Catalytic Activities of Various Moderately Strong Solid Acids and Their Correlation with Surface Polarity Parameters

Abstract

The specific rate constant k′ of the surface-mediated hydride-transfer reaction of 1.4-cyclohexadiene with triphenylmethylium is strongly dependent on the nature of the solid acid catalyst used. The catalysis of this hydride-transfer reaction by 30-moderately strong solid acid catalysts, e.g., silicas, aluminas, aluminosilicates, and titanium dioxide particles, has been studied. Generation of the triphenylmethylium when chlorotriphenylmethane is chemisorbed to the solid acid catalyst was used for the kinetic measurement. The individual, pseudo-first-order rate constant, k, of the hydride-transfer reaction increases linearly with the amount of solid acid catalyst used. The specific rate constant, k′, based on the surface area of the solid acid, was used to quantifiy the catalytic activity of the solid acids. k′ increases in the order silicas < aluminosilicates < titanium dioxides = aluminas. Ln k′ can be be correlated with the corresponding surface polarity parameters of the solid acid, e.g., Reichardt's E T (30), Gutmann's acceptor number AN, and Kamlet-Taft's parameters R (hydrogen-bond-donating acidity) and π* (dipolarity/polarizability). It is shown that the catalytic activity of a moderately strong solid acid catalyst can be characterized by Gutmann's acceptor number AN or the Kamlet-Taft R value. The different classes of solid acids, e.g., silicas, aluminas, and aluminosilicates, respectively, give different dependencies of k′ as a function of their solid acid acidity parameters.

Probing surface acidity, basicity, and dipolarity/polarizability of 12-tungstophosphoric acid by means of solvatochromic dyes

“…5), whereas moderately strong solid acids like silica or alumina do not ionize (C 6 H 5 ) 3 CH. 45 This indicates that a redox process is associated with the chemisorption of (C 6 H 5 ) 3 CH on H 3 PW. However, the fate of the hydride ion could not be determined.…”

“…34 Usually, the triphenylmethylium UV/vis absorption shows a double band with two maxima at l max ¼ 410 nm and 435 nm, the positions of which are independent of the environment used. 45,[54][55][56][57][58] It should be emphasized that when (C 6 H 5 ) 3 C + is adsorbed on H 3 PW two effects in the UV/vis spectrum are observed which are different to its behaviour with other solid acids or Lewis acids in solution. 45,[54][55][56][57][58] The main UV/vis absorption band evidently undergoes a bathochromic shift of about Dl % 10 nm, and secondly an additional UV/vis absorption band at l max ¼ 500 nm appears as a shoulder.…”

“…These Kamlet-Taft parameters a or Gutman's acceptor numbers AN calculated for a series of silicas, aluminas, and aluminosilicates correlate well with the catalytic activity of these solid acids. 45 Till now, there has been a lack of suitable solvatochromic indicators for determining the basicity of strong solid acids. 18 In this paper we will check a special aminobenzodifuranone, 46 dye 3, for probing the surface basicity (b value) of the heteropolyacid.…”

Assessment of the suitability of imidazolium ionic liquids as reaction medium for base-catalysed reactions Case of Knoevenagel and Claisen–Schmidt reactions

“…This signal is different from the H-2 of the starting [(bmim)PF 6 ] and would indicate that hydride abstraction according to Scheme 4 is taking place. The ability of 1,4 cyclohexadiene as hydride donor is very well documented [16].…”

Surface Functionalization of Silica with 2‐Vinylfuran by Cationic Polymerization

“…Thus, it seems that a cationic propagation reaction of the highly reactive monomer 2‐VF on a surface occurs effectively as long as crosslinking in the surrounding solution phase is suppressed 3. Crosslinking takes place preferentially on the silica surface, because the main fraction of the propagating species is located on the surface groups of the silica via an ion pair intermediate (Scheme ) 8,14…”