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RESEARCH NOTES Fast Reaction of Solid Copper(I) Complexes with Hydrogen Sulfide Gas J. Michael Davidson,*,† Craig M. Grant,‡ and Richard E. P. Winpenny‡ School of Chemical Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, U.K., and Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
The low-temperature reactions of H2S (0.2-500 ppm) with Cu2O and a group of copper(I) complexes have been studied. [Cu(OAc)]n, Cu4(OCOCF3)4(C6H6)2, and Cu(hfac)L where hfac ) hexafluoropentanedionate and L ) 1,5-cyclooctadiene (COD) or diphenylacetylene reacted rapidly and quantitatively. Cu2(hfac)2LL where LL ) cyclooctatetraene or norbornadiene and [Cu(OCOC6H5]4 reacted slowly or not at all. Rapid and quantitative conversion is apparently enabled by the displacement of ligands of large molar volume resulting in a porous Cu2S product. Cu(hfac)(COD) is nonporous, and quantitative studies were carried out in a packed-bed reactor for three size ranges of particles. The initial rate is first order in H2S and, according to an approximate estimate, appears to be dependent on the external surface area of the particles. The rates of reaction were faster than those of commonly used absorbents, while the stoichiometric conversion is unusual. Such properties suggest applications in gas cleanup and chemical analysis of H2S. Introduction The reaction of hydrogen sulfide with solid absorbents is particularly significant in the purification of other gases.1 However, little is known of the mechanisms of either the surface reactions or the solid-state transport, although the theory of diffusion reaction in macro-, micro-, and nonporous materials is highly developed and provides a useful background for the discussion of the known experimental facts.2 Typically, H2S reacts with an oxide with displacement of water, as in the case of zinc oxide [reaction (1)]:
ZnO + H2S ) ZnS + H2O
(1)
Zinc oxide having a high porosity and surface area (60-100 m2/g) is very reactive even at subambient temperatures, but the rate slows by 2 orders of magnitude during conversion of 30-40% of the solid, which is the practical limit of the capacity for sufide.3 It has been observed that zinc sulfide is formed as an overlayer,3 and hence the conversion is probably restricted by pore blocking because the molar volume of ZnS (Vsolid ) 2.4 × 10-5 m3/mol) is greater than that of zinc oxide (Vsolid ) 1.45 × 10-5 m3/mol). Zinc carbonate has a slightly greater molar volume (Vsolid ) 2.85 × 10-5 m3/mol) than ZnS but is unreactive even in high surface area form; thus, oxides and carbonates have varying sulfiding capacity.4 Low surface area ZnO is unreactive except at much higher temperatures. It has been difficult to find systematic effects in the reactivity of * To whom correspondence should be addressed. E-mail:
[email protected]. † School of Chemical Engineering, University of Edinburgh. ‡ Department of Chemistry, University of Edinburgh.
