Dehydration silica-alumina

Scheme 1 Structure of MeReOs dispersed on the surface of dehydrated silica-alumina, as established by EXAFS and DFT (5).

According to Leftin and HaU (88b) and Webb (92), a band appears at 423 m/x, when 1,1-diphenylethylene, Ph2C=CH2, vapor is adsorbed under high vacuum on a dehydrated silica-alumina catalyst but not on silica gel Fig, 15. 

Dordmieux-Morin et al. (1991) using H MAS NMR spectroscopy showed that partly dehydrated silica-alumina and strongly dealuminated HY zeolite have similar properties with respect to surface hydroxyls. However, there is a fundamental difference between the dehydration processes of crystalline and amorphous samples due to the differences in the surface structure and OH group location. 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

Dehydration and dehydrogenation combined utihzes dehydration agents together with mild dehydrogenation agents. Included in this class are phosphoric acid, sihca-magnesia, silica-alumina, alumina derived from aluminum chloride, and various metal oxides. 

Adsorption processes use a solid material (adsorbent) possessing a large surface area and the ability to selectively adsorb a gas or a liquid on its surface. Examples of adsorbents are silica (Si02), anhydrous alumina (AI2O3), and molecular sieves (crystalline silica/alumina). Adsorption processes may be used to remove acid gases from natural gas and gas streams. For example, molecular sieves are used to dehydrate natural gas and to reduce its acid gases. 

The dehydration of 1-hexanol to hexene was conducted over heterogeneous sulfated zirconium oxide catalyst [19, 138]. The zirconia was treated with sulfuric acid and is known as super acid catalyst, having well documented performance for many reactions [19]. The reaction conditions are notably milder as for other acid catalysts, such as silica-alumina. 

This method has been applied (M5) for modeling the vapor-phase rate of dehydration of secondary butyl alcohol to the olefin over a commercial silica-alumina cracking catalyst. Integral reactor data are available at 400, 450, and 500°F. Two models considered for describing this reaction are the single site... 

Let us consider the data taken by Laible (LI) on the dehydration of normal hexyl alcohol at 450°F over a silica alumina catalyst. The single- and dualsite surface reaction controlled models applying to alcohol dehydration were discussed in Section V,A,2. We now consider, however, the functional forms given, for example, by Eq. (84), as probably being capable of describing the data, but do not restrict the Ct and C2 plots to a linear pressure dependence as before. Rather, we obtain an empirical pressure dependence from the... 

Silica-alumina mixtures are of great technological importance in the oil industry as catalysts for petroleum processing. The cracking activity is closely linked to surface acidity. Other typical reactions catalyzed by silica-alumina are the dehydration of alcohols and the polymerization of olefins. 

Basila (365) studied the izifrared spectrum of silica-alumina dehydrated at 500°. The technique developed by Peri and Hannan (333) was used. Only one OH stretching frequency at 3745 cm- was observed. This coincides with the absorption of isolated hydroxyl groups on pure silica. The absorption peak is not influenced by the chemisorption of water vapor at 150°. The chemisorbed water retains its molecularity, does not form hydrogen bonds with the isolated silanol groups, and is adsorbed on sites which can be poisoned by treatment with potassium acetate. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

Fiq. 17. Number of protons in silica-alumina (12.5 wt. % AljOj) dehydrated at 500° versus BET surface area. Dashed line is the best straight line representing the data (173). 

Fig. 20. Saturation behavior of NMR absorption (x") and dispersion derivative at resonance (axV Ho) for proton resonance of silica-alumina (sample of area 425 meter / gram) dehydrated at 500°. The audio modulation frequency was 40 c.p.s. Arrows on abscissa indicate values of Hi for which saturation of x" and dx / Ho occurs (17S).

