Carbonate system carbon dioxide hydration

Larson, S.D., Phase Studies of the Two-Component Carbon Dioxide-Water System, Involving the Carbon Dioxide Hydrate, University of Illinois, Urbana, IL (1955). 

There is an abundance of experimental gas partial pressures for gas hydrate equilibria across a broad range of temperatures (Fig. 3.10 Sloan 1998). The lower temperature limit in our model database for these systems is 180 K (Fig. 3.10) because this is the lower limit of our model s ability to estimate aw (Fig. 3.1, Eq. 3.11), which is needed to calculate the solubility product of gas hydrates (Eq. 3.36). In our model, the upper temperature limit for methane hydrate is at 298 K (25 °C), which is the upper temperature limit for FREZCHEM the upper temperature limit for carbon dioxide hydrate is at 283K (10 °C), which is the temperature where liquid C02(l) becomes the thermodynamically stable phase. 

Larson SD (1955) Phase studies of the two-component carbon dioxide-water system involving the carbon dioxide hydrate. PhD dissertation, University of Illinois... 

Carbon-dioxide hydration and its mechanism in living systems are of fundamental importance for bioinorganic chemistry. In 1932 the existence of an enzyme catalyzing CO2 hydration in red blood cells was established. The enzyme was named carbonic anhydrase (abbreviated CA). In 1939 the enzyme was recognized to contain zinc. Because CO2 is either the starting point for photosynthesis or the endpoint of substrate oxidation, carbonic anhydrases are now known to be ubiquitous, occurring in animals, plants, and several bacteria. Different... 

The question of the possible presence of a critical temperature below which a gas hydrate ceases to be thermodynamically stable is important to speculation regarding the occurrence of gas hydrates in the colder reaches of the solar system. It has been suggested that such a quadruple point may exist for carbon dioxide hydrate. 

A trick to accelerate the simulation is to impose order parameters relevant to the description of the crystalline state to drive the system towards nucleation. This method was applied to the study of water nucleation and carbon dioxide hydrate formation using a conventional Monte Carlo method. ° ° Stein-hardt s bond-orientational order parameters Q and W , based on quadratic and third-order invariants formed from bond spherical harmonics (Y/ ,(0, (p)), were employed. These order parameters allow quantitative measures of the local symmetry in liquids and glasses. The rotationally invariant orientational order parameters are defined as ... 

Similar MC calculations were used by Trout s group to study the carbon dioxide-liquid water interface at 220 K and 4 MPa near the phase boundary of a carbon dioxide hydrate (273 K and 4MPa). Nucleation was achieved by seeding the system with a cluster of carbon dioxide hydrate. It was found that a small cluster with diameter <9.6 A dissolved into the solution readily. A hydrate crystal started to grow, however, when a hydrate cluster twice that size (19.3 A) was implanted into the system. The crystal eventually spanned the whole system (Figure 22). Thus the critical nucleus size for hydrate nucleation is estimated to be about 19 A consisting of approximately 200 water molecules. This is a considerably smaller number than that estimated from the local harmonic model of around 600 molecules. The theoretical results refuted the labile cluster hypothesis.This hypothesis speculates the agglomeration... 

The majority of practical micellar systems of Tionnal micelles use water as tire main solvent. Reverse micelles use water immiscible organic solvents, altlrough tire cores of reverse micelles are usually hydrated and may contain considerable quantities of water. Polar solvents such as glycerol, etlrylene glycol, fonnamide and hydrazine are now being used instead of water to support regular micelles [10]. Critical fluids such as critical carbon dioxide are... 

Hindered rotation, 33, 34 internal, 367 Homopolymer, 168, 183 Hot bands, 374 Hot lattice, 4, 11, 21 Hydrates, 7, 9, 21, 31, 41 crystallization, 44 Hydrochloric acid clathrates, 2 in hydroquinone, 7 Hydrogen, bound, 4, 175 bromine hydrate, 35 4- carbon dioxide system, 110 4 carbon monoxide system, 96, 108 chloride hydrate, 35 clathrates, 2 chloride, 30... 

