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Combination of DFT and 13C CP/MAS NMRto determine anisotropy tensors for
xanthates
Anna-Carin Larsson Sven Öberg Division of Chemical Engineering Department of Mathematics
in collaboration with
Zhong-Xi Sun Mats Lindberg Jinan University, China Division of Chemical Engineering
Possible species in a flotation pulp and on a mineral surface
Xanthate (KX) the collector used
Alcohol (ROH) X- + H2O → OH- + ROH + CS2 at pH<7 or
6X- +3H2O → 6ROH + CO32- + 3CS2 + 2CS3
2-
in highly alkaline solutions
Dixanthogen (X2) 2X- + 1/2O2 + H2O → X2 + 2OH- → 2X- + H2O2 at alkaline pH
X2 + 2SO32- → 2X- + S2O6
2-
See e.g. S. R. Rao, Surface Chemistry of Froth Flotation
Possible species in a flotation pulp and on a mineral surface
Monothiocarbonate (KMTC)
Perxanthate (KperX)
Carbonate (ROCO2K) MTC- + 1/2O2 → ROCO2- + S0 has not been found
MTC- → ROH + COS preferential reaction
X- + 1/2O2 → MTC- + S0 or
S2O32- + X- → S2- + S0 + MTC- (not balanced)
X2 + 4OH- → 2MTC- + S0 + S2- + 2H2O
X- + H2O2 → PerX- + H2O
Surface needed for preadsorption of X- or O2.Surface ions (esp. Cu2+) function as catalysts
See e.g. S. R. Rao, Surface Chemistry of Froth Flotation
Possible species in a flotation pulp and on a mineral surface
Insoluble metal xanthates Pb2+ + 2X- → PbX2 or
2Cu2+ + 4X- → 2CuX + X2
S-alkyl-monothiocarbonate (SMTCK) ROCS2-M → RSCOS-MRSCOS-M → RSM + COSRSM + CO2 → RSCO2-M or
ROCOS-M → RSCO2-M
PbX2 See e.g. S. R. Rao, Surface Chemistry of Froth Flotation
A. J. Vreugdenhil, et al., J Mol Struct, 1997, 405, 67-77.
13C CP/MAS NMR and DFT data
KiPrX exp DFT
δiso (ppm) 20.4 22.7 23.4 25.1 79.1 84.1 231.6 251.7
δaniso (ppm) 154.4±4.6 164.3
η 0.60±0.05 0.60
Ω (ppm) 278.2±8.2 295.7
Exp chemical shifts referenced relative to adamantane (38.48 ppm relative to TMS (0 ppm))DFT chemical shifts referenced relative to DFT chemical shift of TMS (0 ppm)
δiso = (δxx + δyy + δzz)/3
δaniso = δzz - δiso
η = (δyy - δxx) / δaniso
Ω = |δzz - δxx|
13C CP/MAS NMR and DFT data
Pb(iPrX)2 exp DFT
δiso (ppm) 20.8 21.3 22.9 22.2 23.6 25.1 74.5/76.6 89.0 79.4/82.2 89.9 226.4 244.4/245.3
δaniso (ppm) 142.8±4.1 152.7/156.2
η 0.73±0.04 0.78/0.74
Ω (ppm) 266.0±7.0 288.3/291.9
Exp chemical shifts referenced relative to adamantane (38.48 ppm relative to TMS (0 ppm))DFT chemical shifts referenced relative to DFT chemical shift of TMS (0 ppm)
13C CP/MAS NMR and DFT data
CuiPrX exp
δiso (ppm) 21.8 23.6 24.9 82.9 223.2
δaniso (ppm) 142.3±8.7
η 0.63±0.10
