Abstract-Chemical processes leading to inactivation of vanadium sulfuric acid catalysts by arsenic(III) oxide vapor were studied. The interaction and solubility in ...
ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 4, pp. 619!623. + Pleiades Publishing, Inc., 2006. Original Russian Text + V. N. Krasil’nikov, A. P. Shtin, V. I. Malkiman, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 4, pp. 627!631.
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Loss of Activity by Vanadium Sulfuric Acid Catalysts under the Action of Arsenic(III) Oxide Vapor V. N. Krasil’nikov, A. P. Shtin, and V. I. Malkiman Institute of Solid State Chemistry, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia UNIKhIM SOZ Federal State Unitary Enterprise, Yekaterinburg, Russia Received September 2, 2005; in final form, December 2005
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Abstract Chemical processes leading to inactivation of vanadium sulfuric acid catalysts by arsenic(III) oxide vapor were studied. The interaction and solubility in the system VO(H2AsO4)2 VOAsO4 H3AsO4 H2O and the composition of oxovanadium arsenate in a mixed oxidation state, used as a starting substance in synthesis of (VIVO)(VVO)2(As2O7)2, were examined. DOI: 10.1134/S1070427206040227
Interpretations of the mechanism by which arsenic(III) oxide is oxidized on vanadium catalysts are also ambiguous. According to [3], oxidation of SO2 and As2O3 involves various activating centers. Van’kov and Perevozchikov [4] regard potassium bisulfate as the most probable oxidant of As2O3 and believe that the presence of arsenic in the active component favors transition of vanadium to the highest oxidation state, V(V). At the same time, Boreskov believes [1] that vanadium(V) is the agent that oxidizes arsenic(III) on the vanadium(V) catalyst.
Vanadium catalysts for conversion of sulfur dioxide are comparatively stable against As2O3 vapor. Nevertheless, the activity of the catalysts gradually decreases upon a prolonged contact with an insufficiently purified gaseous reaction mixture and may be completely lost if concentration of impurities is too high. By the term [catalyst poisoning] is commonly understood the considerable and abrupt loss of activity, caused by contact with trace amounts of a catalytic poison. There exist various concepts concerning the nature of the inactivating effect of arsenic(III) oxide on vanadium sulfuric acid catalysts. In Boreskov’s opinion [1], As2O3 is oxidized in the course of sulfur dioxide conversion to As2O5, which further reacts with the active component to form a compound of composition V2O5 . As2O5, which is volatile at T > 550oC. As a rule, there is no arsenic in the first catalyst bed of a reactor, in which temperature may be as high as 600 3 650oC. However, arsenic is deposited in the subsequent beds, and especially in the third and fourth, in which the temperature commonly does not exceed 450oC, in the maximum amounts, presumably in the form of the compound V2O5 . As2O5. At temperatures lower than 600oC, the surface of catalyst grains can be blocked by arsenic(V) oxide. According to another interpretation, As2O5 reacts with molten potassium bisulfate to give potassium arsenates or sulfate3arsenates [2], in which case sulfur trioxide is released into the gas phase.
The concentration of As2O3 vapor in the contact gas mixtures in manufacture of sulfuric acid depends on the degree of their purification. It is believed that, at plants of nonferrous metallurgy in the Ural region, the most arsenic-contaminated gas mixtures are used by Krasnoural’sk copper-smelting combine (KUMK), because the service life of vanadium catalysts for sulfur dioxide conversion at this plant is strongly limited [5]. To reveal the nature of the inactivating influence exerted by arsenic(III) oxide, we examined grains of SVD catalyst (vanadium sulfate catalyst on diatomite) that had worked for 1.5 years in a reactor for sulfur dioxide conversion under conditions of an increased concentration of As2O3 vapor and completely lost its catalytic activity for this reason. EXPERIMENTAL Samples of grains of an inactivated catalyst were taken at different points of catalyst beds II, III, and 619
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Results obtained in a chemical analysis of grains of an SVD-type catalyst inactivated in operation in a sulfuric acid reactor at KUMK
ÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ ³ As(V) ³ V(IV) ³ V(V) ³ Vtot Bed ÃÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ ³ wt % ÄÄÄÄÅÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ I ³ ³ ³ ³ II ³ 0.98 ³ 2.60 ³ 0.56 ³ 3.16 III ³ 2.68 ³ 3.47 ³ 3.54 ³ 7.01 IV ³ 3.05 ³ 3.44 ³ 3.86 ³ 7.30 ÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ
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IV of the reactor, in which the concentration of arsenic compounds was very high. The grains taken for study were stored under conditions that provided their total isolation from the environment. The phase analysis of the samples was made using a DRON-3 UM X-ray diffractometer with Cu K= radiation and a POLAM S-112 polarization microscope with opticalimmersion control. The chemical analysis of the samples for the content of potassium, vanadium(V) and (IV), sulfur, and arsenic was performed by the standard commonly accepted procedures. Particular attention was given to analysis of the chemical and phase composition and structure of the surface (0.130.2-mmthick) layer of the grains. Individual chemical compounds necessary for the experiments and, in particular, VOAsO4 . 3H2O and VO(H2AsO4)2 were synthesized by previously developed techniques [638] with reagents of, at least, chemically pure grade. The table lists the results of a chemical analysis of grains taken from catalyst beds II, III, and IV of a reactor for sulfur dioxide conversion at KUMK. To the observed increase in the concentration of arsenic(V) on passing from bed II to bed IV corresponds the rise in the total concentration of vanadium, in agreement with the opinion that it is transferred to-
Fig. 1. Schematic cross-sectional view of a grain of inactivated SVD-type catalyst. (1) Cross-section of a grain, (2) deposited crystals of arsenic compounds, and (3) grayblue glass.
