NORMAS DE PERMANENCIA DE LOS ESTUDIANTES
Materia 4: Trabajo de fin de grado
methanol, and pyridine over methanol.69Compound 7.12 is 7.5
7.6
described as an organic analogue of zeolites with hydro- gen-bonded molecular sheets.70Esters, such as ethyl ben- zoate, and ketones, such as 5-nonanone, can be guests.
The octasulfide (where Ar is 3,4-dimethylphenyl;) (7.13) can act as a host for 1,4-dioxane, N,N-dimethylfor- mamide, tetrahydrofuran, toluene, tert-butyl alcohol, ethyl acetate, limonene, and o-xylene.71
Some pyrazolones can also form inclusion compounds. Compound (7.14) (in which Ar is meta- or para-pheny- lene) forms inclusion compounds with methanol, ethanol, isopropyl alcohol, acetone, 2-butanone, tetrahydrofuran, and 1,4-dioxane, often with two guest molecules for one host molecule.72
Carboxylic acids can also form clathrates. Xylene iso- mers can be separated by formation of clathrates with 1,1- binaphthyl-2,2-dicarboxylic acid.73 m-Xylene does not form a complex. The o-xylene complex decomposes at
50–100°C, and the p-xylene complex at 100–120°C. Charge transfer is a factor in the host–guest complex of 4-methyl- 3,5-dinitrobenzoic acid with 2,6-dimethylnaphthalene.74 Charge transfer with tetranitrofluorenone has also been used to remove 60% of the dialkyldibenzothiophenes from petroleum that contains 1920 ppm sulfur, although no in- clusion compound is involved.75As polynitro compounds are often explosive, a better charge acceptor is needed.
The host (7.15) was found by combinatorial synthesis (from a library of 100 salts): It forms host–guest complexes with several alcohols, ketones, xylenes, and others (e.g., 2:3 with methanol, 1:1 with ethanol, 2:2 with acetone, and 2:1 with acetonitrile).76
7.9
7.10
7.12
Cobalt, nickel, and zinc salts of 1,3,5-benzenetricar- boxylic acid are solids with 4- to 5-Å pores, which can of- ten accommodate ammonia, ethanol, or water, but not larger molecules, as guests.77Cobalt and zinc terephtha- lates can also act as hosts for molecules such as pyrazine.78 The purpose in showing these rather exotic structures is to indicate the many possibilities for selectivity for shapes,79 some of which are not possible with zeolites, pillared clays, on others. Sometimes, the host structure adjusts to the guest to give an induced fit. By using this method, it is possible to select compounds based solely on size. For practical use, it may often be necessary to im- mobilize these structures on supports or place them in membranes.
from other sources. Among the examples in the literature are some that deal with atrazine (an herbicide), cholesterol, other sterols, dipeptides, N-acetyltryptophane resolution (L-isomer favored by a factor of 6), adenine, and barbitu- rates.81The polymerizations in the first two examples, are shown in (7.16) (The cross-linking comonomer with the cholesterol-containing monomer was ethylenebis- methacrylate. The cholesterol was cleaved from the poly- mer with sodium hydroxide in methanol.)
III. SEPARATION OF IONS
The desire to attain near-biological specificity in the sepa- ration of ions was mentioned in Chap. 4. Some progress is being made in this direction with ligands that form fairly specific complexes. These may not be inclusion com- pounds, but at least some involve encapsulation of the ion. These have been developed by “tuning” macrocycles by varying ring size, ring substitution, and the donor set of oxygen, nitrogen, and sulfur atoms.82For calix[n]arenes, it can involve varying the ring size, as well as the extent and type of substitution on both the aromatic rings and the hy- 7.15
over sodium of 22,000.85A macrocyclic polyether with at- tached 8-hydroxyquinoline groups (7.18) favors barium over other alkaline earth cations by a factor of more than 10 million.86Compound (7.19) shows a preference of magne- sium over calcium of 590.87 Compound (7.20) has the highest known binding constant for Ag(I), log K 19.6.88 There is also an active search for selective anion accep- tors.89The cryptate (7.21) favors fluoride over chloride by a factor of 40 million.90
Many bodies of water become eutrophic when excess phosphate from detergents and fertilizer washes in. This overenrichment results in undesirable algal blooms. Agents that complex phosphate may allow it to be removed from treated wastewater, recovered, and reused. The first com- plexing agent (7.22) (where X S) complexes dihydrogen phosphate with a K of 820 and acetate with a K of 470; chloride, hydrogen sulfate, nitrate, and perchlorate are held much more weakly.91The K for the second one with phos- phate (7.23; where R is H) is 12,000.92
Strong complexes of phosphates are known for 7.22 compound (7.24) (K 1.9 105 for H
2PO4)93and the zinc(II) complex of the (7.25) (log K 5.8–7.9 for ROPO4
).94
The pyridinium salt (7.26) is a receptor for tricarboxylic acids with log K 4.5–5.0.95 The trisguanidinium salt (7.27) has a binding constant for citrate of 6.9 103.96
