Study #1
“Molecular Sieve Synthesis in Presence of Tetraalkylammonium and Dialkylimidazolium Molten salts” by Sun, Xin, et al.
Molecular sieves are stable, crystalline materials with nanometer-sized pores and which have found many applications in water softening, wastewater treatment, odor control, cracking catalysts in the oil industry and many other fields. They can have a wide range of chemical compositions like silicate, aluminophosphate and all-carbon molecular sieves, to name a few. They can be synthesized using traditional hydrothermal methods or the recent ionothermal methods. In hydrothermal methods, a typical zeolite synthesis consists of a mixture of water, a source of framework atoms, a mineralizing agent and a structure-directing agent (SDA). For the ion thermal methods, an ionic liquid serves as the solvent and/or the structure-directing agent (Sun et al., 1).
Traditionally, tetrapropylammonium bromide ([TPA][Br]) is used as the SDA. In this work, the researchers used three different molten salts Tetrabutylammonium bromide ([TBA][Br]), tetrabutylammonium hexafluorophosphate ([TBA][PF6]), tetrabutylammonium nitrate ([TBA][NO3]) and an ionic liquid, 1-ethyl-3-methylimidazolium bromide ([emim][Br]) were used as SDA. [TPA]Br] was also used to act as the experimental control. The salts contained tetrapropylammonium [TPA] and tetrabutylammonium [TBA] cations to compare the effect of alkyl chain length on the formation of silicate material. Those salts with [TBA] as the cation had different anions to investigate influence of anions on the framework. Two known materials were the products synthesized after this experiment depending on the salt or ionic liquid used: either a porous silicate of MFI-type framework or the densely layered silicate magadiite which is not porous.
Experimental procedure
Materials were prepared hydrothermally based on the method used to synthesize pure silica ZSM-5, a zeolite of MFI-type framework. Water, salt (SDA) or ionic liquid, base (NaOH) and silicon dioxide (SiO2) were combined in a molar ratio 0.072: 0.030: 0.37: 9.2 respectively then stirred for 2 hours. The reaction mixture was charged into autoclaves which were then placed in an oven at 150oC for 44 hours. The precipitate was washed with water, followed by acetone (for [TPA][PF6] only) and air-dried at room temperature. Analysis was done using an X-ray diffractometer and the resulting patterns compared to those available in the literature.
Results
Each of the synthesis mixtures yielded a white product. Only in presence of [TPA][Br] was a product synthesized with X-ray diffractometer patterns (XRD) characteristic of a porous pure silica zeolite with the MFI-type framework. In the presence of either [TBA][Br], [TBA][PF6], [TBA][NO3] or [emim][Br], products synthesized showed X-ray diffractometer patterns characteristic of magadiite (NaSi7O13(OH)3·4(H2O)) which is a dense material with a silicate layer and whose synthesis does not commonly require an SDA. The addition of either [TBA][Br], [TBA][PF6], [TBA][NO3] or [emim][Br] therefore neither enhanced nor inhibited the formation of magadiite; they merely served as co-solvents.
The lengthening of alkyl group from propyl to butyl led to MFI framework not being formed. The larger cation did not provide the necessary van der Waals forces to make MFI.
Comments
The researchers in this experiment should have provided more discussion on the influence of anions in the framework formation. Determining the effect of these anions on the framework was one of the objectives of the experiment but the results are not discussed to this effect. They also could have extended the scope of the experiment by using salts with TPA as the cation but with different anions and determine the influence of anions on the MFI structure.
Small-Angle Neutron Scattering Studies of Model Protein Denaturation in Aqueous Solutions of the Ionic Liquid 1-Butyl-3-Methylimidazolium Chloride.
