Polymer-bound tetraethylammonium

We have investigated the solvolytic stability and reactivity of polymer-bound borohydrides and have evaluated these materials in several applications such as solvent purification, arsine generation, and metal reduction. These polymer-bound borohydrides offer several advantages over sodium or tetraethylammonium borohydride. The primary advantages are the convenience of use and the minimal introduction of ionic species or organic by-products into the treated bulk media. With the polymer-bound borohydrides, the cation is bonded covalently to the insoluble resin while the borohydride anion or its oxidation product (borate) is retained by ionic bonding. Typically, boron at levels of less than 5 ppm is the only impurity introduced into the treated medium. 

Aldehyde Reduction. The results of 2-ethylhexanal reduction in 95% ethanol, 2-ethylhexanol, and hexane with the various types of polymer-bound borohydrides, sodium borohydride, and tetraethylammonium borohydride are shown in Table I. 

However, in the 2-ethylhexanol, the sodium and tetraethylammonium borohydrides are, in general, more reactive than the polymer-bound boro-hydrides. Of the resins, those polymers having the styrene-DVB skeletal backbone (A-26 and IRA-400) offer some advantage over the acrylate-DVB-based resin (XE-279 and IRA-458). Of the styrene-DVB resins, the macroreticular resin (A-26) gives the best reduction, particularly at an elevated temperature. Both sodium borohydride and tetraethylammonium borohydride appeared to have limited solubility at this level in 2-ethylhexanol. However, since 2-ethylhexanol is more viscous and less polar than ethanol, reactions involving the porous ion-exchange resins would be somewhat slower than the corresponding reduction with soluble borohydrides. 

In 95% ethanol, sodium borohydride and the acrylate-DVB-based polymer-bound borohydrides (XE-279 and IRA-458) were not as reactive as tetraethylammonium borohydride or the styrene-DVB-based polymeric borohydrides (A-26 and IRA-400). In the 2-ethylhexanol and hexane evaluation, the macroreticular styrene-DVB polymer-bound borohydride (A-26) was, in general, more reactive than the other resins or sodium or tetraethylammonium borohydrides. 

Solvolytic Stability. Solvolytic stability of all four polymer-bound borohydrides was investigated in 100% ethanol as a function of temperature and in water as a function of pH. Comparative studies for sodium borohydride and tetraethylammonium borohydride were conducted. Solvolytic decomposition of the borohydride group results in the generation of hydrogen gas. Stability measurements were obtained by observing volume of hydrogen gas evolved as a function of time. Percent loss of hydride as a function of time was calculated from resin weight and initial hydride content. 

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