these substances for sulfiding by H2S, but a useful discussion of the effect of molar volume change in the solid phase has been given for one case, which is the reaction of metals with oxygen.5 In this paper, we report specifically on the role of the molar volume change in the solid phase accompanying hydrogen sulfide absorption into some copper compounds. Bulk reactions of solids are often very sluggish although the surface reactions are fast, while solid volume relationships on the basis of formula weights are a useful guide to reactivity.5 We now report sulfiding reactions of a group of compounds of copper(I) subject to a large negative ∆Vsolid and chosen on the basis of the very strong affinity of Cu(I) for sulfur. The equilibrium constant at 298 K for the sulfiding of Cu2O by H2S is 6.7 × 1023; only three elements in particular oxidation states [Ag(I), Hg(II), and Pt(II)]6 show a relatively greater affinity for sulfide vs oxide. Hence, very low levels can be achieved in H2S removal. Copper(I) offers an extensive range of compounds from which test examples can be chosen, including organometallic species with ligands such as alkoxides, carboxylates, and π-acceptor types such as alkenes, arenes, and CO. Prospectively, complexes can be chosen so that the volatile products of the desulfurization reaction are noncontaminating with respect to a process stream. For example, polymeric copper(I) acetate7 ([CuOAc]n) proves to be exceptionally reactive and can be used for H2S removal from a stream containing acetic acid without a contaminating effect [reaction (2)]. The objective of a “tailored absorbent” is significant because in some cases the desulfurization reaction causes alternative contamination of the process stream; e.g., reaction (1) is not suitable for cleanup of previously dried dihydrogen or syngas streams. In reaction (2), because of the decrease
10.1021/ie000966j CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001
Ind. Eng. Chem. Res., Vol. 40, No. 13, 2001 2983 Table 1. Molar Volume Data and Batch-Gas Batch-Solid Reaction Data for Copper(I) Compounds at 295 K compound
105Vsolida
typeb
X2c
Xfinal (time)d
Cu2O [Cu(OCOCH3)]n [CuOCOC6H5]4 Cu(hfac)(COD) Cu(hfac)(DPA) [Cu(hfac)]2(COT) [Cu(hfac)]2(NBD) Cu4(OCOCF3)4(C6H6)2 Cu2S
1.1715
iii i ii i i i ii i
36 49 0 89 98 11 1 67
40 (180) 99 (8) 30 (70) 98 (10) 93 (11) 5 (13) 31 (140) 97 (49)
1.627 9.713 21.99 28.110 16.711 15.516 9.712 1.715
a Molar volume (m3/mol), basis one copper atom. b Reaction types i-iii (see text). c Downstream conversation of H2S after 2 min. d Final conversion of copper and reaction time (% (min)).
in the molar volume of the solid phase accompanied by volatilization of the displaced ligand, we can anticipate that the product layer in either particles or grains should be highly porous, causing little restriction to the transport of H2S or the products.
2CuOAc + H2S ) Cu2S + 2HOAc
(2)
A range of substances was subjected to preliminary screening, primarily to test the above molar volume effect in reactions with H2S, whereby bulky organic ligands are replaced by sulfide. These were, in addition to [CuOAc]n, four complexes of hexafluoropentanedione (Hhfac ) CF3COCH2COCF3), Cu(hfac)(COD)8,9 (COD ) 1,5-cyclooctadiene), Cu(hfac)(DPA)10 (DPA ) diphenylacetylene), Cu2(hfac)2(NBD)11 (NBD ) norbornadiene), Cu2(hfac)2(COT)11 (COT ) cyclooctatetraene), Cu4(OCOCF3)4(C6H6)2,12 [CuOCOPh)]4,13 and Cu2O (prepared by reduction of Fehling’s solution). A more detailed study was carried out using Cu(hfac)(COD) [reaction (3)], chosen because of the large molar volume of its ligands, its ease of handling in air, and ready availability in macrocrystalline form. Other complexes were handled in a dry nitrogen glovebox.