Ω (ppm) 258.5±15.1
Exp chemical shifts referenced relative to adamantane (38.48 ppm relative to TMS (0 ppm))DFT chemical shifts referenced relative to DFT chemical shift of TMS (0 ppm)
Structural fragment
13C CP/MAS NMR and DFT data(iPrX)2 exp DFT
δiso (ppm) 21.7 18.0 22.1 18.7 23.0 20.2 23.2 21.3 81.1 86.9 82.2 87.5 206.5 225.3 207.1 225.4
δaniso (ppm) 141.1±4.0 161.2 140.1±4.3 162.4
η 0.15±0.15 0.09 0.16±0.16 0.03
Ω (ppm) 222.5±12.5 234.1 221.6±13.2 240.8
Exp chemical shifts referenced relative to adamantane (38.48 ppm relative to TMS (0 ppm))DFT chemical shifts referenced relative to DFT chemical shift of TMS (0 ppm)
13C CP/MAS NMR and DFT data
KMTC exp1 DFT
δiso (ppm) 185 198.4
δaniso (ppm) 100 93.1
η 0.3 0.42
Ω (ppm) 164 159.4
KperX exp2 DFT
δiso (ppm) 227.7 247.1
δaniso (ppm) 163.0
η 0.43
Ω (ppm) 279.9
1 Methyl, D. Stueber, et al., SS NMR, 2002, 22, 29-49.2 Ethyl (aq), F. P. Hao, et al., Anal. Chem. 2000, 72, 4836-4845.
Pb(MTC)2 DFT
δiso (ppm) 198.0/198.6
δaniso (ppm) 82.4/82.5
η 0.86/0.87
Ω (ppm) 159.2/159.5
Pb(perX)2 DFT
δiso (ppm) 245.0/246.8
δaniso (ppm) 164.4/159.3
η 0.46/0.43
Ω (ppm) 284.6/273.2
All DFT results are for iso-propyl
13C CP/MAS NMR and DFT data
SMTCK exp DFT (iPr)
δiso (ppm) 173.51 185.1 1752
δaniso (ppm) 642 61.7
η 0.62 0.72
Ω (ppm) 1162 114.81 Methyl, D. Stueber, et al., Inorg Chem, 2001, 40,1902-1911.2 Methyl, D. Stueber, et al., SS NMR, 2002, 22, 29-49.
RSCS2K2 δiso = 247 ppm; δaniso = -178 ppm; η = 0.6; Ω = 317 ppm
RSCOSK2 δiso = 210 ppm; δaniso = -108 ppm; η = 0.7; Ω = 200 ppm
13C CP/MAS NMR and DFT dataROCO2K exp exp DFT
δiso (ppm) 21.6/23.2 22.11 21.0 23.6/24.0 23.7/24.21 21.8 67.3/68.1 67.7/68.41 71.4 157.3 157.81 168.8 158.2 158.71
1572
δaniso (ppm) 76.5±1.6 742 72.2 75.7±1.3
η 0.39±0.05 0.32 0.08 0.32±0.05
Ω (ppm) 129.9±3.0 1202 111.3 125.5±2.8
1 iso-propyl, D. Stueber, et al., Inorg Chem, 2001,40, 1902-1911.2 Methyl, D. Stueber, et al., SS NMR, 2002, 22, 29-49.
Another compound (169.5 ppm)
Exp chemical shifts referenced relative to adamantane (38.48 ppm relative to TMS (0 ppm))DFT chemical shifts referenced relative to DFT chemical shift of TMS (0 ppm)
13C CP/MAS NMR and DFT dataK2CO3 exp exp
δiso (ppm) 169.5 1691/171.22
δaniso (ppm) -52.0±2.4 -491
η 0.54±0.10 0.31
Ω (ppm) 92.1±4.5 821
(KHCO31 δiso = 160 ppm; δaniso = 56 ppm; η = 0.5; Ω =
97 ppm)
1 D. Stueber, et al., SS NMR, 2002, 22, 29-49.2 D. Stueber, et al., Inorg Chem, 2001, 40, 1902-1911.
Strong CP signal but no protons?Absorbs moisture from air?No alkyl chain.
