I n d . E n g . Chem. R e s . 1989,28, 531-535
53 1
MATERIALS AND INTERFACES Reactivity and Low Formaldehyde Release in Durable Pr.ess Agents for Cotton Fabrics? B. A. Kottes Andrews* and Brenda J. Trask-Morrell S o u t h e r n Regional Research Center, M i d - S o u t h Area, Agricultural Research Service, United S t a t e s Department of Agriculture, N e w Orleans, Louisiana 70179
T h e effectiveness of methods for the reduction of formaldehyde in fabrics finished for durable press can vary according t o the reactivity of the cellulose cross-linking agents. Although lowering formaldehyde to nitrogen ratios in cellulose reactants will reduce formaldehyde both in the finishing system and in the fabric, the reduction can be at the expense of the degree of smoothness of the fabric appearance. T h e present research compares the influence on fabric resiliency of reduced methylolation from two types of agents. Agents with reactivity not wholly derived from pendant methylol groups and those with only methylol-initiated reactivity were studied. T h e effectiveness of substitution of the hydroxyl hydrogen in these agents by methyl and hydroxyethoxymethyl groups was investigated also. Determination of formaldehyde release, steady-state, and transport formaldehyde levels is used t o assess effectiveness of the formaldehyde reduction methods. These data are supplemented by information on the durability of the protective alkyl groups through hydrolysis conditions. Over the past several years, the American textile industry has reduced the amount of releasable formaldehyde in durable press fabrics to very low levels, from 1000 pg/g to as low as 100-200 pg/g (North, 1983a,b). This reduction has been a positive response to regulatory pressures and self-imposed industry standards for increased worker safety. Unfortunately, the performance level of the fabrics produced recently has declined also. The present research examines the influence of functionality, degree of methylolation and etherification of dihydroxyethyleneurea- and carbamate-based cross-linkers for cellulose on the balance needed for low formaldehyde release and acceptable smooth-dry performance. In Figure 1 are the generalized reactions between the parent amides and formaldehyde to produce two of the widely used cellulose cross-linkers for durable press fabrics. The reactions are reversible, and the products are not solely the dimethyl01 derivatives but are a statistical distribution of methylolated compounds representing the favored equilibrium positions unique to each amideformaldehyde reaction (Andrews, 1984; Andrews and Reinhardt, 1989; Pasad and Beck, 1986). Formaldehydedihydroxyethyleneurea (DHEU) products are potentially tetrafunctional and formaldehyde-carbamate products are potentially difunctional; these methylolamides can be etherified (capped) with either methanol or a glycol (diethylene glycol, ethylene glycol, or propylene glycol) to reduce formaldehyde release (Andrews and Reinhardt, 1984, 1985; Andrews et al., 1980a,b; North, 1983a,b). This control option currently is used by the industry. Another control option is the use of less than the stoichiometric Presented in part at the 194th National Meeting of the American Chemical Society, Textile Finishing Chemistry Symposium of the Cellulose, Paper, and Textile Division, Aug 3C-Sept 4,1987, New Orleans, LA.
equivalent amount of formaldehyde for complete methylolation (Andrews et al., 1980a,b).
