Fiber Formation by Highly CO₂‐Soluble Bisureas Containing Peracetylated Carbohydrate Groups

Abstract

Supercritical carbon dioxide (scCO 2 ) has attracted interest as an environmentally benign solvent, [1] but its practical usage is limited by the low CO 2 -solubility of polar and high-molecularweight compounds. To enhance the CO 2 -solubility of such compounds, CO 2 -philic surfactants, [2,3] dispersants, [4,5] thickeners, [6] and polymers [5,7] have been designed. Early success was achieved by the use of polyfluoroalkyl groups: various derivatives incorporating these groups showed enhanced CO 2 -solubility. In an attempt to develop "greener" CO 2 -philes, [8][9][10][11] several investigators have explored other highly CO 2 -philic compounds composed solely of carbon, hydrogen, and oxygen. For example, the CO 2 -solubilities of poly(ether carbonate)s are comparable to that of poly(hexafluoropropylene oxide), probably owing, in part, to the concentration of negative charge density on the carbonyl oxygen atom of the carbonate units.[8] The acetate group was also recognized as having the potential to be very CO 2 -philic and has the advantage of easy introduction into polymers and surfactants. [11][12][13][14][15][16][17] Peracetylated sugars, such as sorbitol, [18] sugar amides, peracetylated cyclodextrins, [19] maltose octaacetate, [20] glucose pentaacetate, and galactose pentaacetate [21,22] also exhibit extraordinary solubility in scCO 2 . A Lewis acid-base interaction between the acetyl oxygen atom and the carbon atom of CO 2 , and weak CÀH···O hydrogen bonding between the acetyl methyl group and the oxygen atoms of CO 2 are believed to be responsible for the CO 2 -affinity of the acetyl group. [21,23] We have previously shown that self-aggregating organic compounds containing both hydrogen-bonding urea groups and fluorinated CO 2 -philic tails could modestly increase the viscosity of scCO 2 .[9] Upon depressurization, these solutions produced free-standing foams, which represent organic analogues of silicate aerogels, with submicron-sized fibers and a bulk density reduction of greater than 90 % of the parent material.[9] A critical feature of these systems is the presence of strong and directional hydrogen bonding between the carbonyl oxygen atoms and the NH units in the urea groups of adjacent molecules, which leads to the formation of twodimensional sheet-like structures (Figure 1). [24,25] The sheets likely further associate through noncovalent contacts to form viscosity-enhancing networks in solution and, subsequently, free-standing foams upon removal of the CO 2 . The objective of the present work was to design nonfluorous hydrogen-bonding molecules capable of dissolving in CO 2 and self-associating to form novel materials. An inexpensive and readily available source of multiple hydroxy groups is the family of mono-, di-, and oligosaccharides. These molecules can be readily converted into their peracetylated derivatives, which should have a density of electronegative groups similar to that of perfluoroalkanes. In particular, gluconic acid is readily available from glucose by oxidation and should be easil...

Development of Small Molecule CO2 Thickeners for EOR and Fracturing

“…In four of the twelve compounds studied, the bis-urea was capable of dissolving in CO 2 without being heated and increasing the viscosity of CO 2 by factors of about 3-5 at concentrations of ~5wt% at 25 o C and 4,500 psia. In an attempt to design a non-fluorous analog of these expensive fluorous bis-ureas, CO 2 -philic hydrocarbon groups and CO 2 -philic carbonyl and ether groups were incorporated into the structure [Paik et al, 2007]. It was hoped that these compounds might also dissolve in CO 2 , self-assemble into long cylinders, remain in solution, and thicken CO 2 .…”

Direct Gas Thickener

“…Out of the 12 compounds tested, 4 fluorinated bis-urea compounds were highly soluble in CO 2 without needing heat and capable of improving the CO 2 viscosity by 3-5-fold at 5 wt% of bisurea at 298 K and 31 MPa. In the hope of obtaining a non-fluorous bis-urea, Paik et al [67] attempted to incorporate the CO 2 -philic groups (hydrocarbon, carbonyl, and ether groups) into the molecular structure of the bis-urea as illustrated in Figure 5. However, their results revealed that after forming a transparent solution, microfibres began to form slowly due to the molecules undergoing self-assembly Enhanced Oil Recovery Processes -New Technologies and precipitating out of solution.…”

Chemoselective hydroxyl group transformation: an elusive target

“…For example, to synthesize peracetylated carbohydrate intermediates from D-glucamine without acetylating the amino group, Hamilton and coworkers utilized acetyl chloride under acidic conditions, which prevented acylation of the primary amine by formation of the ammonium chloride salt. 23 The field of carbohydrate chemistry has encouraged the development of strategies for the regioselective acylation of either primary or secondary hydroxyl groups. For example, modified 4-( N,N '-dimethylamino)pyridine (DMAP)- and peptide-based catalysts have been explored to selectively functionalize carbohydrates on their secondary hydroxyl groups.…”