076]Typically, in an alpha (indistinct), this reaction
[00:03:56.015]will result in an alkene, tertiary amine, and water.
[00:04:00.060]Moving to substitution-type degradation reactions,
[00:04:03.060]an SN2-type can happen where nucleophilic hydroxide ions
[00:04:07.270]attached to the alpha carbon of a methylene group can
[00:04:10.550]produce an alcohol.
[00:04:11.556]Meanwhile, the quaternary ammonium is cleaved away
[00:04:14.400]to form a tertiary amine.
[00:04:17.210]If the ionomer has a benzyltrimethylammonium chain,
[00:04:21.940]such as the picture on the left,
[00:04:23.800]two problematic (indistinct), or neutral dipole molecules,
[00:04:26.687]show up because hydroxide ions are complexing with
[00:04:30.150]acidic alpha hydrogens.
[00:04:31.485]The (indistinct) protons are either on the
[00:04:33.750]benzilic methylene groups or the methyl hydrogen.
[00:04:36.460]If the methyl (indistinct) intermediate on the right is made
[00:04:40.711]by a direct attack or by benzilic
[00:04:42.494](indistinct) rearrangement, a tertiary amine and
[00:04:44.920]methanol molecule will be the ultimate results.
[00:04:49.970]Knowing the possible degradation mechanisms
[00:04:52.180]and design criteria, my overall research goal is
[00:04:55.200]the development of a more stable, higher-functioning,
[00:04:58.120]lower-cost membrane via synthesizing and attaching
[00:05:00.656]a side chain with high ion conductivity that is stable
[00:05:05.210]against environmentally nucleophilic attacks.
[00:05:08.770]For the dodecane bromide ion, initially, reagent molar
[00:05:11.740]ratios were closer, and the rate at which
[00:05:13.930]I introduced the reagents to each other was
[00:05:15.690]significantly faster.
[00:05:16.880]Due to the lack of proper product formation, I hypothesized
[00:05:20.700]that hexanediamine may be attacking bromohexane at both ends
[00:05:24.420]because of this wide availability of the reagent.
[00:05:27.740]Therefore, to confirm this was indeed happening,
[00:05:30.170]I went to proton NMR and we saw that tertiary amine peaks
[00:05:34.120]were indeed absent around 2.25.
[00:05:36.450]However, quaternary ammonia peaks around 3.4 were evident,
[00:05:39.250]which confirmed alkylation at both ends.
[00:05:41.660]This product is unideal because without tertiary amines
[00:05:44.020]it cannot attach the desired molecule to my membrane.
[00:05:47.940]After trying to isolate the desired precursor, bismethonium
[00:05:50.930]species crystals were confirmed with
[00:05:52.670]the disappearance of frequency at 3.5.
[00:05:56.180]That frequency is a result of the protons near bromine
[00:05:58.570]experiencing that electronegative pull.
[00:06:01.260]Overall, precursor yield has been very low.
[00:06:03.550]However, without changing a vacuum purification step
[00:06:06.350]to boiling, then no 3.5 peak is present.
[00:06:10.030]Interestingly, the precursor reaction takes place
[00:06:14.770]in product solvents, however, at longer reaction times.
[00:06:17.096]Therefore, I theorized product solvent is used to slow
[00:06:19.850]the reaction kinetics, preventing both ends of the
[00:06:21.900]dibromo alcohol groups from reacting.
[00:06:26.770]For the set-up, starting at the exhaust, I have a bubbler
[00:06:29.330]with mineral oil that is there to
[00:06:31.260]monitor the argon atmosphere.
[00:06:32.880]The reflux condenser will cover volatile solvents as it will
[00:06:35.790]be near its boiling point in the three neck flask.
[00:06:39.082]To optimize product formation, I am dropwise adding
[00:06:42.900]the reagents via a graduated addition funnel
[00:06:45.270]into the reaction vessel
[00:06:46.460]with the temperature probe monitoring the reaction.
[00:06:49.020]Additionally, an argon gas inlet is attached to prevent
[00:06:51.610]any oxidizing water effects,
[00:06:53.050]as both reagents and products are hygroscopic.
[00:06:57.050]Additionally, I've reassembled and used a Buchi Rotavap R124
[00:07:01.620]with a vertical condenser to minimize water's ability
[00:07:05.010]to contaminate the sample.
[00:07:09.440]To summarize, it is seen that solvent choice,
[00:07:12.320]reagent molar ratio, addition sequence,
[00:07:14.560]and system isolation from oxidizing species or water,
[00:07:17.600]all contribute to an optimal product
[00:07:20.080]for some of my precursors.
[00:07:22.510]Also, to further guarantee bromohexane's disappearance,
[00:07:25.730]I have introduced heptane washes based on Hansen solubility
[00:07:28.816]parameters, and the residual reagent has been removed
[00:07:33.942]based on its absent peak.
[00:07:36.830]However, I suspect there may be residual hexanediamine
[00:07:39.856]as analysis of proton NMR would suggest high proton
[00:07:43.840]concentrations only possible with excess reagent.
[00:07:48.260]In the future, I would like to attach various multications
[00:07:51.210]to a polyphenylene membrane in a way similar to what
[00:07:53.320]is reported in literature.
[00:07:54.760]This would involve a nucleophilic substitution
[00:07:57.440]on a brominated group.
[00:07:58.870]Next, to confirm this multication's effectiveness
[00:08:01.480]at blocking hydroxide ion attacks, would be through
[00:08:04.990]its steric effects, charged version and poor leaving groups.
[00:08:08.490]Series of membrane conductivity tests will be conducted.
[00:08:11.420]These tests would occur over the period of a month,
[00:08:14.010]keeping membrane samples in a solution at a similar
[00:08:16.770]alkalinity to what would be experienced in various
[00:08:19.740]applications.
[00:08:21.930]I note that I would now like to thank you for your time
[00:08:24.190]and acknowledge some individuals who have helped make
[00:08:26.480]my research possible.
[00:08:27.920]First and foremost, I want to thank my PI,
[00:08:30.670]Dr. Chris Cornelius for his continual support.
[00:08:33.630]Also, I'd like to acknowledge Thivani Senathiraja,
[00:08:36.670]the other graduate student in my lab, for her support.
[00:08:39.260]The work of the newest members of the Cornelius lab,
[00:08:42.480]undergraduates Casey Holte and Aidan Larson,
[00:08:44.560]have helped with my breakthroughs as well.
[00:08:47.483]Additionally, I want to acknowledge the Nebraska Center
[00:08:50.850]for Energy Sciences Research for providing
[00:08:53.290]the funds necessary for my project.

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Anionic Exchange Membrane (AEM) Stability: Synthesis Optimization of Multication Side Chains

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