Materials and Methods The fabric was an 80 X 80 cotton printcloth, 3.2 oz/yd2 (0.109 kg/m2), desized, scoured, and bleached. The cellulose reactants were prepared in the laboratory by known methods from the parent amide and formaldehyde (Harper et al., 1984). DHEU was prepared from urea and glyoxal. Methyl carbamate was obtained as a commercial product. Formaldehyde was from reagentgrade formalin stabilized with methanol. The forma1dehyde:amide ratios of the reactants were varied by adjusting the amount of formaldehyde used in the methylolation step from 2.5 to 2.0 to 1.7 to 1.5 mol of formaldehydejmol of amide. Etherified products were prepared by reaction of the methylolated reactants with either methanol or ethylene glycol. Both methanol and ethylene glycol were reagent-grade chemicals. The catalyst was a mixture of magnesium chloride hexahydrate and citric acid used in a 20:l gfw ratio. Both were reagent-grade chemicals. The cross-link finishing treatments of cotton were performed on a laboratory scale by the conventional paddry-cure process. All fabrics were padded through squeeze rolls to approximately 85-90% wet add-on from two dips and two nips. After the padding step, the impregnated fabrics were dried at 70 "C for 7 min and cured at 160 "C for 3 min. After the curing step, the finished fabrics were given a neutral afterwash with a nonionic detergent in an apartment size washer and tumbled dry. Wrinkle recovery angles, durable press ratings, and formaldehyde release values (Sealed Jar Test) were obtained by current AATCC test methods (American Association of Textile Chemists and Colorists, 1987). Nitrogen analyses were by the Kjeldahl method. McIlwain's buffer
This article not subject to U S . Copyright. Published 1989 by the American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989
532
DHEU-BASED AGENTS
'KH
'NH
I
-AH 0
ZHCHO
___t
HOCH~~' ~
H
~
H
HC-CH
Ht
0
H
C
I
Table I. Influence of HCH0:DHEU Ratio on Finished Fabric Properties wrinkle recovery angle HCHO: % nitrogen (W + F), deg DHEU" no caD Me cap EG caD no caD Me caD EG caD 1.5 1.43 0.77 0.51 267 249 231 1.7 1.28 0.71 0.48 271 256 236 2.0 1.29 0.69 0.44 275 261 233 2.5 1.11 0.70 0.45 272 265 248
H
0
0
H
H
CARBAMATE-BASED AGENTS
0
II
ROCNHZ
Total reactant conceiitration was 9%.
0
ZHCHO
II C.
ROCN(CH20H)
OHALKYL
cu(BLylm
ALKYl.DUR?€lYWLCLR&UIITE
Figure 1. Generalized reactions between HCHO and DHEU or an alkyl carbamate to form the dimethyl01 compounds.
n
=I :
900
30.1
1
a Reactants were applied in concentrations equivalent t o 6% DHEU.
I I -Me
-EG
CSD
5 WLHO D H E L
H C * O DHEL
Table 11. Influence of HCH0:DHEU Ratio on Finished Fabric ProDerties wrinkle recovery angle HCHO: 70 nitrogen (W + F), deg DHEU" no cap Me cap EG cap nocap Me cap EG cap 1.5 1.27 1.05 1.21 267 261 239 1.7 1.27 1.26 1.08 271 260 238 2.0 1.29 1.27 1.14 275 265 248 2.5 1.23 1.17 1.06 272 268 256
2 0 HCHO DHEU
2 5 HCMO DHEU
Figure 2. Response of HCHO release in fabrics finished with DHEU-based reactants to the combination of variation of HCHO: DHEU ratio in agent preparation and of etherification with either methanol (-Me) or ethylene glycol (-EG).
solutions were used in the hydrolysis studies (Lange, 1961). Steady-state formaldehyde was by the method of Andrews and Reinhardt (1986). Transport formaldehyde was by the method of Reinhardt (Reinhardt, 1983; Andrews et al., 1988). Analyses for methoxyl content were performed by Galbraith Laboratories. (Names of companies or commercial products are given solely for the purpose of providing specific information; their mention does not imply recommendation or endorsement by the USDA over others not mentioned.)
