Researchers have coaxed an acid and a base to work in tandem to trigger chemical reactions.
“Usually, we cannot use an acid and a base in the same pot to catalyze different reaction steps, since they are not compatible with each other,” said N. Raveendran Shiju of the heterogeneous catalysis and sustainable chemistry group at the University of Amsterdam. But Shiju and colleagues Gadi Rothenberg and Albert Alberts, also from U. of Amsterdam; Syed Khalid, Brookhaven National Laboratory; and David Brown, University of Huddersfield in the UK, have succeeded in doing exactly that — aptly named antagonistic catalysis.
In the drawing, a journal cover image, the acid (red) and base (blue) sites are depicted as robots. (The Rock ’em Sock ’em Robots were designed by Marvin Glass and Associates and first manufactured by the Marx toy company in 1964). The antagonist sites are physically separated in private “boxing rings.” This physical separation prevents the acid and base sites from attacking each other, creating a truly bifunctional and tunable acid-base catalyst. Illustration by Itamar Daube.
The work was done in part at Brookhaven Lab’s National Synchrotron Light Source, using beamline X18B.
Using a simple and straightforward approach, the researchers created a bifunctional surface that combines the advantages of bifunctional enzyme-like catalysis with the ease of separation and robustness of traditional heterogeneous catalysis. They then demonstrated the bifunctional catalytic activity in tandem deprotection-aldol reactions and deprotection-Henry reactions — both specialized and powerful chemical reactions. The results were published online in Angewandte Chemie on September 15, 2011.
Because the work is at an early stage, the scientists do not anticipate immediate applications. What the team showed, however, is the feasibility to have base and acid groups on the same solid support without neutralizing each other.
“The complete conversion can now be done in one pot instead of two separate vessels, reducing the number of energy-intensive separation and purification steps” said Shiju.
By changing the ratio of polyacid and amine groups, the catalyst can be made predominantly basic, predominantly acidic, or equally acidic and basic. Similar types of catalysts can be synthesized using other heteropolyacids, widening the scope of antagonistic catalysis.
Mesoporous silica was first grafted with aminopropyl groups, making a basic catalyst. Then, phosphotungstic acid was immobilized on some of these groups. By varying the amount of phosphotungstic acid, the researchers could tailor the ratio of acid to base on the active site.
Characterizing the materials before and after each step confirmed that the pore structure remained unaltered, keeping the chemically important mesopores that give the catalyst a highly active surface area. Nuclear magnetic resonance spectroscopy (carried out in EPSRC solid-state NMR service, Durham University, UK) confirmed that the so-called Keggin structure of the phosphotungstic acid was maintained after immobilization.
The researchers then applied the catalyst in the one-pot tandem conversion of dimethoxymethylbenzene to trans-1-Nitro-2-phenylethylene. The results showed that both the amino groups and the acid polyanions retained their reactivity.
The same catalyst also catalyzed another acid-base tandem sequence — deacetalization followed by aldol condensation with malononitrile — showing a wide application scope.
Summary Slide (.pdf)