Silica and Alumina

Silica-supported Lewis acids (e. g. ZnCl2, Znl2, or TiCl4) were also studied in Diels-Alder reactions of furan [37,38]. Reactions were performed at 25 °C and the best results were obtained in the absence of solvent. The combination of dienophile and supported Lewis acid had a great influence on the outcome of the reactions. Reaction with the chiral dienophile (-)-8-phenylmenthyl acrylate gave the endo adduct in 68 % d. e. and the exo adduct in 70 % d. e. [37]. 

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The method for preparing (-)-menthol (73) from (+)-citroneUal (68), which can be fractionally distilled from citroneUa oU, is cyclization by the ene-reaction. The reaction can be done thermally or using alumina and silica catalysts (145—147). 

Note The reagent can be employed on silica gel, alumina and silica gel plates but not on amino or polyamide layers. The dipping solution can also be employed as a spray solution. 

Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

Phosphoric acid esters having a low content of arsenic can be obtained by treating with 0.1-10% adsorbents such as activated clay, active carbon, alumina, and silica gel to decrease the arsenic content. Thus, 100 parts lauryl phosphate containing 10.3 ppm As and 2 parts activated clay were mixed at 60-70°C for 2 h and filtered to give lauryl phosphate only containing 0.6 ppm As [28]. 

The catalysts mentioned above are soluble. Certain cross-linked polystyrene resins, as well as alumina and silica gel, have been used as insoluble phase-transfer catalysts. These, called triphase catalysts, have the advantage of simplified product work up and easy and quantitative catalyst recovery, since the catalyst can easily be separated from the product by filtration. 

The author wishes to thank Dr. J.B. Cohen for supplying samples of Pt and Pd on alumina and silica and Drs. J. Schwank and A.K. Dayte for samples of Ru and Au on magnesia and silica. This work was supported by the US Department of Energy under Contract DMR-76ER02995 and has make use of the resources of the ASU Faoiltity for High Resolution Electron Microscopy, supported by NSF grant DMR 8306501. 

Supported platinum. The STEM and TPD data for platinum supported on alumina and silica are summarized In Table I. The platlnum-slllca samples show a high degree of variability In size and mass. This variability Is Indicative of the mobility of platinum on silica at elevated temperatures, l.e., 500°C. These samples were of little... 

The powder is principally composed of zinc oxide (Table 6.2). Magnesium oxide is found in all current commercial brands in amounts that range from 3 to 10%. Alumina and silica are sometimes to be found. Present day compositions show less variation than formerly when bismuth, calcium and barium oxides, or sometimes no additives, were to be found in commercial examples (Paffenbarger, Sweeney Isaacs, 1933). 

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

Eluotropic series can be expressed more precisely by the Snyder parameter e°, given in Table 4.2 for alumina and silica gel. It is important to take into account eluotropic series and know some rules when solvents are used according to this table [16,17] ... 

EPR studies of host-guest complexes of carotenoids Measuring distances between carotenoid radicals and distant metals in matrices by using ESEEM methods and pulsed EPR relaxation techniques EPR studies of radical cations on activated alumina and silica-alumina... 

Radical anions of tetracyanoethylene and trinitrobenzene have been observed on Ti02 and MgO. For nitro-benzene compounds adsorbed on well-degassed MgO, Tench and Nelson (90) reported spectra similar to those observed for the radical anions on alumina and silica-alumina. After discussing several possibilities for the source of electrons in MgO they suggested that some of the lattice oxide ions may act as electron donors. One of the difficulties with this proposal is the failure to detect the resulting... 

In the cracking of benzene to acetylene over alumina- and silica-supported nickel catalysts it was observed that the selectivity of the reaction, expressed as the ethyne/ ethene ratio, was dramatically affected (from 1 9 to 9 1) by controlling the micro-wave energy input (i. e. 90% selectivity) [83]. 

Note The positions of alumina and silica gel are dependent on water content. 

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

Figure 8.12 Raman spectra of alumina- and silica-supported molybdena catalysts after impregnation of the supports with solutions of ammonium heptamolybdate, (NH4)6Mo7024 4 H20 of different pH values, and after calcination in air at 775 K. See Table 8.3 for a list of characteristic Raman frequencies of molybdate species. The sharp peaks in the spectra of the calcined MoOySiOj catalyst are those of crystalline Mo03 (from Kim el at. [43J).

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