After removing ROCO2K(dissolves in EtOH)
Summary of 13C isotropic chemical shifts exp DFT(TMS) DFT(KX)
KX 231.6 251.7 231.6
KperX 227.71 247.1 227.0
Pb(perX)2 245.9 225.8
PbX2 226.4 244.8 224.7
CuX 223.2
X2 206.8 225.4 205.3
KMTC 1852 198.4 178.3
Pb(MTC)2 198.3 178.2
SMTCK 1752 185.1 165
K2CO3 169.5
ROCO2K 157.8 168.8 148.7
K → Pb/Cu gives lower chemical shift valuesPbX2 and Pb(perX)2 can be distinguished by
anisotropyKMTC and Pb(MTC)2 can be distinguished by
anisotropy
1 Ethyl (aq), F. P. Hao, et al., Anal. Chem.
2000, 72, 4836-4845.2 Methyl, D. Stueber, et al., Inorg Chem,
2001, 40, 1902-1911.
Effect of alkyl chain length
3 mM Heptylxanthate (HX)pH 8 226.4 +182 + 173.2 + 166.7 ppm
3 mM Amylxanthate (AX)pH 8 226.4 + 182 + 173.2 + 166.7 ppm
3 mM Ethylxanthate (EX)pH 7.5 226.4 + 172.4 + 166.3 ppm
PbS
PbX2 at 226.4±0.5 ppm
Surface-PbX2 at 226.4±2 ppm,δaniso = 142±2 (142.8±4.1 for precipitate);η = 0.9±0.1 (0.73±0.04 for precipitate)
Adsorption of 13C-enriched xanthates
Broader lines than for PbX2 precipitate indicate surface bonded complex
More PbX2 forms (or stays stable) when thealkyl chain is longer.Shorter chains decompose easier.
Effect of alkyl chain length cont.
PbS
K2CO3 at 169.5 ppm
PbCO3 at 167 ppm (166.41)RSCO2K at 175 ppm2, 173.5 ppm3, 173.8 ppm4
Pb(RSCO2)2 at 173 ppm?KMTC at 185 ppm2
Pb(MTC)2 at 182 ppm?
1 H. W. Papenguth, et al., Am. Mineralogist, 1989, 74, 1152-1158.2 Methyl, D. Stueber, et al., SS NMR, 2002, 22, 29-49.3 Methyl, D. Stueber, et al., Inorg Chem, 2001, 40, 1902-1911.4 Ethyl, D. Stueber, et al., Inorg Chem, 2001, 40, 1902-1911.
Adsorption of 13C-enriched xanthates
3 mM Heptylxanthate (HX)pH 8 226.4 +182 + 173.2 + 166.7 ppm
3 mM Amylxanthate (AX)pH 8 226.4 + 182 + 173.2 + 166.7 ppm
3 mM Ethylxanthate (EX)pH 7.5 226.4 + 172.4 + 166.3 ppm
Effect of time
3 mM Ethylxanthate (EX)pH 7.1
PbS
Adsorption of 13C-enriched xanthates
After 5h there is a lot of PbX2 (226 ppm) and only a small amount of PbCO3 (167 ppm).Some MTC- (185 ppm) may be present but obscured by a spinning sideband.Some S-alkyl-MTC- (174 ppm) is present.
As time goes PbX2 is decomposed and after 45 h most of it has decomposed to PbCO3.
Conclusions
DFT calculations and experiments to determine chemical shift tensors giveresults which are in very good agreement
Combination of chemical shifts and chemical shift anisotropy can be used todistinguish surface species
13C NMR is sensitive enough to distinguish between different surface species
Xan are very easily decomposed (for comparison DTP almost does notdecompose at all) leading to loss of hydrophobicity
Conclusions
Longer alkyl chains adsorb more easily and stays stable for a longer time
The decomposition of EtXan on PbS could be followed
A route for the decomposition of Xan species were suggested:Pb-X → Pb-MTC → Pb-O2CSR → PbCO3
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