Abstract

Organic synthetic research laboratories generate a large amount of waste. Some of the waste is the silica gel used as the stationary phase in column chromatographic separations. Here, the authors discuss the possibility of recycling silica gel wastes thermally at 600 °C, at which temperature the remnant adsorbed organic material combusts. It could be shown that the recycled silica gel maintains its adsorption characteristics. The process could be repeated 10 times with any discernable deterioration of the separation properties of the silica gel for the product mixture of the reactions used in this research. In those cases where triphenyl oxide remained on the silica gel after the separation of the reaction mixture, such as after Wittig olefination and Appel-type reactions, an increase of phosphorus content was noted in the silica gel after thermal treatment. The original and recycled silica gel was partly analyzed by Brunauer-Emmett-Teller (BET) surface measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and in inductively coupled plasma optical emission spectroscopy (ICP-OES). The process significantly reduced waste production in our laboratory and also led to a reduction in costs associated with acquiring new silica gel and with the management and the disposal of spent silica gel. A simple environmental impact assessment has been carried out.

Highlights

  • In organic synthetic chemistry, silica gel is used extensively as the stationary phase in column chromatographic separations of reaction mixtures

  • It is generally thought that the triarylphosphine oxide is converted to phosphoric acid upon combustion and after cooling and hydration [9,10], where it is likely that amounts of polyphosphoric acids remain, where in our case phosphoric acids can be esterified with silanol groups

  • We found that after the thermolysis, the recycled silica gel gradually took up water so that by day 8 about 7.9 w%, by day 10 9.1 w% of the mass of the silica gel is water, either bound to the surface of the silica gel, inserted into the silica gel by siloxane cleavage or as interstitial water

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Summary

Introduction

Silica gel is used extensively as the stationary phase in column chromatographic separations of reaction mixtures. It has been noted that silica gel will lose adsorbed water at its surface already below 150 °C and that silanol groups will convert to siloxanes at above 300 °C [5,6,7] and that this may lead to structural instability [4] and to lower separation properties of the heat-treated silica. It has been warned against using such recycled silica gel for the work-up of untested reactions in research. The current work looks at the use of multiple recycled silica gels

General
Thermal Recycling of the Silica Gel
Results and Discussion
Environmental Impact Assessment
Conclusions

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