Sugars such as fructose and glucose, can be complexed with the arylboronic acid (7.28), presumably through the formation of cyclic boronates.97 (Although not ions, the sugars are included here because of the similarity of the complexing agent to those effective for ions.) A bisboronic acid of a -oxobis [porphinatoiron(III)] forms complexes with glucose and galactose, with association constants of
7.17
7.19
7.22
7.23 7.24
working fluids, polyvinyl alcohol from textile desizing, and in the removal of polymeric materials from drugs, such alkaloids. In the refining of vegetable oils, it removes 100% of the phospholipids, 80–85% of the free fatty acids, and most of the pigments.104This process eliminates wa- ter–acid treatment to remove the phospholipids, base to re- move the free fatty acids, and deodorization by vacuum stripping. It can also be used for small organic molecules (e.g., 2-phenylethanol) held in micelles (in water) that are too large to pass through the pore.105
Reverse osmosis106uses membranes with 5- to 20-A pores or no pores at all. Some authors add a category called nanofiltration that is roughly between ultrafiltration and re- verse osmosis. A membrane with 20-Å pores retained 90–98% of sugars and magnesium sulfate, but passed sodium chloride.107 Acidic copper sulfate-containing wastewater was concentrated by reverse osmosis, then passed through a nanofiltration membrane to recover the copper sulfate. The recovered water was recycled to the process. This is an appropriate option for electroplating waste. At low pH, acetic acid and lactic acid are largely undissociated and pass through such a membrane. At higher pH, they do not go through. These acids can also be separated by Nafion membranes,108 weak base ion-ex- change resins,109and by extraction with long-chain tertiary amines.110Cheese whey, which contains 4–6% sodium chloride, and soy sauce, which contains 18% salt, can be desalted in this way. Such a process can replace more ex- pensive salting-out procedures, in which salts are added to cause a product to separate.111The fatty acids, oils, and fats left in the wastewater from oil-processing plants can be re- covered in this way.112An aromatic polyamide membrane for desalination by reverse osmosis had improved flux, with no loss in ion rejection after treatment with hydroflu- oric acid.113
Electrodialysis uses stacks of pairs of anion- and cation- exchange membranes in deionizing water and in recovery of formic, acetic, lactic, gluconic, citric, succinic, and glu- 7.27
7.28
104–105.98Fructose can be separated from glucose by using a liquid membrane with a similar boronic acid in microp- orous polypropylene.99 When glucose isomerase is in- cluded in the liquid, the output can be more than 80% fruc- tose. High-fructose corn syrup is used widely as a sweetener.
IV. MEMBRANE SEPARATIONS
Membranes can be used to separate molecules that differ in size, polarity, ionic character, hydrophilicity, and hy- drophobicity.100Their use is less energy-intensive than dis- tillation. They can often separate azeotropes and close-boil- ing mixtures. They can sometimes replace traditional methods, such as solvent extraction, precipitation, and chromatography, that can be inefficient, expensive, or may result in the loss of substantial amounts of product. Ther-
permeable to anions.118 They are used in electrochemical processes, such as the electrolysis of aqueous sodium chlo- ride to produce chlorine and sodium hydroxide. They are re- placing polluting mercury cells in this application.
Membranes can be made in various shapes. The plate and frame types have less surface area per unit of space than the spirally wound ones; hollow fiber ones119have the most. In commercial reverse osmosis, spirally wound ones are used in 75% of the cases and hollow fibers in 25%. Fouling can be a problem with membranes. A prefilter120is used to remove larger particles. The liquid can be flowed across a surface instead of directly at it to reduce fouling. High-frequency flow reversal and local shear enhancement (possibly by ultrasound) can reduce fouling.121Mechanical vibration at 60 Hz is said to increase the flow rate five- fold.122If the particles causing the fouling have a net sur- face charge, they can be repelled by a membrane with the same surface charge.123The use of an electromagnetic field with a conventional reverse osmosis membrane reduced fouling severalfold and doubled the flux.124 Sulfonated polysulfone membranes are fouled less by proteins.125The fouling of membranes by natural organic matter is delaying the acceptance of the technology for purifying drinking wa- ter.126The problem is being studied intensively. A clever system has been used to prepare a self-cleaning membrane for the concentration of a solution of casein. Trypsin is at- tached to the aminated polysulfone membrane with glu- taraldehyde.127 Any proteins that settle on the membrane are digested. When the fouling impurities in drinking water are better identified, such an enzymatic technique may be applicable. If an inorganic membrane becomes fouled with organic matter, the latter can be burned off. If the mem- brane fouled very often, this could be inconvenient for a water treatment plant.