Results and Discussion Both reduction of formaldehyde content and etherification (capping) of N-methylol groups of cyclic urea- and carbamate-based cellulose cross-linking agents have been used separately as techniques for decreasing formaldehyde release in durable press fabrics finished with these agents (Andrews, et al., 1980a; Harper et al., 1984). However, the potential for reduction of formaldehyde release by combining these techniques has not been explored fully. Formaldehyde Reduction/Etherification in DHEU-Based Agents. In Figure 2 is shown the response of formaldehyde release in fabrics finished with DHEUbased agents to both variation of formaldehyde in agent preparation and to etherification. Reactants were applied to fabrics at a 9% total solids concentration with 2.7% magnesium chloride/citric acid catalyst. A t a 9% total solids concentration of reactant, the percentage of DHEU in solution decreases as the ratio of forma1dehyde:DHEU is increased. Without etherification, formaldehyde release in unwashed fabrics decreases dramatically as form-
aldehyde was decreased from an excess of 2.5 to 2 mol of formaldehyde/mol of DHEU. (The ratio of forma1dehyde:DHEU in dimethyloldihydroxyethyleneurea (DMDHEU) is 2:l.) Formaldehyde release decreased further as the formaldehyde in agent preparation was decreased to 1.5. However, with etherified agents, there appears to be little immediate advantage in the reduction past the equivalent ratio of 2:l. A comparison at constant DHEU concentration, not shown, followed the same trends. Formaldehyde release from etherified agents was slightly higher in fabrics from application at constant DHEU concentration because part of the formaldehyde in a 9% total solids concentration is not replaced by the etherifying agent. The influence of the forma1dehyde:DHEU ratio from treatments at the 9% total solids concentration on textile performance characteristics is shown in Table I. Bound agent, indicated by nitrogen content, is about the same from treatments with agents from 1.7 and 2.0 ratios. However, the effect of replacement of part of DHEU with alcohols is apparent in the lowered nitrogen contents; because of the tetrafunctionality of DMDHEU, wrinkle recovery does not fall off until ratios are below 1.7. Ethylene glycol produced a poorer product; both bound agent and wrinkle recovery angles were less for a similar forma1dehyde:DHEU ratio for fabric finished with these etherified products. If the agents are applied at constant DHEU concentration, finished fabrics with properties more equivalent to those from DMDHEU treatments are obtained with the reduced formaldehyde/etherified agents. These data are seen in Table 11. In order to examine the necessity of increasing the degree of etherification to the maximum, DHEU-based reactants were prepared from two levels of etherification with methanol or ethylene glycol. The results are shown in Table 111. Either 2.5 or 5.0 mol of alcohol/mol of DMDHEU was used for etherification (Andrews and Reinhardt, 1989; Beck et al., 1988). These ratios provided for partial or complete etherification. There appears to be little advantage in increasing etherification past the partial stage. Decreased formaldehyde release from ethylene glycol probably is from decreased reactivity. Formaldehyde Reduction/Etherification in Carbamate-Based Agents. In Figure 3 is shown the response
Ind. Eng. Chem. Res., Vol. 28, No. 5 , 1989 533 Table 111. Influence of Alkylation on Fabric Properties from DHEU-Based Finishes 70 nitrogen
alkylation equiv in 2.5 HCH0:DHEU reactanto methylol methylol + ring OH
EG cap 0.45 0.40
Me cap 0.70 0.61
HCHO release unwashed, WgJg Me cap EG cap 27 1 139 227 107
durable press rating EG cap Me cap 3.3 3.7 2.4 3.6
OTotal reactant concentration was 9%. For alkylation equivalent to N-methylol groups, 2.5 mol of alcohol or glycol was used; for alkylation equivalent to N-methylol groups and ring hydroxyls, 5.0 mol of alcohol or glycol was used. 1000
.
