Richard T. Carlin

The Room Temperature Ionic Liquid 1‐Ethyl‐3‐methylimidazolium Tetrafluoroborate: Electrochemical Couples and Physical Properties

Room temperature molten salts composed of the 1-ethyl-3-methylimidazolium cation and a chloroaluminate anion have received much attention for use in a variety of commercial applications such as batteries, photovoltaics, metal deposition, and capacitors. The room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF{sub 4}) was demonstrated as a versatile electrolyte by examining three representative electrochemical couples: ferrocene and tetrathiafulvalene oxidations and lithium ion reduction. Square-wave voltammetric data for ferrocene oxidation were fit to a reversible one-electron process using the COOL algorithm to give a half-wave potential of 0.490 V vs. Al/Al(III) and a diffusion coefficient of 5.1 {times} 10{sup {minus}7} cm{sup 2}/s. The two-electron oxidation of tetrathiafulvalene was reversible and proceeded through two consecutive one-electron steps; although data collected at lower square-wave frequencies indicated a slow precipitation of the TTF{sup +} species. Lithium ion was reduced to lithium metal at a Pt electrode following the addition of water to the EMIBF{sub 4} electrolyte, whereas lithium ion reduction at an Al wire produced the {beta}-LiAl alloy. Conductivities and kinematic viscosities of EMIBF{sub 4} were measured from 20 to 100 C and had values of 14 mS/cm and 0.275 cm{sup 2}/s, respectively, at 25 C.

Complexation of Cp2MCl2 in a chloroaluminate molten salt: relevance to homogeneous Ziegler-Natta catalysis

Abstract Ethylene polymerization via Ziegler-Natta catalysis occurs in the ambient-temperature molten salt AlCl 3 ·MEIC (MEIC = 1-ethyl-3-methylimidazolium chloride) employing Cp 2 TiCl 2 as the catalyst and AlCl 3 - x R x (R = Me, Et) as a cocatalyst. Catalysis occurs only in melts with AlCl 3 :MEIC molar ratios > 1. Cp 2 ZrCl 2 and Cp 2 HfCl 2 with AlCl 3 - x R x cocatalysts are not catalytically active in acidic melts. 1 H NMR studies indicate formation of a strong 1:1 complex between Cp 2 TiCl 2 and AlCl 3 , while Zr and Hf form much weaker 1:1 complexes due to strong Zr-Cl and Hf-Cl bonding. This stronger M-Cl bonding for Zr and Hf is proposed to preclude the initiation reaction for ethylene polymerization in the molten salt.

Nucleation and Morphology Studies of Aluminum Deposited from an Ambient‐Temperature Chloroaluminate Molten Salt

Aluminium deposition from AlCl 3 :MEIC (1-methyl-3-ethylimidazolium chloride) has been studied employing an inverted optical microscope to perform in situ optical observations during the deposition process at a 250 μm diam tungsten electrode. Thin, continuous aluminum coatings with nuclei (or cluster) sizes below optical microscopic resolution are produced from a 1.1:1.0 AlCl 3 :MEIC molten salt using constant potential deposition at potentials ≤-0.2 vs. an Al(III)/Al reference electrode

Microelectrode Evaluation of Transition Metal‐Aluminum Alloy Electrodepositions in Chloroaluminate Ionic Liquids

Chronoamperometric data collected at a 250 μm tungsten microelectrode were analyzed under near-steady-state conditions to determine the composition of MAl x alloys (M = Co, Ni, Fe, Cu, and Ag) electrodeposited from 1.5:1.0 AlCl 3 :1-ethyl-3-methylimidazolium chloride room temperature ionic liquids. The analysis method relied on the fact that these alloys are produced by an underpotential deposition mechanism. Results were consistent with previous studies showing that the CoAl x , FeAl x , and CuAl x systems tended to produce alloys with x 1. Analysis of the NiAl x data was complicated by kinetic phenomena, while data analysis of the AgAl x system was precluded by dendritic growth of the electrodeposit. All the alloy systems showed complex anodic stripping voltammetric behavior, and the nature of the oxidation processes (e.g., metal anodization, alloy anodization, or selective dealloying) are different for electrodeposits produced in specific potential regimes. Nonlinear curve fitting of the chronoamperometric data to the appropriate short-time and long-time equations gave diffusion coefficients from 3.9 x 10 -7 to 8.3 x 10 -7 cm 2 s -1 for the transition metal ions in the ionic liquid electrolyte at ca. 22°C.