Metallic solutions, 120 Methacrylonitrile, 155 Methane, -f carbon dioxide system, 97 in clathrates, 30, 41 hydrate, 33, 34, 47... 

In steam-condensate systems, we are primarily concerned with the effect of carbon dioxide on pH levels. For example (and assuming all carbon dioxide is fully hydrated to carbonic acid) ... 

The RuClj/aq. HCl system is effective for hydration of phenylpropiolic acid to give acetophenone and carbon dioxide via the P-keto acid (Eq. 6.53) [86]. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

The system illustrated by (272) forms the basis of a model for the zinc-containing metalloenzyme, carbonic anhydrase (Tabushi Kuroda, 1984). It contains Zn(n) bound to imidazole groups at the end of a hydrophobic pocket, as well as basic (amine) groups which are favourably placed to interact with a substrate carbon dioxide molecule. These are both features for the natural enzyme whose function is to catalyze the reversible hydration of carbon dioxide. The synthetic system is able to mimic the action of the enzyme (although side reactions also occur). Nevertheless, the formation of bicarbonate is still many orders of magnitude slower than occurs for the enzyme. 

Ng, H.-J. Robinson, D.B. (1985). Hydrate Formation in Systems Containing Methane, Ethane, Propane, Carbon Dioxide or Hydrogen Sulfide in the Presence of Methanol. Fluid Phase Equilibria, 21, 145-155. 

Blockages of valves and pipes can also occur by gas hydrates. Such adducts can be formed by a number of gases with water. In Fig. 7.1-5 the pressure-temperature diagram of the system propane/water with an excess of propane is presented. The line, (g), shows the vapour-pressure curve of propane. Propane hydrate can be formed at temperatures below 5.3°C. At pressures below the vapor pressure of propane a phase of propane hydrate exists in equilibrium with propane gas (Fig. 7.1-5, area b). At higher pressures above the vapor pressure of propane and low temperatures a propane hydrate- and a liquid propane phase were found (area d). In order to exclude formation of gas hydrates these areas should be avoided handling wet propane and other compounds like ethylene, carbon dioxide [14], etc. 

Carbonate is an important ligand not only in chemistry but also biochemistry and geochemistry. The reason for this is to be found in the wide occurrence in Nature of carbon dioxide and its ready hydration to give carbonate based systems. Thus there are many naturally occurring carbonates e.g. calcite, malachite and these have been reviewed.103 The range of review literature prior to 1970 is extensive and the reader is encouraged to consult this.104,105... 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

There are only few data sets of aqueous solubility for systems with hydrates (1) methane and ethane solubility in water as a function of temperature ramping rate (Song et al. 1997), (2) carbon dioxide solubility in water by Yamane and Aya (1995), (3) methane in water and in seawater (Besnard et al., 1997), (4) methane in water in Lw-H region [see Servio and Englezos (2002) and Chou and Burruss, Personal Communication, December 18,2006, Chapter 6], As a standard for comparison, Handa s (1990) calculations for aqueous methane solubility are reported in Table 4.3. 

The most productive two-phase (H-V or H-Lhc) equilibrium apparatus was developed by Kobayashi and coworkers. The same apparatus has been used for two-phase systems such as methane + water (Sloan et al., 1976 Aoyagi and Kobayashi, 1978), methane + propane + water (Song and Kobayashi, 1982), and carbon dioxide + water (Song and Kobayashi, 1987). The basic apparatus described in Section 6.1.1.2 was used in a unique way for two-phase studies. With two-phase measurements, excess gas was used to convert all of the water to hydrate at a three-phase (Lw-H-V) line before the conditions were changed to temperature and pressures in the two-phase region. This requires very careful conditioning of the hydrate phase to prevent metastability and occlusion. Kobayashi and coworkers equilibrated the hydrate phase by using the ball-mill apparatus to convert any excess water to hydrate. 