In summary, the NMR data indicate that the protons remaining on silica gel and silica-alumina after dehydration at 500° are present as SiOH groups which are distributed randomly about the surface of the solid. 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

The reaction mechanism of amine deamination and disproportionation has been put forward by analogy with other eliminations, namely dehydration and dehydrochlorination [149,155], its characteristic feature being the cooperation of acidic and basic sites. In the deamination, /3-hydrogen and the NR2 group (R is hydrogen or alkyl) are eliminated by an E2-like mechanism on alumina, but by an El-like mechanism on silica-alumina. The nature of the acidic sites is not clear, protons from surface hydroxyls or aluminium ions are possible candidates. However, it seems... 

The same catalysts as for dehydration are suitable for the dehydrosulphidation, i.e. alumina, silica—alumina, zeolites, metal oxides and metal sulphides. (For a comparison of their activities, see ref. 247.)... 

When acetaldehyde and ammonia in a 3 1 mole ratio are fed over dehydration-dehydrogenation catalysts, such as PbO or CuO on alumina, ThOj, or ZnO or CdO on silica-alumina, or CdF2 on silicamagnesia at 400-500oC. and atmospheric pressure, an equimolar mixture of 2-and 4-picolines can be obtained in 40-60% yields. When a mixture of acetaldehyde, formaldehyde, and ammonia in about 2 1 1 mole ratio is passed over such catalysts, pyndine and 3-picohne are produced their ratios are usually 1 0.8, bnt the amounts of pyiidine can be increased by changes in the feed. 

Dehydration of ethanol has been effected over a variety of catalysts, among them synthetic and naturally occurring aluminas, silica-aluminas, and activated alumina (315—322), hafnium and zirconium oxides (321), and phosphoric acid on coke (323). Operating space velocity is chosen to ensure that the two consecutive reactions,... 

The reaction is catalyzed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include silica—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

The work of Misono et al. (55) illustrates how acid strength distributions for silica-alumina catalyst can be deduced from catalytic titration measurements by use of an appropriate series of reactants. Surface concentration of amine, pyridine in this case, was adjusted by proper choice of amine partial pressure and desorption temperature while carrier gas flowed over the catalyst sample. At each level of chemisorbed pyridine, pulses of the reactants were passed over silica-alumina at 200°C and the products analyzed. The reactants were t-butylbenzene, diisobutylene, butenes, and f-butanol. It was concluded that skeletal transformations require the presence of very strong acid sites, that double-bond isomerization occurs over moderately strong acid sites, and that alcohol dehydration can occur on weak acid sites. 

A unique way of identifying acid sites in amorphous silica-alumina was tried by Bourne et al. (128). These authors decided to synthesize, then characterize, two extreme types of acid site structures that they felt existed in commercial silica-aluminas. The two catalyst types consisted of low concentrations (<1.4% wt) of aluminum atoms incorporated (a) on the surface of silica gel (termed aluminum-on-silica) and (b) within the silica lattice (termed aluminum-in-silica). From infrared measurements of pyridine chemisorbed on the two materials, they conclude that dehydrated aluminum-on-silica contains only Lewis acid sites and that dehy-... 

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

The further development of the theory of nonuniform surfaces in the U.S.S.R. was helped by the mathematical methods of Zel dovich and Roginskil (200,201,331). A. V. Frost analyzed some work on the subject (mostly Russian) in a recent review (10) and concluded that an equation derived by him on the assumption that the reactants are adsorbed on a uniform surface and that no significant interactions take place between the adsorbed molecules, satisfactorily described many reactions on non-uniform surfaces including cracking of individual hydrocarbons and petroleum fractions, hydrogen disproportionation, and dehydration of alcohols. From the experimental results it was concluded that the catalytic centers on the surface were not identical with the adsorption centers. The catalysts used consisted of different samples of silica-alumina and pure alumina. 

Simultaneous dehydrogenation and dehydration of cyclohexanol over silica-alumina cracking catalyst at 340-375° was reported (245) to give a 78 to 95% yield of hydrocarbons. Subsequent hydrogenation resulted in the formation of 45% methylcyclopent.ane (245). 

The dehydration reaction is performed over a suitable sold acid catalyst (alumina or silica-alumina) at typically 250°C. The equiUbrium is established and the products separated from any unconverted ethanol by distillation. The ethanol is recycled Figure 10.9. 

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