gether with arsenic in the form of a volatile compound V2O5 . As2O5 [1, 9]. At the same time, the high concentration of vanadium(IV) in the grains (especially in their surface layer) gives reason to assume that arsenic(V) compounds containing reduced vanadium can be formed in the grains. In 0.13 0.2-mm-thick sections from the surface of grains, the concentration of As(V) was as high as 11.6 312.2 wt %. Grains of a freshly prepared SVD catalyst are yellow and have a rough surface; the coloration of grains of an inactivated catalyst is light blue or gray-blue, and the grain surface is smooth and hard. An analysis of sections of the grain surface under a microscope demonstrated that the crust covering the grains is a gray-blue glass well soluble in water. According to the results of a chemical analysis, the content of vanadium(V) in this glass is virtually zero, that of vanadium(IV) is close to 7.5 wt %, and the content of arsenic(V), 21 wt %. After the glass is removed by dissolution in water, regularly arranged black cylindrical outgrowths composed of coarse well-formed lamellar crystals become visible on the grain surface (Fig. 1). A characteristic feature of these crystals is a clearly pronounced dichroism (their coloration varies from greenish brown, nearly black to light green with their rotation in polarized light under a microscope). This fact indicates the possible presence of vanadium in two oxidation states, (V) and (IV), in their structure. Optical properties of this kind are observed for crystals of a phase of composition (VIVO)(VVO)2(As2O7)2, synthesized by partial oxidation of VO(AsO3)2 with atmospheric oxygen upon heating [6] and by dehydration of (VIVO)(VVO)2(HAsO4)4 . 5H2O and (VIVO)(VVO)2(HAsO4)4 in an inert medium at 450oC [7, 8]. The contents of vanadium(V), vanadium(IV), and arsenic(V) in aggregates of crystals detached from the surface of grains from catalyst bed IV of the reactor are 10.6, 4.9, and 30.2 wt %, respectively; the average concentration of potassium does not exceed 0.5 wt % (VV/ VIV) ~2]. This indicates that potassium is not contained in the compound that forms outgrowths on the surface of catalyst grains. In the atmosphere of saturated water vapor, inactivated grains very rapidly disintegrate, with crystals in the surface outgrowths being decomposed into light yellow and blue phases, whose crystal-optical constants and interplanar spacings coincide with published values for VOAsO4 . 3H2O (light yellow coloration) [10] and VO(H2AsO4)2 (blue) [11]. Consequently, the compound found in the outgrowths on the surface of inactivated catalyst grains is close in chemical nature and composition to the above-described oxovanadium
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arsenates, i.e., it includes vanadium in two oxidation states, V(V) and V(IV). The following transformations were observed in a saturated water vapor for (VIVO)(VVO)2(As2O7)2: (VIVO)(VVO)2(As2O7)2 + 2H2O = (VIVO)(VVO)2(HAsO4)4,
(1)
(VIVO)(VVO)2(As2O7)2 + 7H2O
.
= (VIVO)(VVO)2(HAsO4)4 5H2O.
