The effect of using a methanol–water solvent mixture on pH oscillations in the palladium-catalyzed phenylacetylene oxidative carbonylation reaction

Abstract

The palladium-catalyzed phenylacetylene oxidative carbonylation reaction has been shown to produce regular oscillations in pH, reaction heat, and turbidity when conducted in batch with methanol as solvent. Previous studies of the catalytic system showed that water plays an important role in the behavior of the catalytic cycle. Investigating this role further, the reaction was performed in methanol-water mixtures. Experiments demonstrated that, under the conditions studied, oscillations in pH are feasible when the water concentration in the system is increased (0-30 V%). At the same time oscillations visually changed, being transformed from regular oscillations to more stepwise behavior as the proportion of water is increased. By establishing the relationship between the recorded pH and HI concentration it was found that increased water concentration suppresses HI formation and hence slows the autocatalysis in the reaction. It was also found that oscillations begin when the HI concentration is in the region of 1 9 10 -3 mol L -1 . Additionally, as the amount of water in the system increases a slight decrease in the concentration of HI when oscillations start was noted.

From small molecules to polymeric catalysts in the oscillatory carbonylation reaction: multiple effects of adding HI

“…A detailed study of the effect a methanol-water solvent composition has on recorded pH and actual concentration of acid present in solution has been previously published for the PdI 2 /PhAc oscillatory carbonylation system. 12 Samples were taken at the end of the experimental runs (unless otherwise stated within the main text) and filtered over silica prior to being diluted 1 : 2 with methanol. The samples were analysed for starting material conversion and product distribution by a Varian Saturn 2200 gas chromatograph fitted with a VF-5ms column (30 m) and equipped with a mass spectrometry detector (GC-MS).…”

“…Oscillations in the PhAc/PdI 2 system are sustained in a batch reactor for days of monitoring with the possibility of varying the amplitude and period of oscillations by adjusting the temperature of reaction, 10,11 concentration of the catalyst, substrate or even water content. 12,13 Besides the sustained oscillations, the PCOC reaction has demonstrated a temperature-dependent product selectivity, which is potentially useful for industrial application due to a reduction of costs for isolation and purification, 14 applicable to a number of substrates, as demonstrated by Gorodsky. [15][16][17] Recently oscillations in pH were reported for PCOC, where an alkyne-functionalised polymeric substrate (poly(ethylene glycol)methyl ether acetylene, abbreviated as PEGA) was employed in place of PhAc, extending the opportunities of application of PCOC in smart materials.…”

Combining polymer-bound catalyst with polymeric substrate for reproducible pH oscillations in palladium-catalysed oxidative carbonylation of alkynes

“…Observed trends suggest that Chi-Pro-Pd catalysis proceeds in a slower manner as the catalyst remains heterogenous, bound to the polymer structure, and the reaction requires liquid to solid diffusion, reaction on the solid surface and subsequent diffusion back into the liquid phase. On the other hand, higher conversions and lower pH recorded for Chi-Pd suggest catalyst leaching and a homogeneous PdI 2 (from leached Pd) catalytic path [21,26,44].…”

Broadening the scope of Pd-catalyzed oscillatory carbonylation reactions: solvent, substrate, catalyst

“…The rate of the pH drop is an indication of the rate of initial autocatalytic hydrogen ion formation and observed trends could suggest that this rate decreases in 1-BuOH and further slows down in 1-HexOH, indicating the effect of the increase in aliphatic alcohol chain length. While this may be the case, further studies seeking to correlate recorded pH with hydrogen ion concentration are needed to confirm this given that the pH of non-aqueous solutions is being recorded using aqueous pH [35,36]. Oscillations in pH were recorded in MeOH, EtOH, 1-PrOH and 1-BuOH.…”