Table IV. Influence of the Forma1dehyde:DHEU Ratio on Formaldehyde Release of Fabrics after Each Processing Stage HCHO release, pg/g DHEU deriv uad drv cure wash 1.7 HCH0:DHEU Ratio 524 methylol 1579 1376 821 Me cap 1334 800 254 59 EG cap 1168 844 54 69
750
8 W Y)
5
500
W
a 0
I 0
methylol Me cap EG cap
1.8 HCH0:DHEU Ratio 2429 2246 1124 1983 1145 320 1863 1377 107
735 74 96
methylol Me cap EG cap
1.9 HCH0:DHEU Ratio 3165 2594 1150 2923 1569 350 2871 1997 86
622 81 112
methylol Me cap EG cap
2.0 HCH0:DHEU Ratio 3270 2431 1103 3245 1513 417 2918 1872 59
732 105 83
I 250
0 NO
- M ~
Cap 1 5 HCHO
MC
No Cap
- M e -EG
1 7 HCH0:MC
No
-Me
-EG
Cap
2.0 HCHO M C
Figure 3. Response of HCHO release in fabrics finished with methyl carbamate based reactants to the combination of variation of HCH0:MC ratio in agent preparation and of etherification with either methanol (-Me) or ethylene glycol (-EG).
of formaldehyde release in fabrics finished with carbamate-based agents to both reduction of formaldehyde in agent preparation and to etherification. These reactants were applied at a 9% total solids concentration with 2.7% magnesium chloride/citric acid catalyst, as were the DHEU-based reactants (Figure 2). The same trend in fabrics from non-etherified methyl carbamate finishes as with similar DHEU-based finishes is seen. There is an increase in formaldehyde release with increasing formaldehyde used in methylolation, but there is little difference among fabrics from etherified finishes. There appears to be little immediate advantage in formaldehyde release characteristics of the finished (precure) fabrics from reduction past the equivalent ratio of 2:l. However, if the fabrics are to be used in postcure operations, the formaldehyde release characteristics of the fabric after the drying stage (sensitized fabric) become important. In postcure operations, the garments are fabricated from dried-only fabrics and the curing step is performed on garments. Formaldehyde release data after each processing stage, pad, dry, cure, and wash, are shown in Table IV. From these data, it can be seen that forma1dehyde:DHEU ratios less than 1.8 are necessary to reduce formaldehyde release in the sensitized fabrics treated with etherified DHEU-based reactants to levels below 1000 pg/g. (With carbamate-based reagents, even the combination of etherification and reduced HCH0:carbamate ratios is not sufficient. Formaldehyde release from sensitized fabric from a methylated 1:l HCHO/carbamate product was 2971 pg/g.) When durable press appearance properties of the fabrics finished with DHEU- and carbamate-based agents are compared, the effects of hydroxyl functionality can be seen. These data are in Table V. Varying the formaldehyde: amide ratio from 1.5 to 2.5 produced little difference in the appearance properties of fabrics finished with DHEU-based reactants. However, with carbamate-based reactants, appearance properties decreased with decreasing forma1dehyde:amide ratios. Differences were even more
Table V. Influence of HCH0:Amide Ratio on Appearance Properties of Fabrics with DHEU-Based Finishes and Carbamate-Based Finishes wrinkle recovery durable press rating angle (W + F), deg methyl methyl carbacarbaHCH0:amide" mate DHEU mate DHEU 1.5 243 249 2.9 2.0 249 261 3.2 2.5 262 265 3.2 "Total reactant concentration was 9%.
1 -I
2
PH
2
4
7 1 0
FULLY METHYLATED DMDHEU
2
4
7 1 0
PARWLV METHYLATED DMWU
2
4
7 1 1
DIMETWLOL 4.5-DMOEV
4
7
4 ,I-DMOEU
Figure 4. Fraction of methoxyl retained on hydrolysis at pH 2, 4, 7, and 10 for fabrics finished with fully methylated DMDHEU, partially methylated DMDHEU, dimethylol-4,5-dimethoxyethyleneurea (dimethylol-4,5-DMOEU), and 4,5-dimethoxyethyleneurea (4,5-DMOEU).
