Heterogeneous acid–base reactions

Key point: The surfaces of many catalytic materials and minerals have Brønsted and Lewis acid sites. Some of the most important reactions involving the Lewis and Brønsted acidity of inorganic compounds occur at solid surfaces. For example, surface acids, which are solids with a high surface area and Lewis acid sites, are used as catalysts in the petrochemical industry for the interconversion of hydrocarbons. The surfaces of many materials that are important in the chemistry of soil and natural waters also have Brønsted and Lewis acid sites. Silica surfaces do not readily produce Lewis acid sites because –OH groups remain tenaciously attached at the surface of SiO2 derivatives; as a result, Brønsted acidity is dominant. The Brønsted acidity of silica surfaces themselves is only moderate (and comparable to that of acetic acid). However, as already remarked, aluminosilicates display strong Brønsted acidity. When surface OH groups are removed by heat treatment, the aluminosilicate surface possesses strong Lewis acid sites. The best-known class of aluminosilicates is the zeolites (Section 14.15), which are widely used as environmentally benign heterogeneous catalysts. The catalytic activity of zeolites arises from their acidic nature and they are known as solid acids. Other solid acids include supported heteropoly acids and acidic clays. Some reactions occurring at these catalysts are very sensitive to the presence of Brønsted or Lewis acid sites. For example, toluene can be subjected to Friedel–Crafts alkylation over a bentonite clay catalyst:

When the reagent is benzyl chloride Lewis acid sites are involved in the reaction, and when the reagent is benzyl alcohol Brønsted sites are involved.

Surface reactions carried out using the Brønsted acid sites of silica gels are used to pre pare thin coatings of a wide variety of organic groups using surface modification reactions such as

Thus, silica gel surfaces can be modified to have affinities for specific classes of molecules. This procedure greatly expands the range of stationary phases that can be used for chromatography. The surface –OH groups on glass can be modified similarly, and glassware treated in this manner is sometimes used in the laboratory when proton-sensitive com pounds are being studied. Solid acids are finding new applications in green chemistry. Traditional industrial proc esses generate large volumes of hazardous waste during the final stages of the process when the product is separated from the reagents and byproducts. Solid catalysts are easily separated from liquid products and reactions can often operate under milder conditions and give greater selectivity.

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مواضيع ذات صلة

Symmetry operations, elements and point groups

The stability field of water

Standard potentials and spontaneity

Superacids and superbases

Acidic and neutral solvents

Basic solvents

Solvents as acids and bases

Interpretation of hardness

The classification of acids and bases

Halogens as Lewis acids

Group 14 Lewis acids

Group 13 Lewis acids