2Cu(hfac)(COD) + H2S ) Cu2S + 2Hhfac + 2COD (3) Experimental Section The Pyrex batch-gas, batch-solid recycle reactor (BRR) and the open flow reactor have been described previously.3,14 However, in the open-flow experiments the chromatograph used was a dual-detector Perkin-Elmer model 8700 instrument equipped with a sulfur flame photometric detector and a microvolume thermal conductivity detector having limits of detection of H2S of 0.1 and 50 ppm, respectively. Preliminary absorption tests were carried out using the BRR and 0.5-2.0% H2S in He to establish the rate behavior of the reactions, the conversion (from the gas-phase analysis), and the stoichiometry (by trapping the volatile products in a solid CO2 trap at -78 °C for subsequent identification of the mixtures by NMR spectroscopy). BRR reaction data are given for all of the Cu(I) compounds studied in Table 1, where three broad classes of reactivity are indicated. As an example, Cu(hfac)(COD) (0.379 g, 1.0 mmol) was treated with 0.938 mmol of H2S in helium (1.85% recycling at 1 L/min at 22 °C) and consumed 0.491 mmol. Gas chromatographic analysis of the downstream unreacted gas was steady, and hence the reaction was complete, prior to the fourth sample (12 min). The mixed
organic products were trapped at -78 °C (0.291 g, theory 0.316 g) and shown by NMR spectroscopy to be a 1:1 mixture of Hhfac and COD. The open-flow reactor (8 mm internal diameter) was used for more detailed study of the rate of reaction of Cu(hfac)(COD) with H2S in a N2 carrier (∼1 L/min; 0.2500 ppm H2S; 268.2-298.6 K). The absorbent was first sublimed in vacuo to obtain crystals of more than about a millimeter size which could then be crushed and sieved into narrow size ranges (105-300, 300-450, and 710-1000 µm mesh); these were supported in the reactor in an inert diluent packing of glass beads of similar size in order to impose a reproducible flow regime. These reactions were carried out at 0 °C using an initial concentration of H2S of 230 ppm, or less, in an attempt (partly successful) to measure the reaction rate at differential conversion of both solid and gas. In a high-conversion, preparative-scale experiment, Cu(hfac)(COD) (1.111 g without diluent) was treated with flowing 0.2% H2S in N2 at 22 °C/1 atm for 1 h and yielded 0.238 g of Cu2S (theory 0.233 g, identified by X-ray powder diffraction). An experiment was also carried out to monitor the progress of the reaction front by means of the color change (chrome yellow to black). Using 0.989 mmol of Cu(hfac)(COD) as undiluted 300450 µm particles as a packed section 11.5 mm deep in an 8 mm i.d. tube, a nitrogen carrier stream (22 ppm H2S) was passed at 2.66 × 10-4 mol/s. After 7 h and the reaction of 1.46 × 10-4 mol of H2S, the front was at a depth of 3.3 mm, corresponding to 28% conversion or 2.8 × 10-4 mol of Cu(hfac)(COD). The sharpness of the front and its position indicates the complete and rapid conversion of the absorbent even at ppm levels of H2S. Because of the marked color change, it was possible to observe the slow development of the front even using 0.2 ppm H2S. Results and Discussion We have investigated the general effect of the ∆Vsolid in gas-solid reactions using a range of copper(I) complexes, some of which proved to be very reactive for absorption of H2S, which was removed to levels below the limit of detection in flowing gases. The molar volumes relevant to this work are listed in Table 1 for comparison with that of Cu2S (Vsolid ) 1.7 × 10-5 m3/ mol on the basis of one copper atom). More detailed studies were carried out using Cu(hfac)(1,5-cyclooctadiene); its molar volume is 2.19 ×10 -4 m3/mol, and comparison with that of Cu2S provides a good test of the effect of large negative ∆Vsolid on the permeability of the product layer. A single crystal of Cu(hfac)(COD) was observed in a microscope during saturation sulfiding by 0.2% H2S in N2 and a 40 µm dimension appeared to be unchanged, suggesting that, because the bulk volume is not altered, there must be a substantial increase in the porosity of the solid. The fast reaction of Cu(hfac)(COD) with H2S and the quantitative conversion to products seem remarkable in view of the very low surface area of nonporous particles (typically 0.08 m2/g; BET method by N2 adsorption) and presumably reflect the increase in porosity at high conversion. The surface area of the fully sulfided Cu2S product remained low (1.1 m2/g). Rate data and some derived results are shown in Figure 1 and, for comparison, one run using zinc oxide (60 m2/g) at similar reaction conditions is included; the conversion of ZnO was only 22% after 1 h, and the rate was then very slow.
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Figure 1. Rate data for sulfiding reactions of Cu(hfac)(COD) (+, ×, 0, ]) and zinc oxide by hydrogen sulfide at 0 °C.
