Catalyst Activity and Selectivity for Liquid-Phase Oxidation of Sodium Sulfide catalyzed by various Materials

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1 Catalyst Ativity and Seletivity for Liquid-Phase Oxidation of Sodium Sulfide atalyzed by various Materials Tohru SAKAI and Nobu ihi OHI Department of Chemial Engineering, Toyama University, Takaoka, Japan The liquid-phase oxidation of sodium sulfide atalyzed by various materials suh as 4 sulfur dyes, 6 vat dyes, CuSO,, MnS O,, CoSO,, FeSO, pyroatehol, and sulfate lignin have been studied, and atalyst ativity and seletivity for this oxidation of the employed atalysts have been investigated Measurement of time-ourse of onentration of liquid ompounds was arried out with a stirred vessel under aeration, while that of a absorption rate of oxygen was perfor/med in a bathwise operation Oxidation produts were Na2 S, Na2 SO, and mainly Na2 S2 3 At first, Na2 S, Na2 SOa and Na2S23 were formed by oxidation simultaneously, and Na2S23 was also formed by the reation of Na2 S 2 with Na2 S3 subsequently In general, atalyst ativity of sulfur dyes was higher than that of vat dyes, and that of metal sulfates suh as MnSO,, CuSO,, and CoSO, was partiularly high Metal sulfates had a high seletivity for the formation of Na2 S2 3 A presumption of reation mehanism was somewhat desribed 1 INTRODUCTION I) The waste water from dye-works and the blak liquor m the digestion of kraft pulp 2) proess ontain sodium sulfide whih has a toxiity and a high reduiblity These strongly alkaline solutions ause a lowering of the dissolved oxygen in water, inrease in hemial oxygen demand, evolution of hydrogen sulfide as the solution beomes aidi, and orrosion of metals Therefore, it is preferable that sodium sulfide is reovered as sodium thiosulfate, having a high additional value for hemials, produed by the oxidation of it or is onverted to harmless substanes by the oxidation to prevent water pollution In the previous paper of authors, the liquid-phase oxidation of sodium sulfide with oxygen atalyzed by sulfur dye Sulfur Blak B has been studied kinetially on the basis of the harateristis of this reation obtained experimentally In the present paper, we shall disuss on atalyst ativity and seletivity for this oxidation reation atalyzed by some sulfur dyes and redox atalysts suh as metal sulfates -28-

2 Catalyst Ativity and Seletivity for Liquid-Phase Oxidation of Sodium Sulfide atalyzed by various Materials Tohru SAKAI and Nobuihi OHI 2 EXPERIMENTAL Measurement of the rate of oxidation with respet to liquid ompounds was arried out with a stirred vessel under aeration, while that of absorption rate of oxygen was performed in a bathwise operation Only the outlined experimental proedure using the stirred vessel under aeration will hereinafter be desrided The weighted sodium sulfide and atalyst* were harged together with distilled water m the stirred vessel under purified nitrogen atmosphere After the solution temperature in the vessel reahed a desired level with agitation, oxygen gas was introdued to the bottom of reator through a ring sparger at the predetermined rate of gas flow Thus, the oxidation reation started Aliquots of reation solution were then withdrawn at regular intervals, and zin arbonate powder as a reation-stopping reagent was immediately added in the sampled solution to preipitate soluble sulfides Sodium sulfide and sulfur ompounds formed by this oxidation were determined by means of iodometry and high-performane liquid hromatography For futher details of experimental proedures and analytial tehniques the pre- vwus paper should be reffered to 3 RESULTS AND DISCUSSION 3 1 Time-ourse of onentration of liqu id ompounds With regard to the liquid-phase oxidation of sodium sulfide atalyzed by sulfur dyes or metal sulfates, the time-ourse of onentration of liquid ompounds was followed The typial representations whih illustrate the obtained results are shown in Figs 1 to 4 From these figures, it was proved that sodium sulfide was simultaneously hanged to 2r , r , I O Na,s O Na,s,o,!;:, Na,so, [) Na,s, ' ' "' 2 1 \ \ \ f- -- \ 1\o o- 'o I oo /D/1 2 4 ::D::-:-g-:1;!:[) d- no- 6 Fig 1 The ourse of oxidation of sodium sulfide atalyzed by Sulfur Red Brown 6RN Reation onditions; Na2 S: 2 g-mol/ l, Oxygen partial pressure: 1 atm, Sulfur Red Brown 6RN (Catalyst): 1 g/1, ph in reation solution: 1288, Reation temp: 25 C, Rate of stirring: 112 rpm, Rate of oxygen gas flow: 1 ml/min Fig 2 The ourse of oxidation of sodium sulfide atalyzed by Vat Brill ian! Blue 4G pdr Fig 1 *Dye manufatured by Mitsui Toatsu Chemials In were employed -29 -