It has been shown that, in supercritical carbon dioxide, increases in water concentration result in increases in enzyme activity. The amount of added water needed for this increase varies and can depend on many factors, such as reaction type, enzyme utilized, and initial water content of the system. This is true until an optimal level is reached. For hydrolysis reactions, activity will either continue to increase or maintain its value. For esterification or transesterification reactions, once the optimal level of hydration has been reached, additional water will promote only side reactions such as hydrolysis. Dumont et al. (1992) suggests that additional water beyond the optimal level needed for enzyme hydration may also act as a barrier between the enzyme and the reaction medium and thereby reduce enzyme activity. Mensah et al. (1998) also observed that water above a concentration of 0.5 mmol/g enzyme led to lower catalytic activity and that the correlation between water content of the enzyme and reaction rate was independent of the substrate concentrations. 

A co-ordinated hydroxide ligand will still possess some of the nucleophilic properties of free hydroxide ion, and this observation proves to be the basis of a powerful catalytic method, and one which is at the basis of very many basic biological processes. In general, hydrolysis reactions proceed more rapidly if a water nucleophile is replaced by a charged hydroxide nucleophile. This is readily rationalised on the basis of the increased attraction of the charged ion for an electrophilic centre. However, in many cases the chemical properties of the substrate are not compatible with the properties of the strongly basic hydroxide ion. This is exactly the situation that biological systems find themselves in repeatedly. For example, the uncatalysed hydration of carbon dioxide is very slow at pH 7 (Fig. 5-61). 

By the use of a model system, Kimura et al. [17] tried to mimic the function of the two mechanistically most typical zinc(II) enzymes. Carbonic anhydrase (CA, EC 4.2.1.1) catalyses the reversible hydration of carbon dioxide to bicarbonate ion and its zinc(II) active site is bound to three histidine residues and a water molecule. Carboxypeptidase A (CPA, EC 3.4.17.1) catalyses the hydrolysis of the hydrophobic C-terminal amino acids from polypeptides, and its active-site zinc(II) is bound to two histidine residues, a glutamine residue and a water molecule which is hydrogen bound to a glutamine residue (Scheme 19). 

The hydrate and phenol clathrate equilibrium data of the water-carbon dioxide, phenol-carbon dioxide, and water-phenol-carbon dioxide systems are presented in Table 1 and depicted in Figure 2. In order to establish the validity of the experimental apparatus and procedure the hydrate dissociation pressures of carbon dioxide measured in this work were compared with the data available in the literature (Deaton and Frost [7], Adisasmito et al. [8]) and found that both were in good agreement. For the phenol-carbon dioxide clathrate equilibrium results, as seen in Figure 2, the dramatic increase of the dissociation pressures in the vicinity of 319.0 K was observed. It was also found in the previous study (Kang et al. [9]) that the experimental phenol-rich liquid-phenol clathrate-vapor (Lp-C-V) equilibrium line of the binary phenol-carbon dioxide system could be well extended to the phenol clathrate-solid phenol-vapor (C-Sp-V) equilibrium line (Nikitin and Kovalskaya [10]). It is thus interesting to note that a quadruple point at which four individual phases of phenol-rich liquid, phenol clathrate, solid... 

Soils vary greatly in composition and reactivity. Many complex and dynamic processes occur continuously in most soils composed primarily of mineral and organic matter, water, and air. The soil atmosphere is composed of oxygen, carbon dioxide, nitrogen, and several minor gases whereas the mineral fraction varies in amounts of sand, silt, and clay and in types and amounts of clay minerals. Moreover, hydration and base saturation of the clay minerals also vary considerably. The organic matter and mineral colloids present in the soil contribute directly and indirectly to the extremely active nature of pesticide-soil systems. Since soil water contains many soluble compounds, it serves as an essential medium for many chemical and physical processes. The extreme complexity of these soil systems has been the primary reason that so few fundamental studies have been undertaken involving the ultimate fate of pesticides in soils. 

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