Hydrothermal acidic

Oefner, P. J., Lanziner, A. H., Bonn, G., and Bobleter, O., Quantitative studies on furfural and organic-acid formation during hydrothermal, Acidic and alkaline-degradation of deuterium-xylose. Monatshefte Fur Chemie 1992, 123 (6-7), 547-556. 

For such treated samples it is not easy to discriminate between two possible effects of dealumination, namely, the removal of some acid sites and the decrease in microporosity due to the deposition of aluminum-containing debris in the pores. Thus, hydrothermally dealuminated FER, hydro thermally dealuminated acid-washed FER. acid-washed FER, and CsFER were compared under the same experimental conditions (62). The results indicate the following order of isobutylene selectivities untreated FER < acid-treated FER < hydrothermally treated FER < hydrothermally acid-treated FER < CsFER 61). These results, obtained with noncoked catalysts, reinforce the interpretation in terms of shape selectivity. The hydrothermally acid-treated sample has acid sites located only in the micropores, and the aluminium debris in the micropores creates an additional constraint playing a role identical to that of Cs" in FER. 

Shimada, K., 1960. Properties and industrial application of amorphous silica connected with volcanic activities. XXIV. Effect of hydrothermal acid treatment on the transformation of silica gel. J. Chem. Soc. (Japan), Pure Chem. Sect. 81 422. 

For emerald, a hydrothermal reaction arrangement is used, analogous to that of Figure 4. Here the mineralizer is strongly acid, containing 8A[ NH Cl [12125-02-9] and HCl [7647-01-1] to give pH between 0.2 and 4.5. 

Fig. 1. General process operations for hydrothermal synthesis. Feedstocks may be oxides, hydroxides or salts, gels, organic materials, or acids or bases. The atmosphere within the reactor may be oxidising or reducing. To convert MPa to psi, multiply by 145.

Lead Sesquioxide. Lead sesquioxide (lead trioxide), Pb202, is an amorphous, orange-yeUow powder soluble ia cold water. It decomposes ia hot water and ia acids to lead salts plus Pb02. Lead sesquioxide can be prepared from lead dioxide by hydrothermal dissociation (43). 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

Sodium decreases the hydrothermal stability of the zeolite. It also reacts with the zeolite acid sites to reduce catalyst activity. In the regenerator, sodium is mobile. Sodium ions tend to neutralize the strongest acid sites. In a dealuminated zeolite, where the UCS is low (24.22°A to 24.25°A), the sodium can have an adverse affect on the gasoline octane (Figure 3-7). The loss of octane is attributed to the drop in the number of strong acid sites. 

Fig. 14. Equipment for the hydrothermal method, with hydrohalic acids as solvents.

Vanadium borophosphates 107-109 that contain cluster anions with different ring sizes can be synthesized by hydrothermal reactions of vanadium(III) oxide, boric acid and an appropriate phosphate salt [145,146]. The ring size of the cluster anion with the composition [(VO)2BP20iol 4-6 is determined by the size of the cation that is, however, included only in the interior of the cages with n = 5 and 6 (Fig. 28). 

Amend, J. P. and Shock, E. L. (1998). Energetics of amino acid synthesis in hydrothermal ecosystems. Science 281,1659-1662. 

In order to prepare ZSM-5 zeolite nanocrystals, an A1 source of aluminium isopropoxide was added into solution A, and hydrothermal synthesis of the solution A containing Si and A1 sources was carried out in an 0-15/cyclohexane solution at 120 degree C for 50 h. Figures 4 show ac-NHj-TPD spectra and a SEM photograph of the ZSM-5 zeolite nanocrystals. Nanocrystals with a diameter of approximately 150 nm were observed, and the NH3-TPD spectrum showed desorption of NHj above 600 K, indicating that the nanocrystals possessed strong acid sites. 

Hydrothermal synthesis of titanium dioxides using acidic and basic peptizing agents and their photocatalytic activity on the decomposition of orange II... 

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