pronounced if comparisons were made on the basis of equivalent amide concentrations (not shown). Influence of Etherification on Hydrolysis Resistance of DHEU-Based Reactants. In Figure 4 are shown
534
Ind. Eng. Chem. Res., Vol. 28, No. 5 , 1989
Table VI. Stability t o Hydrolysis As Measured by HCHO Characteristics HCHO release, Fg/g, after hydrolysis at DHEU derivative for finish orig PH 2 PH 4 PH 7 PH 9 551 172 159 932 960 DMDHEU 56 14 22 186 148 partially methylated DMDHEU 39 0 0 0 0 fully methylated DMDHEU 73 44 39 41 71 dimethylol-4,5-dimethoxyethyleneurea
pH 10 819 109 30 31
Table VII. Stability to Hydrolysis As Measured by HCHO Characteristics free formaldehyde characteristics," p g / g , after hydrolysis at PH 2 PH 4 pH 7 PH 9 pH 10 DHEU derivative for finish SS T SS T SS T SS T ss T 0 24 13 11 DMDHEU 1 0 18 9 10 7 2 0 0 37 3 3 0 0 6 0 partially methylated DMDHEU 5 8 11 32 9 6 0 0 5 0 fully methylated DMDHEU 0 17 13 3 0 3 0 17 0 5 dimethylol-4,5-dimethoxyethyleneurea
SS = steady-state formaldehyde; T = transport formaldehyde.
I] 21 -.
2 FULLY METHYLATE[ DMDHEU
7
to
'ARTIALLY ETHYLATE IMOHEU
M H E U
DIMETHYLOL 4 . 5 - D M O E U 4 5-OMOEU
1,3-
45i)HEU
7 10
Figure 5. Fraction of nitrogen retained on hydrolysis at pH 2, 7, and 10 for fabrics finished with fully methylated DMDHEU, partially methylated DMDHEU, dimethyloldihydroxyethyleneurea (DMDHEU), dimethylol-4,5-dimethoxyethyleneurea(dimethylol4,5-DMOEU), and 4,5-dimethoxyethyleneurea (4,5-DMOEU),1,3dimethyl-4,5-dihydroxyethyleneurea(1,3-dimethyl-4,5-DHEU), and 4,E~dihydroxyethyleneurea(4,5-DHEU).
the fractions of methoxyl retained on hydrolysis for fabrics finished with fully methylated (tetramethylated) DMDHEU, partially methylated DMDHEU, and two reactants methylated only on the 4,5-position: 1,3-dimethylol-4,5-dimethoxyethyleneurea(dimethylol-4,5DMOEU), and 4,5-dimethoxyethyleneurea(4,5-DMOEU). The latter is capable of reacting only through the 4,5position. The fabrics were hydrolyzed at 60 "C for 30 min at the indicated pHs in buffer solutions, followed by three 5-min cold-water rinses in deionized water and line drying. There was a high percentage of methoxyl retention by the partially and fully methylated DMDHEU derivatives. Complete methylation did not offer much advantage over partial methylation. From the reduction in the methoxyl content retained in the two finishes from the agents methylated only in the 4,5-positions, it appears that the methoxyls on the 1,3-position are more stable than those on the 4,5-positions. Further, from the differences between the methoxyl fraction retained between the two 4,5-dimethoxyethyleneurea agents, reaction with cellulose through a 1,3-hydroxymethyl group appears to increase 4,5-methoxy durability to hydrolysis. If the fraction of nitrogen retained through hydrolysis is examined, a similar but expanded pattern can be seen. These data are in Figure 5 . When judged by nitrogen retention, only the finishes with etherification on the 4,5-position or with reactivity solely through the 4,5-pos-
ition show susceptibility to hydrolysis. If the formaldehyde release characteristics are examined, a somewhat different picture emerges. Formaldehyde release values after hydrolysis are given in Table VI. With DMDHEU and partially methylated finishes, formaldehyde release increases with neutral-to-alkaline hydrolysis. Formaldehyde probably is removed through demethylolation with acid hydrolysis. This increase in formaldehyde release was noted during laundering under alkaline conditions (Reinhardt and Andrews, 1988). Both tetramethylated DMDHEU and dimethylol-4,5-dimethoxyethyleneurea showed no increase from the low original values on hydrolysis. However, the nitrogen data (Figure 5) suggest that some cross-links are completely removed by acid hydrolysis. In Table VI1 are the steady-state and transport formaldehyde levels in the same fabrics. These free and surface formaldehyde characteristics are very low and little changed by hydrolysis. The levels before hydrolysis were 30 bg/g or lower, probably below the level of statistical significance (Andrews, 1987; Andrews et al., 1988).