Study #2
“Small-Angle Neutron Scattering Studies of Model Protein Denaturation in Aqueous Solutions of the Ionic Liquid 1-Butyl-3-Methylimidazolium Chloride” by Baker, Gary A., and William T. Heller
The possible application of enzymatic transformations and clean biotechnological process by means of ionic liquid (IL) based media has continued to drive research in this are. Relatively few biochemical characterizations of protein molecules in systems containing ionic liquids (ILs) exist in literature and therefore the effects of various ILs on enzymatic reactions are not known. It is therefore important to establish a general understanding of structure-function relationships for enzymes in IL media. This calls for systematic studies in this field.
This work presents the details of the researchers’ investigation into the behaviors of two proteins within the aqueous mixtures of the entirely water miscible IL, 1-butyl-3-methylimidazolium chloride, [bmim]Cl. The measurements were done using the small- angle scattering technique, using neutrons and X-rays (SANS and SAXS, respectively) and circular dichroism spectroscopy (CD). The two model proteins used were equine heart cytochrome c and human serum albumin, HSA which are of two of the most thoroughly characterized proteins. Both equine heart cytochrome c and HSA were found to be denatured by high concentrations of [bmim]Cl. The effect of [bmim]Cl on the proteins was not particularly pronounced until the aqueous solution reached 50% [bmim]Cl (Baker & Heller, 7).
Experimental Procedure
Lyophilized powders of essential fatty acids-free HSA and cytochrome c from equine heart were used. They were obtained from Sigma (St. Louis, MO, USA) and used without further purification. High-quality [bmim]Cl was synthesized according to established literature procedures. For the small- angle scattering measurements, stock solutions of HSA and cytochrome c were prepared by dissolving protein in 20mM Tris buffer at pH 7.4. Each protein stock was then filtered using a sterile syringe filter equipped with 10-mm diameter inorganic membrane to remove large particulates and proteinaceous aggregates. The filtered protein stock concentrations were determined to be 9.0 and 25.1mg/mL for HSA and cytochrome c, respectively using UV–vis absorption. Samples were then diluted to half the original concentration using [bmim]Cl and Tris buffer to yield protein in buffer solutions of 0%,10%,25% and 50%. Small angle-scattering and CD measurements were then collected as follows. CD spectra (300-500) were obtained at room temperature using a 1-mm path length cuvette on a JASCO J-810 spectropolarimeter (JASCO Inc., Easton, MD). For the SANS data, Intensity patterns for samples and appropriate backgrounds were collected using a wavelength of 6Å with a wavelength spread, change in λ/λ, of 0.15. SAXS data were collected for the proteins in 0, 10, and 50 vol. % [bmim]Cl. The scattering data were reduced using the standard procedures to correct for background and detector sensitivity. The data was then azimuthally averaged to produce the unidimensional intensity profile and compared with a model intensity profile.
Results
Protein CD measurements at 10% [bmim]Cl was identical to that of aqueous phosphate buffer control indicating minimal change to its structure. At 25% [bmim]Cl CD measurements showed decreased activity and at 50%[bmim]Cl, the spectrum is identical to that of urea denatured protein. The small-angle scattering measurements showed small changes to the protein structure at [bmim]Cl concentrations below 25% but denaturation occurred at 50%. SANS and SAXS results demonstrated that on denaturation, the polypeptide structure was transformed from a compact structure to a high unfolded structure.
Comments
By using concentrations of [bmim] that were so spread out, accuracy could have been compromised while at the same time not providing exhaustive findings. The researchers should have used more concentration rates at closer intervals to maximize the level of accuracy.
Works Cited
Baker, Gary A., and William T. Heller. “Small-Angle Neutron Scattering Studies of Model Protein Denaturation in Aqueous Solutions of the Ionic Liquid 1-Butyl-3-Methylimidazolium Chloride.” Chemical Engineering Journal 147 (2009): 6-12. Print.
Sun, Xin, et al. “Molecular Sieve Synthesis in Presence of Tetraalkylammonium and Dialkylimidazolium Molten salts.” Chemical Engineering Journal 147 (2009): 2-5. Print.