Boria

The 3M Company manufactures a continuous polycrystalline alurnina—sihca—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal sihca and dimethylform amide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extmded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-p.m diameter aluminum borosihcate fibers, which are suitable for fabrication into textiles for use at temperatures up to 1427°C. 

B. Stanmore, D. N. Boria, and L. E. Paulson, Steam Dying of Eignite A Beview of Processes and Peformance, DOE/GEETC/Rl-82/1 (DE82007849), U.S. Department of Energy, available from National Technical Information Service, Washington, D.C., 1982. 

E. Adem, G. Burillo, E. Munoz, I. Rickards, L. Cota and M. Avalos-boria, Polym. Degrad. Stab., 81 (2003) 75. 

Bulk alumina and india are isostructural, with a linear structure OMOMO, while B2O3 molecule is V-shaped. The Ga203 can present the both types of isomers, the V-shaped structure being a little more stable than the linear one [1], These very different structural features (shape, electronegativities, etc.) of group lllA oxides may help explain their specific properties that fail to strictly follow any simple rule. Their amphoteric character (except for boria) that is not easy to evaluate, has been confirmed and quantified by the experimental microcalorimetric results. 

Sato et al. [195] have studied the surface borate structures and the acidic properties of alumina-boria (3-20 wt.%) catalysts prepared by impregnation method using B(MAS)-NMR measurements and TPD of pyridine, as well as their catalytic properties for 1-butene isomerization. The number of Brpnsted acid sites was found to increase with increasing boria content, and the catalytic activity was explained by the strong Brpnsted acid sites generated by BO4 species on the surface of alumina. 

It follows from all the above considerations that the acidic character of the surface is necessary for the esterification reaction. This view is supported by the parallel found by some workers [405,406] between the rate of esterification and that of other typical acid-catalysed reactions. A linear correlation was established between the rate of acetic acid—ethanol esterification and that of deisopropylation of isopropylbenzene on a series of silica—alumina, alumina—boria and alumina catalysts [406] a similar relation was found between the rate coefficient of the same esterification reaction and the cracking activity of a series of silica—alumina catalysts prepared in a different way [405]. 

Cr/alumina can be modified like Cr/silica. Adding titania is not particularly useful, but replacing the hydroxyls with fluoride does boost the activity by as much as 10-fold (62). An example is shown in Fig. 22, where activity is plotted versus the amount of fluoride impregnated onto a highly porous alumina. Too much fluoride tends to sinter the alumina and destroy the activity. Other modifications which improve the activity of Cr/alumina include adding chloride, sulfate, boria, phosphate, or 1-5% silica (62, 78). 

A detailed examination of the effects of operation conditions on boria on alumina catalyst performance and lifetime is reported in this paper, in an attempt to further elucidate the important parameters controlling optimisation and maintenance of caprolactam yield. 

Characterisation by XRD showed that boria (B2O3), was the only crystalline phase present on the surface of the alumina after calcination. 

Fig-1, (a) oxime conversion, (+ ) ceprolactam selectivity and ( ) caprolactam yield with time using 0.1 g boria on alumina catalyst at a reaction temperature of 300 C. 

The effect of temperature on the conversion, selectivity and yield after 3 hours on stream is shown in figure 2. In each case a catalyst mass of 0.1g boria on alumina catalyst was tested. With increasing reaction temperature the oxime conversion increased, however, maxima in lactam selectivity and yield were observed at a reaction temperature of 300 C. At higher temperatures excessive coking and side reactions were thought to occur,... 

Fig.4. (a) Oxime conversion, (b) Caproiactam selectivity, (c) Caproiactam Yield using 0.2g (.) boria on alumina, M regenerated boria on alumina and (+) alumina catalyst at 300 C for 30 hours. 

In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation. 

Alumina is one of the most widely used catalyst supports in the petroleum industry because it is robust, porous, relatively inexpensive, and—what is especially important—it is capable of contributing acid-catalyzed activity that can be tailored to suit the requirements of a diverse array of catalytic processes. These include reforming (52, 55), hydrotreating (84, 55), and paraffin isomerization (56-55). Since pure alumina is relatively inactive for the skeletal isomerization reactions that are necessary in such processes, its acid activity is promoted through the addition of catalyst components such as fluoride, chloride, phosphate, silica, or boria. After a discussion of pure alumina itself, we will review pertinent studies of surface acidity and catalytic activity of the promoted aluminas. 

The acid function of the catalyst is supplied by the support. Among the supports mentioned in the literature are silica-alumina, silica-zirconia, silica-magnesia, alumina-boria, silica-titania, acid-treated clays, acidic metal phosphates, alumina, and other such solid acids. The acidic properties of these amorphous catalysts can be further activated by the addition of small proportions of acidic halides such as HF, BF3, SiFit, and the like (3.). Zeolites such as the faujasites and mordenites are also important supports for hydrocracking catalysts (2). 

Ray et al.29 used V205/Cr2 03 catalysts supported on A1203 for the ammoxidation of xylenes. The maximum yields increased from 40% for o-xylene to 70% for m-xylene and, using a catalyst with boria addition, to 89% for p-xylene. From t.p.r. and e.s.r. measurements the authors conclude that a new compound VCr04 is present which is responsible for the catalytic activity. 

Cruz Reyes, X., Avalos Boria, M., L6pez Cordero, R., and L6pez Agudo, A., Appl. Catal. 120,147 (1994). 

M L. Hair and W. Herd, Reactivity of Boria-Silica Surface Hydroxyl Groups , J. Phys. Chem., 77 [16] 1965-9(1973). 

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