Summary A systematic study of the influence of available functionality and of etherification of N-methylol compounds on the performance of fabrics finished with these compounds was conducted. It was shown that the forma1dehyde:amide ratio need not be less than 2:l for preparation of low formaldehyde release etherified agents for use in precure finishing operations; a formaldehyde:amide ratio of 2:l is preferable for acceptable appearance properties, particularly with difunctional agents. In postcure finishing operations, forma1dehyde:amide ratios should be less than 1.8:l for DHEU-based agents. In studies of hydrolysis under both acidic and alkaline conditions, it was established that the methoxyls of amidomethylol moieties are more resistant to hydrolysis than ring methoxyls in DHEU derivatives. In addition, cellulose reactivity primarily through pendant N-methylol groups produces a more hydrolysis-resistant product than reactivity primarily through ring hydroxyls. Finally, partial methylation of DMDHEU imparts sufficient hydrolysis resistance to durable press finishes without the loss of reactivity toward cellulose found in the tetramethylated product. Acknowledgment The authors thank John E. Helffrich, 111, and Dione Joseph for their laboratory assistance. The authors also
I n d . Eng. Chem. Res. 1989,28, 535-541
thank Anastasia Hammond for performing the nitrogen analyses and Raymond Richard for determining the wrinkle recovery angles. Registry No. Dimethylol-4,5-DMOEU, 4211-44-3; 4,5DMOEU, 3891-44-9; DMDHEU, 1854-26-8; 1,3-dimethyl-4,5DHEU, 3923-79-3; 4,5-DHEU, 3720-97-6; DMDHEU (tetramethyl ether), 4356-60-9; HCHO, 50-00-0.
Literature Cited American Association of Textile Chemists and Colorists AATCC Technical Manual; AATCC: New York, 1987; Vol. 62. Andrews, B. A. Kottes Use of Reversed-Phase High-Performance Liquid Chromatography in Characterizations of Reactants in Durable-Press Finishing of Cotton Fabrics. J. Chromatogr. 1984, 228, 101-110. Andrews, B. A. Kottes An Update on Testing for Formaldehyde Release. Textile Chem. Color. 1987, 29, 19-26. Andrews, B. A. Kottes; Reinhardt, R. M. Process to Produce Durable Press Low Formaldehyde Release Cellulosic Textiles. US Patent 4,488,878, Dec 18, 1984. Andrews, B. A. Kottes; Reinhardt, R. M. Agents to Produce Durable Press Low Formaldehyde Release Cellulosic Textiles: Etherified NjV-Bis(Hydroxymethy1)Carbamates. US Patent 4,539,008, Sept 3, 1985. Andrews, B. A. Kottes; Reinhardt, Robert M. Assessment of Steady-State Free Formaldehyde in Durable Press Cotton Fabrics, T e x t . Res. J . 1986, 56, 115-120. Andrews, B. A. Kottes; Reinhardt, Robert M. Reaction Equilibria and Textile Performance from Low Formaldehyde Release Carbamate Finishing. T e x t . Res. J . 1989, 59, 53-60.
