Solid Bronsted acidity

Perhaps the best known catalytic property of clays is their Bronsted acidity, which as described in Section 3 can be controlled by ion exchange. The most widely studied organic reactions using clays as solid Bronsted acid catalysts are described in this section. 

Chemisorption of metallocenes on conventional Bronsted acid surfaces often results in formation of catalytically inert p-oxo species via M—CH3 protonolysis. Recently, superacidic solid Bronsted acids,... 

Simple use of conventional solid acids and bases affords the efficient production of furfurals. First, a soHd base catalyzes the isomerization of the aldose, such as glucose, xylose, and rhamnose, into the corresponding ketose, for example, fructose, xylulose, and rhamnulose. Second, a solid Bronsted acid dehydrates the ketose into furfurals (HMF, furfural, and MF). Such an approach is impossible for Hquid acid-base pairings because of their self neutralization. 

Strong, solid Bronsted acid catalyst. It has a wide range of applications including alkene alkylations, Beckmann rearrangements and esterification reactions its uses in Friedel-Crafts reactions are described in section 16.5.1. 

Envirocat EPIC General acid catalysis Beckmann rearrangements Acylations with carboxylic acids and alkylations with alkenes Strong, solid Bronsted acid Improved product quality Easier separation... 

A second example of acid and base catalysts that can be used in a one-pot multireaction is the use of montmorillonite and hydrotalcite. The Ti-exchanged montmorillonite (Ti +-mont) is a solid Bronsted acid catalyst as a result of the presence of Ti + centers in the interlayer spaces. (37)a, (64). In contrast, hydrotalcite (HT) is considered a promising base catalyst because the HT surface displays a tunable basicity. Since large HT particles cannot enter the narrow interlayer of the Ti +-mont, these catalysts can be used in the same reactor, allowing both... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Reaction of 1 mole of aminals 352 with 4 mol of methyl 3-aminocrotonate in the presence of the solid acids montmorillonte clay (Kio) and ZF520 zeolite as strong Bronsted acidic catalysts, gave 1,4-dihydropyridines 353 and 2-methyl-4//-pyrido[l, 2-n]pyrimidin-4-one (99MI8). 

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Two such well studied systems are pyridine chemisorbed on alumina (15) and pyridine chemisorbed on silica-alumina (16). It had been previously shown that alumina contains only sites which adsorb pyridine in a Lewis acid-base fashion whereas silica-alumina has both Lewis and Bronsted acid sites. These two different kinds of sites are distinguishable by the characteristic vibrational bands of pyridine adducts at these sites (see Table I). Photoacoustic and transmission results are compared in Table II. Note that the PA signal strength depends on factors such as sample particle size and volumes of solid sample and transducing... 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

The reachon of benzene with ethylene or propylene to form ethylbenzene or isopropylbenzene (cumene) is an industrially important transformahon, with ethylbenzene as the key building block for polystyrene and cumene as the feedstock for phenol produchon [55]. Fthylbenzene was originally produced with a Lewis acid catalyst consishng of AlCfi or a Bronsted acidic solid phosphoric acid (SPA) catalyst [56]. Both catalyst systems suffered from equipment corrosion so, in the 1980s the Mobil-Badger vapor phase alkylation process was introduced, which... 

An area of broad interest in catalysis is the search for viable replacements for the widely used Bronsted liquid acids such as HF and H2SO4 and solid Lewis acids such as AICI3 and MgCl2. The liquid acids are corrosive and also costly, because of the need to work up the products by neutralization and repeated washing. In many cases, the contamination of the products by these acids induces degradation over time and limits the application of the products. The cost of multi-step washing can be quite high. Acidic ionic liquids therefore offer potential alternatives for such reactions (//). 

Some ionic liquids have tunable Lewis acidities and basicities. The tuning can be achieved simply by varying the anion fraction in the overall ionic liquid composition. In some cases, Bronsted acidity can also be introduced into stable ionic liquids. Many publications show the broad applicability of acidic or basic ionic liquid media in catalysis replacing corrosive liquids and solid catalysts. 

Similar to the method of modifying neutral ionic liquids with strong Bronsted acids, a Lewis basie ionic liquid was also obtained by adding conventional base, such as NaOH, into the neutral ionie liquid. However, the solubility of the base in the ionie liquid is not high, and part of the [BMIM]PF6 was converted to a solid phase, probably beeause of the anion exehange between PF and OH 107,108). 

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