Residual metal ions can cause significant problems with operations. For instance, hydroxylamine has been found to react exothermically with Ti, Fe, Cu, Ni, Cr, and Mn, which may be components of metal reactors and fittings; to avoid contamination by metal ions a process involving hydroxylamine was developed using a glass microreactor (1). Contamination by 15 ppm of residual copper ions in new lines decomposed some of the performic acid charged in a process (2). MnO2 and Fe(III) each react with H2O2 (3, 4). Pt is used for hydrogenation, but if residual Pd is present hydrogenolysis may also occur. Removing residual metal ions from reactors can be essential to ensure safe operating conditions and completion of reactions.
Many ions are more soluble under acidic conditions than under basic conditions, as for FeCl3 vs. Fe2O6. Organic acids are often used to solubilize metal ions. ∝-Hydroxy acids are more acidic than the non-hydroxylated analogues (5); carboxylic acids have often been used to clean reactors. ∝-Hydroxy acids may also be good ligands because they can form stable 5-membered rings in octahedral coordination complexes (6).
Citric acid has been used to solubilize ions of Fe, Ca, and other metals. Citric acid forms soluble chelates of Fe, Cu, Mg, Mn and Zn (7). Citric acid has been used synthetically to solubilize Ti(IV)(8), Os(VIII) (9), and ruthenium (10). A citric acid (0.3 M) – oxalic acid (0.2 M) combination has been used to decontaminate nuclear reactors, and oxalic acid can be used to remove MnO2 and rust from iron (11). Malic acid was used to solubilize Al(III) (12).
Under basic conditions (or at least conditions that are not strongly acidic), chelants such as EDTA and nitrilotriacetic acid (NTA) will complex with metals. Glyphosate (Roundup) forms a 5-membered ring with ions of Mn (13). EDTA and NTA strongly complex with Ni(II) and Cu(II), and can corrode reactors and fittings (14).
Matching the metal ion, the right oxidation state, the complexing reagent, and the right pH can all be important. For instance, a solution of 2,4,6-trimercaptotriazine sodium salt was ineffective in removing Cu(I), but after the process streams were sparged with air the Cu(II) generated was effectively removed by TMT treatment. Trivalent ions such as Al(III), Cr(III), and Fe(III) were not bound to TMT (15). (Since TMT precipitates metal ions it is not appropriate for removing trace metals from reactors.) Cu(II) oxalate precipitates from solutions made strongly acidic with HCl (16).
All such reagents can be corrosive. It is wise to test for the corrosivity of solutions to glass and metal reactors by soaking representative pieces of glass or metal coupons in the solutions that could be used for cleaning.
A general procedure for cleaning new equipment has been described (17). Cleaning the surface of reactors is called pickling, and then the reactors are passivated (oxidized) to prevent corrosion of the surface. With usage sediment can accumulate in the jackets of reactors, decreasing the efficiency of heat transfer. Some companies have provided contract services to clean the jackets of reactors (18).
1) Vörös, A.; Baán, Z.; Mizsey, P.; Finta, Z. Org. Process Res. Dev. 2012, 16, 1717.
2) Thanks to Paul Jass for this personal communication.
3) Decomposition of excess hydrogen peroxide to work up an epoxidation: Vaino, A. R. J. Org. Chem.2000, 65, 4210.
4) Swaddle, T. W. Inorganic Chemistry: An Industrial and Environmental Perspective; Academic Press; 1997, p. 252.
5) Some pKas of selected carboxylic acids: oxalic, 1.23, 4.19; citric, 3.08, 4.74; glyoxylic, 3.2; malic, 3.46; glycolic, 3.83; acetic, 4.75; peracetic, 8.2.
6) Swaddle, T. W. ibid, p. 245.
7) http://www.citric-acid.us/Citric_Acid_uses.html
8) Working up a Kulinkovich reaction: Young, I. S.; Haley, M. W.; Tam, A.; Tymonko, S. A.; Xu, Z.; Hanson, R. L.; Goswami, A. Org. Process Res. Dev. 2014, XX, XXXX; DOI: 10.1021/op500135x.
9) For achiral dihydroxylation: Dupau, P.; Epple, R.; Thomas, A. A.; Fokin, V. V.; Sharpless, K. B. Adv. Synth. Catal. 2002, 344(3+4), 421.
10) Cleaning nuclear reactors: Row, T. H. Nucl. Sci. Abstr. 1967, 21, 27690; Reference 7 in Couturier, M.; Andresen, B. M.; Jorgensen, J. B.; Tucker, J. L.; Busch, F. R.; Brenek, S. J.; Dubé, P.; am Ende, D. J.; Negri, J. T. Org. Process Res. Dev. 2002, 6, 42. The Pfizer researchers used acetonitrile as solvent to contain volatile and toxic RuO2 in solution.
11) Kumar, V.; Goel, R.; Chawla, R.; Silambarasan, M.; Sharma, R. K. J. Pharm. Bioallied Sci. 2010 Jul-Sep; 2(3), 220–238; doi: 10.4103/0975-7406.68505
12) Workup of a Friedel-Crafts alkylation: Wu, G.; Wong, Y.; Steinman, M.; Tormos, W.; Schumacher, D. P.; Love, G. M.; Shutts, B. Org. Process Res. Dev. 1997, 1, 359.
13) http://cabecahealth.com/wp-content/uploads/2013/05/FoodPlaguePrimer.pdf
14) Swaddle, T. W. ibid, p. 269.
15) Malmgren, H.; Bäckström, B.; Sølver, E.; Wennerberg, J. Org.Process Res. Dev. 2008, 12, 1195.
16) Sedergran, T. C.; Anderson, C. F. U.S. 4,675,398, 1987 (to E. R. Squibb & Sons).
17) Mukherjee, S. “Preparations for Initial Startup of a Process,” Chemical Engineering 2005, 112(1), 36.
18) For instance, the OptiTherm service from GE Water and Power Technologies: https://www.gewater.com/kcpguest/documents/Capability%20Profiles%20FAQ_Cust/Americas/English/cp102.pdf.