(2)
Reaction (1) is an intermediate stage of reaction (2), and, therefore, the best way to synthesize hygroscopic (VIVO)(VVO)2(HAsO4)4 is thermolysis of (VIVO)(VVO)2(HAsO4)4 . 5H2O with slow heating to 120 3130oC. AS a result of contact with an excess amount of water at room temperature, (VIVO)(VVO)2(HAsO4)4 . 5H2O decomposes to give crystals of VOAsO4 . 3H2O and VO(H2AsO4)2. A comparison of the crystal-optical properties and X-ray diffraction patterns of (VIVO)(VVO)2(HAsO4)4 . 5H2O, (VIVO)(VVO)2(HAsO4)4, (VIVO)(VVO)2(As2O7)2, and crystals from the outgrowths on the surface of grains of an inactivated SVD catalyst demonstrated that each of these compounds may be present in the outgrowths. Apparently, an arsenate of composition (VIVO)(VVO)2(As2O7)2 is originally formed on the grain surface and then, in storage in air, it absorbs water vapor and transforms into (VIVO)(VVO)2(HAsO4)4 or (VIVO)(VVO)2(HAsO4)4 . 5H2O. A similar transformation may occur in washing grains with water to remove the glass and crystal aggregates from their surface. In addition to (VIVO)(VVO)2(As2O7)2, presence of oxovanadium(V) orthoarsenate of composition VOAsO4 or V2O5 . As2O5 was revealed [1], even though not indisputably, by X-ray phase analysis in out-growths on grains of an inactivated catalyst that worked in the middle part of catalyst bed IV. Figure 2 compares X-ray diffraction patterns of (VIVO)(VVO)2(HAsO4)4 . 5H2O and a preparation composed of ground crystal aggregates scraped-off from grains of an inactivated SVD crystal, preliminarily washed in water to remove the glass. The X-ray diffraction pattern of the preparation of crystal aggregates differs from that for (VIVO)(VVO)2(HAsO4)4 . 5H2O only in the presence of a line (marked by arrow) coinciding in position with the strongest line in the X-ray diffraction pattern of (VIVO)(VVO)2(As2O7)2 [6, 7]. The X-ray diffraction pattern of the aggregates cut-off from the grains not washed with water contained only lines of (VIVO)(VVO)2(As2O7)2, whereas RUSSIAN JOURNAL OF APPLIED CHEMISTRY
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Fig. 2. X-ray diffraction patterns of (1) (VIVO)(VVO)2(AsO4)4 . 5H2O and (2) crystal aggregates removed from the surface of grains of the SVD catalyst. Arrow denotes the strongest line belonging to (VIVO)(VVO)2(As2O7)2.
Fig. 3. Content of the liquid phase of H3AsO43H2O vs. x of the section (1 3
vanadium(V) and vanadium(IV) in the system VO(H2AsO4)23VOAsO43 in the isomolar set of compositions x)VO(H2AsO4)2 + xVOAsO4 . 3H2O.
after washing, lines of (VIVO)(VVO)2(HAsO4)4 . 5H2O and ( VIVO)(VVO)2(HAsO4)4 appeared in the pattern. To reveal the possibility of formation of oxovanadium arsenates in a mixed oxidation state (IV, V), other than (VIVO)(VVO)2(HsO4)4 . 5H2O, which serves as the main starting substance in synthesis of (VIVO)(VVO)2(As2O7)2, the interaction and solubility in the system VO(H2AsO4)23VOAsO43H3AsO43H2O No. 4
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was studied at room temperature. The starting mixtures of solid oxovanadium(IV) and oxovanadium(V) arsenates comprised an isomolar set of compositions from the section (1 3 x)VO(H2AsO4)2 + xVOAsO4 . 3H2O at a total molar content of the arsenates equal to 0.01 M. The initial concentration of arsenic acid was constant (2 M). Analysis of the solubility curves of vanadium for the system under study (Fig. 3) shows that, at x > 0.6, the concentration of vanadium(IV) in solution abruptly decreases and then remains constant at x > 0.7. The curve describing the dependence of the concentration of vanadium in solution on x shows two weak bends at x = 0.4 and 0.75. According to the results of a microscopic analysis, the bottom phase is constituted at x = 0.13 0.5 by blue crystals of VO(H2AsO4)2 and by a new compound, whose formation is evidenced by the presence of black squared plates. At x = 0.6 3 0.7, the bottom phase has the form of black squared plates that well reflect light; at x > 0.7 yellow crystals of VOAsO4 . 3H2O are additionally present. The liquid phase acquires a dark green coloration, which is the most intense at x = 0.6 3 0.7. Microscopic, X-ray phase, and chemical analyses revealed that the new phase is identical to the previously obtained [7] compound of composition (VIVO)(VVO)2(HAsO4)4 . 5H2O. The study performed demonstrated that no other oxovanadium arsenates in mixed oxidation state are formed in the given system. The most probable agent that oxidizes As2O3 on vanadium catalysts is vanadium(V). This is confirmed by the results of a study of reaction of As2O3 with K4V2O3(SO4)4 and K3VO(SO4)3, compounds simulating the active component of vanadium catalysts [8], in sealed argon-filled quartz ampules at 300 3 450oC: K4V2O3(SO4)4 + As2O3 = {4K2VO(SO4)2 + As2O5}, (3) K3VO(SO4)3 + As2O3 = {4K2VO(SO4)2 + 2K2S2O7 + As2O5}.
(4)
Under the same conditions, vanadium(V) oxide reacts with arsenic(III) oxide [12]: 2V2O5 [4V(V)
+
+ As2O3 = V4As2O13 2As(III)
=
4V(IV)
+
(5)
2As(V)].