3 Bulletin of Faulty of Engineering Toyama University 198 sodium sulfite, sodium disulfide, and sodium thiosulfate, and sodium thiosulfate was also onseutively formed by reation of sodium sulfite with sodium disulfide regardless of kinds of atalyst 4) Although Cu( II), Co( II), Mn( II), and Fe (II) ions are well-known atalysts whih have been employed in the reation of sulfites with oxygen to form sulfates, these metal ions didn't almost promote the oxidation of sodium sulfite until a onsiderable derease m onentratiqn of sodium sulfide <> \ o, '\ o'o 1 +' +' m u u [9- _o_o- :::-tl-!:::, -!:::,_ - I I o -::ar-fll JI"a Reation time [min] Fig 3 The ourse of oxidation of sodium sulfide Fig 4 The ourse of oxidation of sodium sulfide atalyzed by Threne Blak BBN pdr Fig 1 atalyzed by Mn S4 Fig 1 exept onentration of atalyst Conentration of Mn SO (Catalyst): 7X1Q-5 g-mol/ l 3 2 Comparison of harateristis of atalysis The harateristis of atalysis of atalysts suh as 4 sulfur dyes, 6 vat dyes, Cu SO" Mn SO,, CoSO,, Fe SO,, pyroatehol, and sulfate lignin will be onsidered m some detail We take up both atalyst ativity and seletivity as the harateristis At first, the results obtained on atalyst ativity are shown in Tables 1 and 2 In general, atalyst ativity of sulfur dyes was higher than that of vat dyes Sulfur Brilliant Green 4GF and Sulfur Red Brown 6RN whih ontained CuSO, added in thionation Table 1 Comparison of atalyst ativity of sulfur dyes and vat dyes Dye Absorption rate of oxyqen* Initial rate of disapperane of sodium sulfide " o, (mt-stp/t min} r (Na2S) X 1 ' [q-mol/t-min) l!,{sulfur Blak 8 { Sulfur Brilliant Green 4Gf' Sulfur Red Brown 6RN Vat Brill1ant Blue 4G pdr Th<ene "ue BCS pdr Ill Threne Blak BBN pdr Threne Brilliant Violet RR pdr Threne Yellow GCN pdr Threne Br1lllant P1nk R pdr Threne Blue RSN pdr ' 2 l " Reation onditions Na1S:2 g-mol/i, Oxygen partial pressure:lo atm, Dye:Ol gjl, Reation temp:25, ph in reation solution:l288, Rate of stirrinq:l12 rpm, Rate of oxygen gas flow:looo ml/min Measurement of absorption rate of oxygen was performed in a bathwise operation Measurement of this rate was arried out with a stirred vessel under aeration - 3 -