In the open-flow experiments, feed and product analysis was carried out using the sulfur flame photometric detector with chromatograph sampling about every 4 min. Values of Ftotal/nCu were in the range of 9.22-11.8 s-1 with H2S concentrations of 102-231 ppm. At higher H2S concentration (500 ppm), the reaction was too fast for rate measurements using our existing method. Figure 1 shows reaction rate at 0 °C vs time plots calculated on the basis of the initial number of moles of copper(I) for three particle size ranges and two H2S feed concentrations. These plots of the specific rate, molH2S/molCu‚s, appear nearly linear and allow extrapolation to find both the initial rate and the time for complete conversion of the solid (τ). Some confirmation of the soundness of this linear empirical correlation can be gained from the fact that the average of the area under the straight lines for these runs was 0.52 molH2S/molCu which is close to the stoichiometry of eq 4. From two runs using 300-450 µm particles, the reaction appears to be first-order in H2S, while an increase in the rate with a decrease in the particle size is apparent from the group of three runs at very similar conditions. The solid conversion prior to the first gas analysis was in the range 3-15%. Most models for diffusion reaction describe the performance in terms of a well-defined geometry, such as uniform spheres.2 In such an analysis, the evaluation of τ is very useful. For nonporous spheres, the sharp interface model then applies, and the core radius (rc) and solid conversion (Xsolid) can then be found [e.g., (t/τ ) 1 - (rc/R)2 ) Xsolid for the fast surface reaction with external film masstransfer control]. Practical quantities of uniform spheres are difficult to obtain, and for the present we can only achieve a crude approximation by equating sphere diameters to the means of the size ranges of the sieved
particles (e.g., the equivalent radius of particles of mesh size 105-300 µm was taken to be R ) 0.0101 m and thus aexternal ) 0.00925 m2/g). With this approximation, for the runs presented in Figure 1, the initial rates can then be compared on the basis of moles of H2S consumed per unit external surface area of the solid for which the first-order rate constants (k, m/s) for the four experiments are in fair agreement, indicating control of the initial rate by either first-order surface reaction or diffusion of H2S through an external boundary layer film followed by fast surface reaction. Using the usual correlations17and the properties of bulk N2 for those of the mixtures, with NRe ) 3, R ) 4.27 × 10-4 m, u ) 1.3 m/s, and ) 0.5, a mass-transfer coefficient of 0.4 m/s is found. Thus, the intial rates at 0 °C appear to be about an order of magnitude lower than the maximum possible mass-transfer rate in the system. However, the activation energy, determined from initial rates measured using 300-450 µm particles at five temperatures in the range of 268.2-298.6 K, was 16 kJ/mol, which is low for chemical control. In view of the crude approximations inherent in the above calculations, further fitting of the data to conversion-time models is not justified, although it appears that the rate per unit surface area of the core does not change much. Single crystals of Cu(hfac)(COD) are easily grown in vacuo, and it may be possible to use these to measure the reaction rate with an improved description of the particle geometry. Qualitative comparisons of the sulfiding rates were made for some other copper(I) complexes and for Cu2O (Sg ) 1.3 m2/g) using the BRR. The form of the concentration-time curves for H2S is somewhat dependent on the molar ratio of the absorbent to that of