Reactions (3) and (4) are written with account of the constant mass of the samples; the reaction products are enclosed in braces because they form blue
glasses and were not isolated in the form of crystals. As the concentration of As2O3 is lowered, green hygroscopic glasses containing vanadium in two states, V(V) and V(IV), are formed in the reaction mixtures. The interaction of sulfur dioxide with oxovanadium(V) orthoarsenate at 400 3 450oC occurs in accordance with the most probable reaction
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2VOAsO4 + SO2 = (VO)2As2O7 + SO3 .
(6)
The composition of the X-ray amorphous phase (VO)2As2O7 was determined from the results of chemical [quantitative analysis for the content of V(V), V(IV), and arsenic] and gravimetric analysis (evaluation of the sample mass in the course of the reaction). Thus, reactions (3)3(6) show that vanadium(V) in the active component of sulfuric acid catalysts is the only oxidizing agent both for SO2 and for As2O3. Raising the reaction temperature leads only to elimination of sulfur trioxide (reversible process) in the first case, and to removal of vanadium from the active component and to volatilization of vanadium at high temperatures, presumably in the form of VOAsO4 or V2O5 . As2O5, and its transfer to reaction zones having lower temperatures [1]. Sulfur dioxide reduces vanadium even in the structure of VOAsO4. Therefore, this compound, present on the surface of catalyst grains in catalyst beds III and IV, can react with sulfur dioxide to give compounds of vanadium(IV) or vanadium in a mixed oxidation state (IV, V) and, in particular, (VIVO)(VVO)2(As2O7)2. In any case, formation of oxovanadium arsenates leads to removal of vanadium from the active component and, consequently, adversely affects the performance of the catalysts. CONCLUSIONS (1) A study of grains of an SVD catalyst that lost its activity upon prolonged operation under conditions of insufficient purification of contact gases to remove arsenic(III) oxide vapor revealed aggregates of crystals composed of arsenates of oxovanadium in a mixed oxidation state, (VIVO)(VVO)2(As2O7)2 and a gray-blue glass with an exceedingly high content of vanadium(IV) and arsenic. (2) In activation of sulfuric acid vanadium catalysts lose their activity upon a prolonged contact with As2O3 vapor is caused by blocking of the working surface of their grains by compounds of arsenic with vanadium(V) and (IV) and vanadium in a mixed oxidation state (V, IV). The formation of com-
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pounds of arsenic and vanadium on the surface of catalyst grains leads to an irreversible change in the composition of the active component and, accordingly, to inactivation of the catalyst. As2O3 and SO2 are oxidized at the same active vanadium centers. REFERENCES 1. Boreskov, G.K., Kataliz v proizvodstve sernoi kisloty (Catalysis in Manufacture of Sulfuric Acid), Moscow: Goskhimizdat, 1954. 2. Kholostov, S.B., Ketov, L.N., and Bankovskaya, V.R., Zh. Prikl. Khim., 1981, vol. 26, no. 7, pp. 1900 1904. 3. Adadurov, I.E., Pershin, P.P., and Novikova, T.N., Zh. Prikl. Khim., 1934, vol. 7, no. 1 2, pp. 18 27. 4. Van’kov, B.P. and Perevozchikov, L.A., Available from ONIITEKhIM, 1985, Cherkassy, no. 66 khp 85 Dep. 5. Kopylov, N.I. and Kalinskii, Yu.D., Khim. Inter. Ustoich. Razv., 1997, no. 5, pp. 221 258.
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6. Krasil’nikov, V.N. and Shtin, A.P., Zh. Prikl. Khim., 1991, vol. 64, no. 9, pp. 1818 1822. 7. Krasil’nikov, V.N. and Shtin, A.P., Zh. Neorg. Khim., 1993, vol. 38, no. 7, pp. 1118 1120. 8. Krasil’nikov, V.N., Oxosulfovanadates and the Chemical Nature of the Active Component in Vanadium Sulfuric Acid Catalysts, Doctoral Dissertation, Yekaterinburg: Inst. of Solid State Chemistry, Ural Division, Russian Acad. Sci., 2003. 9. Petrovskaya, G.I., Gerke, L.S., and Novikova, T.N., Khim. Prom!st., 1999, no. 2, pp. 12 15. 10. Chernorukov, N.G., Egorov, N.P., and Korshunov, I.A., Zh. Neorg. Khim., 1978, vol. 23, no. 10, pp. 2672 2675. 11. Aranda, M.A.G., Attfield, J., Bruque, S., and Martinez-Lara, M., J. Solid State Chem., 1991, vol. 91, no. 1, pp. 25 31. 12. Haddad, A., Jouini, T., and Piffard, Y., J. Solid State Chem., 1992, vol. 54, no. 1, pp. 57 63.
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