Tables for
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.7, pp. 310-311

Section Isocracker sludge

J. A. Kaduka,b,c*

aDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,bDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and cPoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail: Isocracker sludge

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An isocracker is a refinery unit which simultaneously carries out cracking and isomerization reactions to produce more high-octane gasoline. A black deposit isolated from such a unit was surprisingly crystalline (Fig. 3.7.9[link]; files NALK157.gsas, NALK157.raw and padv.prm). It was easy to identify small concentrations of elemental sulfur, pyrrhotite-4M (now called pyrrhotite-4C), haematite, lepidocrocite and dolomite, but the major peaks did not match well those of any entry in the PDF.

[Figure 3.7.9]

Figure 3.7.9 | top | pdf |

The phases identified in a deposit from a refinery isocracker. At the time, the (NH4)Fe2S3 was a new phase, identified by analogy to KFe2S3, rasvumite.

It seemed likely that a mineral-related phase would serve as a structural prototype for an apparently new phase, so two separate searches for mineral-related phases with one of their three strongest peaks in the d-spacing ranges 7.09 ± 0.03 and 5.57 ± 0.03 Å were combined. The two hits in the search list were both uranium minerals. These seemed un­likely in a refinery deposit(!). Widening the search ranges to 7.09 ± 0.10 and 5.57 ± 0.07 Å yielded rasvumite, KFe2S3 (PDF entry 00-033-1018), as the second entry in the hit list.

The fit to the major peaks in the deposit was reasonable, but there should not be any potassium in a refinery deposit and none was detected in a bulk chemical analysis. When the jar containing the deposit was opened, it smelled strongly of ammonia. Ammonium and potassium ions are about the same size and often form isostructural compounds. The infrared spectrum of the deposit was dominated by bands of ammonium ions.

The potassium in the structure of rasvumite (PDF entry 01-083-1322, used as a reference) was replaced by nitrogen. Analysis of potential hydrogen-bonding interactions yielded approximate hydrogen positions in the ammonium ion. These positions were refined using a density-functional geometry optimization. This model yielded a satisfactory Rietveld refinement (Fig. 3.7.10[link]) and the quantitative analysis 45.7 (2) wt% (NH4)Fe2S3, 12.8 (4) wt% S8, 22.0 (6) wt% lepidocrocite (γ-FeOOH), 5.5 (5) wt% haem­atite (α-Fe2O3), 6.6 (3) wt% pyrrhotite-4C (Fe7S8) and 6.6 (3) wt% dolomite [CaMg(CO3)2; limestone environmental dust]. The powder pattern and crystal structure of (NH4)Fe2S3 are now included in the PDF as entry 00-055-0533.

[Figure 3.7.10]

Figure 3.7.10 | top | pdf |

The final Rietveld plot from refinement of the isocracker deposit.

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