4 Catalyst Ativity and Seletivity for Liquid-Phase Oxidation of Sodium Sulfide atalyzed by various Materials Tohru SAKAI and Nobuihi OHI Table 2 Comparison of atalyst ativity of metal sulfates an:l the other atalysts vo, [m!-stp/! min] r (Na2S) ')( 1 4 [g-mol/! min] Sulfur Blak B MnS4 FeS4 uso oso, Pyroatehol Sulfate lignin Without atalyst lob * * * unobserved Table 1 Conentration of atalyst: 7 OX1 4 g-mol/z or 7 1(1 -s g-mol/z (*mark) exept onentration of atalyst proess, and Vat Brilliant Blue 4G whih ontained phthaloyanine Blue ( C3 2 H16 Cu) added in the same proess, had partiularly high ativity The order of atalyti ativity of metal sul1fates and the other redox atalysts was obtained as follows : MnSO > CuSO > CoSO> Pyroatehol > Sulfur lignin > FeSO Partiularly, atalyst ativity of MnSO was extremely high Next, for these atalysts the ratio of formation of oxidation produts estimated on the basis of sulfur amounts were determined at 25, 5, and 75% onversion of sodium sulfide to the oxidation produts The obtained results are shown in Table 3 It was Table 3 Ratio of the oxidation produts on the bas is of sulfur amounts for various atalysts Produt ratio [ % ] at 25 onversion at 5 onversion at 75 onversion Sulfur Blak B Sulfur Brilliant Green 4GF Sulfur Red Brown 6RN Val: Brilliant Blue 4G pdr Threne Blue BCS pdr Threne Blak BBN pdr 32 8 o Threne Brilliant Violet RR pdr Threne Yellow GCN pdr Threne Brilliant Pink R pdr Threne Blue RSN pdr MnS o 84 FeS CuS B oso, Sulfate lignin found that produt ratios were about 55 to 85% for sodium thiosulfate, about 1 to 35% for sodium sulfite, and about to 25% for sodium disulfide, respetively, without regard to kinds of atalyst The hief produt was therefore sodium thiosulfate, and the yield of sodium thiosulfate inreased with an inrease in onversion of sodium sulfide The following features of _the seletivity for this oxidation were also proved Sulfur dyes were superior to the formation of sodium disulfide than the other atalysts, vat dyes were exellent in the formation of sodium sulfite than that of sodium disulfide, and metal sulfates had a high seletivity for the formation of sodium thiosulfate

5 Bulletin of Faulty of Engineering Toyama University Presumption of reation mehanism In general, sulfur dyes have the struture of polysulfides together with the fundamental ring struture made up of thiazole ring or thiazine ring whih ats as a hromophore It seems that as shown in Eqs ( 1 ) and ( 2) -s-s- -SH HS- (1) - NH -- H (2) polysulfides and the part of indophenol in the fundamental ring struture would partiipate in a oxidation- redution reation Whereas vat dyes have a quinon type ring and the part of atalysis of the dyes may be represented by a oxidation- redution reation as shown in Eq ( 3 ) Red, = "' Oxd Ho-Q -oh ( 3 ) Sodium sulfide is oxidized by the dyes and metal wns to form oxidation produts, and the dyes and metal ions redued at that time are reoxidized by oxygen to produe their's original forms On the basis of the kineti study reported in the previous paper, it ould also presumed that this reation has a omplex reation mehanism involving a hain reation step 4 CONCLUSION With regard to the liquid- phase oxidation of sodium sulfide atalyzed by varwus materials suh as sulfur dyes, vat dyes, metal sulfates, and the other redox atalysts, the harateristis of atalysis of the employed atalysts were onsidered The obtained informations were as follows (1 ) In general, atalyst ativity of sulfur dyes was higher than that of vat dyes (2 ) Catalyst ativity of metal sulfates suh as MnSO,, CuSO,, and CoSO, was partiularly high ( 3 ) Metal sulfates had a high seletivity for the formation of sodium thiosulfate Referenes 1 ) Maeda, Y, Y Nishiumi and K Ihikawa ; Kogyo Yosui, 1967, No 16, 6 2) Bilberg, E and P Landamark ; Norsk Skogind, 15, 221 ( 1961) 3 ) Ohi, N; "Showa 53 nendo Kagaku Kenkyuhi Hojokin (Ippan Kenkyu(B) ) No Kenkyu Seika Hokokusho ; p 4 ( 1979) 4 ) Urano, K and Y Nakamihi; Nippon Kagaku Ka ishi, 1978, No 1, 53 ( A part of this paper was presented at the 13th Autumn Meeting of The So of Chern Engrs, Japan, at Nagoya, Ot, 1979) ( Reeived Otober 25, 1979) -32 -