Microporous membranes that are not wetted by the liq- uid on either side can be used to concentrate solutes to high levels at low temperature and pressure. The volatile com- ponent crosses the vapor gap to the receiving liquid. This technique of osmotic distillation has been used to concen- trate fruit and vegetable juices, as well as pharmaceuticals, sugars, proteins, and salts of carboxylic acids.128
The pores of microporous membranes can be filled with liquids to form liquid membranes that can be used in sepa- rations.129Transport through such membranes is often fa- cilitated by the addition of a carrier to the liquid to improve the separation factor or the flux rate. A solution of a long- chain tertiary aliphatic amine in kerosene in the pores of a
ied the carrier-mediated transport of ions through liquid membranes in polymer pores using salicylimine–uranyl complexes.131Dihydrogenphosphate ion moved through 140 times as fast as chloride ion. Catecholamines have been extracted using a boronic acid calixarene in 2-nitro- phenyl octyl ether (7.29).132(The analogue with an octade- cyloxy group meta to the boronic acid group was about nine times as effective as the one shown.)
This is analogous to the extraction of sugars with boronic acids described earlier. The recovery of phenylala- nine from a fermentation broth has been simplified by us- ing a microporous poly(tetrafluoroethylene) membrane with tri-n-octylmethylammonium chloride in toluene in the pores.133 Phenylalanine can also be separated using the quaternary ammonium salt with 2-nitrophenyl octyl ether in a cellulose triacetate membrane.134Kerosene flowing in hollow fiber membranes can remove 99.9% of organic pol- lutants, such as benzene, p-dichlorobenzene, chloroform, and carbon tetrachloride, from wastewater outside the fibers.135
Anions can be separated by varying the applied poten- tial across a porous polypropylene membrane with polypyrrole in the holes.136Chloride ion had high perme- ability, whereas sulfate and benzoate had low permeability. Almost all gas separations are performed with non- porous membranes.137The rate of movement through the membrane depends on the diffusivity and solubility of the gas in the polymer.138 Diffusion is higher for small molecules. It is faster in a rubbery polymer than in a glassy polymer. The selection of nitrogen over pentane in natural rubber is 10, but in polyvinyl chloride, it is 100,000. The rate of movement of the gas through the polymer is in- versely related to the thickness. For a rapid rate, the poly- mer film should be as thin as possible. For a mechanically strong defect-free film, the limit is about 20 m. In prac- tice, a very thin film is placed on a thicker porous structure of the same or a different material. To obtain a satisfactory flux rate, the surface area is made as large as possible, of- ten by the use of hollow fibers. The flux rate can vary with the temperature and pressure. The driving force for move- ment through the membrane is a pressure, concentration, or pH gradient. The separation factor, , usually becomes lower as the flux rate increases. If the separation factor is low, it may be necessary to use more than one stage. For practical use, the membrane must be mechanically, ther- mally, and chemically stable over a long period and have a high selectivity for the desired separation.
Common industrial applications include the following separations: (a) nitrogen and oxygen from air, (b) hydrogen from refinery gases, (c) hydrogen from ammonia plant off- gas, (d) carbon dioxide from methane (for natural gas and landfill gas), (e) carbon monoxide from hydrogen (for ad- justment of the ratio for hydroformylation of olefins), (f) helium from methane, (g) hydrogen sulfide from methane (for natural gas), and (h) organic vapors from air.139Gas separation membranes offer low-energy consumption, re- duced environmental impact, suitable cost effectiveness at low gas volumes, low maintenance costs, and ease of oper- ation, with space and weight efficiency.140Because the units are modular, it is easy to expand a plant. Some exam- ples will be given to illustrate the diversity of approaches used. Nitrogen from commercial hollow fiber membrane systems is available in 95–99.9% purity.141Membrane- based nitrogen costs one-third to one-half as much as liquid nitrogen. The best separation factors for oxygen over nitro- gen (8–12, and in one case 26) appear to be with aromatic polyimides and polytriazoles.142The permeability was low where the -valve was 26 at 30°C and this dropped to 13 for the same membrane at 100°C. This membrane had a separation factor of 169 for hydrogen over nitrogen at the higher temperature. Polyaniline membranes separate oxy-
valve of oxygen over nitrogen to 7–15.145The flux rate can also go up. The problem with these systems is one of lim- ited stability, partly owing to the oxidation of the cobalt complex. Zeolites and carbon molecular sieves are also used to separate oxygen from nitrogen.146The latter have an -valve of 9–16, but are vulnerable to moisture. To avoid this, hollow fiber membranes of the molecular sieve have been coated with copolymers of tetrafluoroethy- lene.147The coated material gave an -valve of 200 for hy- drogen over methane. Air Products has used Li3Co(CN)5 2 HCON(CH3)2to adsorb more oxygen reversibly than a car- bon molecular sieve.148It lost 15% of its activity over 545 cycles in 17 days. It also degraded in moist air. For com- mercial use, the half-life must be greater than half a year.
There is also interest in separating carbon dioxide from nitrogen in flue gases. Both have important industrial uses. The separation factor for carbon dioxide over nitrogen with polycarbonate and polysulfone membranes149is 35–40; with polyethylene glycol in cellulose acetate150is 22; with polyether imides containing polyethylene oxide units151is 70; with an amine modified polyimide152 is 814; with a copolymer of dimethylaminoethyl methacrylate and acry- lonitrile153 is 60–90; and with 40% 3-methylsulfolane in poly(trimethylsilylmethyl methacrylate)154 is more than