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H2S, but three classes of reactivity can be recognized (Table 1). These are as follows: Type i. Highly reactive absorbents form a sharp front and exhibit breakthrough only a short time before the front reaches the end of the particulate packing. Thereafter, the concentration of H2S in the product stream rises until it has reached a steady value when the reaction has ceased. Type ii. Absorbents of low reactivity allow immediate breakthrough of a low-conversion partly reacted gas. Thereafter, the concentration of H2S throughout the BRR loop declines to a steady value when the reaction has ceased, often with incomplete reaction of the solid. Type iii is intermediate between types i and ii. The absorbent shows an initial high reactivity with high conversion across the reactor. A rapid decline in the reactivity of the absorbent leads to a sharp increase in the H2S concentration followed by a slow decline to a lower value, usually with incomplete conversion of the solid. Several metal oxides and carbonates show this behavior. In Table 1 are given the reaction type, the H2S conversion 2 min after the reactor was switched on line, and the final conversion of copper(I) when reaction had ceased. These are good measures of the initial rate and frontal characteristics and the absorption capacity, respectively. Evidently, a group of complexes for which ∆Vsolid is very negative [Cu(hfac)(COD), Cu(hfac)(DPA), Cu4(OCOCF3)4(C6H6)2, and [CuOAc]n] are highly reactive type i species. Using an excess H2S, solid conversions were nearly quantitative within a few chromatographic cycles. Polymeric copper(I) acetate appeared to be the most reactive of the absorbents and solid conversion was complete within about 6 min. By comparison, tetrameric copper(I) benzoate gave a slow reaction with 30% conversion in 70 min; in this case ∆Vsolid relates to the mixture of Cu2S with benzoic acid which is not volatile, and hence the product will not be so highly porous. The reactions of both dinuclear complexes were surprisingly slow in view of the structural similarity to Cu(hfac)(COD). Cu2(hfac)2(COT) showed a slight initial absorption, probably because of surface reaction, but otherwise was almost entirely unreactive. Cu2(hfac)2(NBD) gave only 31% conversion after 140 min. Clearly, further studies will be required to identify the factors influencing the rates of these reactions. In particular, it must be noted that in several cases the mechanism must accommodate a stoichiometry in which two copper-containing molecules react with one H2S molecule at fast rates, whereas both of the dinuclear complexes tested had poor reactivity. Presumably, attachment of the cyclopolyalkene ligands to more than one copper atom in the lattice prevents its displacement. It is also noteworthy that solution reactions of H2S with odd-electron chromium species have 2:1 stoichiometry and are third order.18 Copper(I) oxide reacted in a manner very similar to ZnO; both are type iii, having positive ∆Vsolid. Upon treatment with H2S (1.8% He with 0.5% added H2O), about 30% of the copper(I) reacted within 20 min, after which the rate was very slow, reaching 40% conversion in 3 h. Conclusions It has been shown that, for a group of copper(I) complexes, a fast and quantitative low-temperature gas-solid reaction with hydrogen sulfide is enabled when the displacement of ligands results in a substan-
tial decrease in the molar volume of the solid phase. Clearly, such reactivity must be associated with the high porosity of the resulting Cu2S product. Nevertheless, the dinuclear Cu(I) complexes of COT and NBD were much less reactive, indicating that structural, electronic, and stereochemical factors can also be important. The observed quantitative reaction of Cu(hfac)(COD) in a packed bed also implies negligible readsorption of the displaced ligands onto the unreacted fluorinated complex. This raises the possibility of application of the reaction in analysis of H2S at ppm levels by means of photometric determination of hexafluoropentanedione. This may be advantageous because the sensitivity of the infrared analysis of H2S is low while the ultraviolet determination is subject to significant interferences. Acknowledgment We thank the EPSRC (U.K.) for a postdoctoral fellowship (to C.M.G.). Nomenclature aexternal ) external surface area of