Ionic liquid–polymer gel catalytic membrane

A novel catalytic membrane for heterogeneous hydrogenation is fabricated by incorporating palladium into a gas-permeable ionic liquid–polymer gel composed of 1-n-butyl-3-methylimidazolium hexafluorophosphate and poly(vinylidene fluoride)–hexafluoropropylene copolymer.

Lithium and Sodium Standard Reduction Potentials in Ambient‐Temperature Chloroaluminate Molten Salts

By adding to buffered neutral (MEIC = 1‐methyl‐3‐ethylimidazolium chloride) ambient‐temperature molten salts, it is possible to measure directly the Li+/Li(s) couple at a tungsten electrode. From open‐circuit potential measurements performed in a large number of buffered melts, the Li+/Li(s) standard reduction potential is found to be −2.066 (±0.005) V vs. Al(III)/Al in a reference melt. Similarly, by adding to buffered neutral (DMPIC = 1,2‐dimethyl‐3‐propylimidazolium chloride), a Na+/Na(s) standard reduction potential of −2.097 (±0.050) V is directly determined.

Anodization and Speciation of Magnesium in Chloride‐Rich Room Temperature Ionic Liquids

Magnesium anodization was examined in room temperature AlCl 3 :EMIC and AlCl 3 :DMPIC ionic liquids, where EMIC = 1-ethyl-3-methylimidazolium chloride and DMPIC = 1,2-dimethyl-3-propylimidazolium chloride. For all melts, the AlCl 3 :organic chloride mole ratio was <1, yielding chloride-rich (i.e., basic) compositions. The rate of magnesium anodization was limited by diffusion of chloride ions to the electrode surface. From the Cottrell slopes for magnesium anodization at a Mg disk electrode and for chloride oxidation at a Pt disk electrode, the chloride stoichiometry of the anodization process in AlCl 3 :EMIC was determined to be 4.1 (± 0.5), corresponding to the formation of soluble MgCl 4 2 . Similar chloride stoichiometry was found in AlCl 3 :DMPIC. MgCl 2 buffers the melt to approximate neutrality from the basic side, dissolving as MgCl 4 2- . Magnesium metal was chemically stable in basic AlCl 3 :DMPIC, but it reacted completely and irreversibly with basic AlCl 3 :EMIC to produce colored organic byproducts. Some comments are made on the acidity of NiCl 2 and CdCl 2 in the basic melts.

Evidence for hydrogen bonds in 1,2-dimethyl-3-propylimidazolium chloride and its chloroaluminate molten salts

The crystal structure of l,2-dimethyl-3-propylimidazoIium chloride (DMPICl) reveals the formation of hydrogen bonds between the ring, methyl, and methylene hydrogens and the chloride ion. Although the aromatic rings are found in stacks, aromatic-aromatic interactions are precluded by the 6.98 Å distance between the rings. From IR and1HNMR spectroscopies, it is determined that only the hydrogen bonds between the ring hydrogens and the chloride ion persist in the room-temperature molten salts obtained from combining DMPIC1 with AlCl3.

Asymmetric electrode kinetics induced by concurrent metal-ligand bond dissociation

Abstract The electrochemistry of the Cu(II)/Cu(I) couple under nitrogen and carbon monoxide has been investigated in the ambient-temperature molten salt AlCl 3 :MEICl (MEICl = 1-methyl-3-ethyl-imidazolium chloride) at a 250 μm tungsten disk electrode. Under nitrogen, the couple exhibits reversible electrode kinetics; however, under carbon monoxide, a Cu(I)CO complex is formed and the Cu(II)/Cu(I) couple displays asymmetric, quasi-reversible electrode kinetics. Pulse voltammetric data were fit with a nonlinear least-squares fitting program to give an apparent standard rate constant ( k 0 a ) of 1.5 × 10 −3 cm s −1 and an anodic transfer coefficient (β) of 0.12–0.17 for the oxidation of the Cu(I)CO complex. The change from reversible to quasi-reversible electrode kinetics is attributed to the concurrent dissociation of the Cu(I)CO bond during the electron transfer process.

Electrochemical Quantification of Carbon Monoxide Complexation to Copper(I) in a Chloroaluminate Molten Salt

The reversible complexation of carbon monoxide to Cu(I) in the 1.5:1.0 AlCl 3 :BuPyCl(BuPyCl=1-butylpyridinium chloride) molten salt at ambient temperature was investigated using voltammetric and potentiometric techniques. The infrared spectrum of the carbon monoxide complex exhibits a strong υ(CO) band at 2142 cm -1 . Staircase cyclic and normal pulse voltammetries reveal that the Cu(II)/Cu(I) couple is reversible under nitrogen but becomes quasi-reversible or hampered by homogeneous kinetics under carbon monoxide