Lab Notes - Recycling palladium from Pd(PPh3)4

[00:00:00]Hi guys, here is MIH. About half a year ago, I received a few grams of tetrakis(triphenylphosphine)palladium(0) or Pd(PPh3)4 from another friend doing chemistry, and I intended to use the compound as a catalyst in Suzuki and Sonogashira coupling reactions in the future.

[00:00:18]However, when I checked upon it recently, the compound has turned to a dirt brown color instead of the bright yellow color when I just received it, indicating that it has oxidized and decomposed by air.

[00:00:28]This isn't a surprise to me because Pd(PPh3)4 is easily oxidized by air and should be stored in an inert gas atmosphere under low temperatures. But, this vial is only charged with a some propane and sealed with a rubber septum. So, it's completely normal that some air leaks in and decomposes my catalyst. The catalyst is now unusable, but it still contains nearly 10% by mass of precious palladium, and just throwing it out would be a tremendous waste.

[00:00:54]The standard recycling procedure I found online was to simply fire the powder at red heat until most organics oxidizes, which leaves behind a palladium-containing residue that can be leached out with acids. However, this method releases a ton of toxic organophosphorus compounds, which is a major safety and environmental concern.

[00:01:13]Hence, I decided to investigate on a safer and environmentally friendly way of recycling these spent catalysts, and managed to do this with a single step of boiling in ethanol. Note that this method probably only works with Pd(PPh3)4 and doesn't apply to other spent palladium catalysts. First, I pour all my catalyst powder in a beaker, and it weighs 1.67 g. The beaker was previously used in test runs, so it has some palladium stains on its sides. Around 20 mils of ethanol is then added to the beaker, and I use the ethanol to wash out remaining catalyst in the bottle.

[00:01:47]Nothing seems to happen now, and the orange catalyst powder simply suspends in solution as Pd(PPh3)4 isn't soluble in ethanol. However, when the mixture is heated on the hot plate until boiling and vigorously stirred, the color deepened to a dark brown. Here, Pd(PPh3)4 is oxidized by oxygen in the air to give triphenylphosphine oxide and black palladium metal particles. And ethanol serves to dissolve the triphenylphosphine oxide byproduct and enable fresh Pd(PPh3)4 surfaces to be exposed to air, greatly accelerating the rate of oxidation.

[00:02:21]After half an hour, the ethanol boiled dry and the beaker now contains an entirely black mixture. I add some more ethanol to it and pass it through a simple paper filter to isolate the palladium powder from the oxidation products. After washing and drying, the palladium powder is scraped from the filter paper into the beaker.

[00:02:39]Using a sintered glass funnel is probably better to minimize loss during transfer, but my glass funnel was clogged and unusable.

[00:02:46]After some effort, I managed to transfer most of the palladium into the beaker and around 15 mils of 10% hydrochloric acid is added to wash off remaining impurities. The acid is decanted and 5 mils of 10% hydrochloric acid is then added, followed by a few drops of 70% nitric acid.

[00:03:04]The colorless solution immediately turned yellow as palladium started to dissolve in the aqua regia mixture.

[00:03:10]More acid is added and the beaker is placed on hot plate to accelerate the dissolution.

[00:03:14]Normally, bulk palladium only dissolves slowly in such a dilute aqua regia mixture, but the powder dissolves rather rapidly and only after 10 minutes an amber red liquid containing mostly palladium (II) chloride is obtained.

[00:03:28]I allow the acid to boil dry and try to weigh out my yield of palladium chloride, but my balance isn't precise enough to give effective readings.

[00:03:36]Later, I used my jewelry scale to determine that I made 0.25 g of palladium (II) chloride, which is essentially a quantitative yield starting from the 1.67 g of Pd(PPh3)4 catalyst.

[00:03:48]As you can see, this method avoids the use of high temperature incineration to remove organic matter and thus is a lot more convenient and less hazardous than the classic method documented in literature.

[00:03:59]If any of you happens to have a bottle of old PDPP H3 for lying around, I'd love to hear your experiences using this method to recycle plating. See you soon.