particles (m2/g) BET ) Brunauer-Emmet-Teller BRR ) batch recycle reactor Ftotal ) total molar flow rate (m3/s) k ) first-order rate constant for reaction at the external surface (m/s) nCu ) number of moles of Cu(hfac)(COD) (mol) NRe ) particle Reynolds number rc ) core radius in the sharp interface model (m) R ) radius of spherical particles (m) Sg ) surface area of solids (m2/g) u ) gas velocity (m/s) Vsolid ) molar volume of the solid phase on the basis of one copper atom (m3/mol) ∆Vsolid ) molar volume change of reaction in the solid-phase atom (m3/mol) ) porosity of a packed bed τ ) time for complete reaction of a particle
Literature Cited (1) Carnell, P. J. H. Feedstock Purification. In Catalyst Handbook, 2nd ed.; Twigg, M. V., Ed.; Wolfe Publishing: London, 1989; Chapter 5. (2) Doraiswamy, L. K.; Kulkarni, B. D. In Chemical Reaction and Reactor Engineering; Carberry, J. J., Varma, A., Eds.; Marcel Dekker: New York, 1987; Chapter 5. (3) Davidson, J. M.; Lawrie, C. H.; Sohail, K. Kinetics of the Absorption of Hydrogen Sulfide by High Purity and Doped High Surface area Zinc Oxide. Ind. Eng. Chem. Res. 1995, 34, 29812989. (4) Davidson, J. M.; Sohail, K. Low-Temperature Reaction of Metal Oxides, Carbonates and Basic Carbonates with Hydrogen Sulfide and Carbonyl Sulfide. Industrial Chemistry Engineering 1995 Research Event; Preprinted Papers 517-519. (5) Cottrell, A. An Introduction to Metallurgy, 2nd ed.; Edward Arnold: London, 1975. (6) Phillips, C. S. G.; Williams, R. J. P. Inorganic Chemistry; Oxford University Press: Oxford, U.K., 1965; Vol. I, Chapter 16. (7) . Mounts, R. D.; Ogura, T.; Fernando, Q. Crystal Structure of Copper(I) Acetate. Inorg. Chem. 1974, 13, 802-805. (8) Chi, K. M.; Shin, H.-K.; Hampden-Smith, M. J.; Duesler, E. N. The Chemistry of β-Diketonate Copper(I) CompoundssIII. The Synthesis of (β-Diketonate)Cu(1,5-COD) Compounds, the Solid State Structure and Disproportionation of Hexafluoroacetylacetonato(1,5-cyclooctadiene)copper(I), (hfac)Cu(1,5-COD). Polyhedron 1991, 10, 2293-2299. (9) Kumar, R.; Fronczek, F. R.; Maverick, A. W.; Kim, A. J. Bond Breaking in the Chemical Vapor Deposition Precursor (1,1,1,5,5,5-Hexafluoro-2,4-pentandionato)(η2-1,5-cyclooctadiene)-
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copper(I) Studied by Variable-Temperature X-ray Crystallography and Solid State NMR Spectroscopy. Chem. Mater. 1994, 6, 587595. (10) Chi, K.-M.; Shin, H.-K.; Hampden-Smith, T. T.; Kodas, T. T.; Duesler, E. N. Synthesis and Characterisation of (β-Diketonato)copper(I) Alkyne Complexes: Structural Characterisation of (Hexafluoroacetylacetonato)(diphenylacetylene)copper(I). Inorg. Chem. 1991, 30, 4293-4294. (11) Doyle, G.; Eriksen, K. A.; van Engen, D. Alkene and Carbon Monoxide Derivatives of Copper(I) and Silver (I) β-Diketonates. Organometallics 1985, 4, 830-835. (12) Rodesiler, P. F.; Amma, E. L. Preparation and X-ray Molecular Structure of an Unusual Copper(I) Polynuclear Species: Tetrakis(copper trifluoroacetate)dibenzene. J. Chem. Soc., Chem. Commun. 1974, 599-600. (13) Drew, M. G. B.; Edwards, D. A.; Richard, R. Crystal and Molecular Structure of Tetrakis[Copper(I) benzoate]. J. Chem. Soc., Dalton Trans. 1977, 802-805. (14) Chambers, R. P.; Dougharty, N. A.; Boudart, M. A. A
Reliable Noncontaminating Recirculation Pump. J. Catal. 1965, 4, 625-626. (15) Berry, L. G.; Mason, B.; Dietrich, R. V. Mineralogy, 2nd ed.; Freeman: San Francisco, CA, 1983. (16) Davidson, J. M.; Grant, C. M.; Parsons, S.; Winpenny, R. E. P. Unpublished results. (17) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; MIT Press: Boston, 1970. (18) Capps, K. B.; Bauer, A.; Ju, T. D.; Hoff, C. D. Mechanistic Study of the Reaction of •Cr(CO)3C5Me5 with H2S Yielding HCr(CO)3C5Me5, HSCr(CO)3C5Me5 and C5Me5(CO)2CrdSdCr(CO)2C5Me5. Kinetic Evidence for the Formation of the Substituted Radical Complex •Cr(CO)2(H2S)C5Me5. Inorg. Chem. 1999, 38, 6130-6135.
Received for review November 15, 2000 Revised manuscript received March 21, 2001 Accepted March 26, 2001 IE000966J