535
Andrews, B. A. Kottes; Reinhardt, Robert M.; Trask-Morrell, B. J. Influence of Prolonged Storage on Formaldehyde Liberation from Durable Press Textiles. Text. Res. J . 1988, 58, 255-263. Andrews, B. A. Kottes; Harper, Robert J., Jr.; Vail, S. L. Variables That Influence Formaldehyde Release from Cottons Finished for Durable Press. Text. Res. J. 1980a, 50, 315-321. Andrews, B. A. Kottes; Harper, Robert J., Jr.; Smith, Richard D.; Reed, John W. Lowering Formaldehyde Release with Polyols. Text. Chem. Color. 1980b, 12(11), 25-29. Beck, Keith, et al. GC/MS Analysis of Durable Press Agents. Text. Chem. Color. 1988, 20(3), 35-39. Harper, R. J., Jr.; Andrews, B. A. K.; Harris, J. A.; Reinhardt, R. M.; Vail, S. L.; Bathija, A. P.; Ulsamer, A. G. Formaldehyde Release from Labeled and Unlabeled Cross-linked Cotton and CottonPolyester Fabrics. Agricultural Research Service, Agricultural Research Results, ARR-S-l7/May 1984. Lange, N. A. Handbook of Chemistry, 10th ed.; McGraw-Hill: New York, 1961; p 952. North, Bernard F. New Ultra-Low H,CO Resins. Am. Dyestuff Reptr. 1983a, 72(5), 32-35. North, Bernard F. Treating Cellulose Textile Fabrics with Dimethylol Dihydroxyethyleneurea-Polyol. US Patent 4,396,391, . . Aug 2, 1983b: Pasad. Dilirx Beck. Keith Quantitative Analvsis of Commercial Durable-Press Finishes by HPLC. T e x t . Chem. Color. 1986, 18(5), 27-32. Reinhardt, Robert M. Vapor Transport of Formaldehyde to Cotton. Text. Res. J . 1983,53, 175-180. Reinhardt, Robert M.; Andrews, B. A. Kottes Ind. Eng. Chem. Res. 1988, 27(3), 35-39.
Received f o r review August 24, 1988 Accepted January 13, 1989
High-Temperature H2S Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Dioxide Sorbents Susan Lew, Kandaswami Jothimurugesan,+ and Maria Flytzani-Stephanopoulos* D e p a r t m e n t of Chemical Engineering, Massachusetts I n s t i t u t e o f Technology, Cambridge, Massachusetts 02139
Bulk Zn-Ti-0 binary oxides and ZnO were synthesized in macroporous and dispersed form by the amorphous citrate technique. T h e addition of T i 0 2 into ZnO stabilized ZnO against reduction t o volatile elemental zinc at 650 " C in a simulated coal gas mixture of 1%H$-13% H2-19% H20-67% N2. T h e intrinsic sulfidation kinetics of zinc titanates were similar to zinc oxide at 650 "C. Cyclic sulfidation-regeneration experiments were performed in a packed-bed microreactor. The H2S removal efficiency of Zn-Ti-0 materials was as high as that of ZnO removing H2Sto less than 1-5 ppm until breakthrough. Although zinc titanates typically had low sulfidation conversion (50-60%), this problem could be eliminated by preparing the sorbents from a chloride precursor. High-temperature desulfurization of coal-derived fuel gas offers improvements on the thermal efficiency of emerging technologies using coal gasification such as integrated gasification combined-cycle power generation employing gas turbines, and gasifier-molten carbonate fuel cell power plants. For these technologies, highly efficient sulfur removal from several thousand parts per million (ppm) down to 1ppm for fuel cell power plants or 100 ppm for gas turbines is needed. Commercial desulfurization processes are based on liquid scrubbing at or below ambient temperatures, resulting in considerable thermal efficiency loss as well as costly wastewater treatment. Early work on hot gas desulfurization employed single oxide sorbents, especially iron oxide and zinc oxide
-
-
*To whom all correspondence should be addressed. Present address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803.
0888-5885/89/2628-0535$01.50/0
(MERC, 1978; Grindley and Steinfeld, 1981). From the standpoint of high H2S removal efficiency, zinc oxide is more attractive than iron oxide because of more favorable sulfidation thermodynamics. However, the sulfidation kinetics of ZnO are much slower than those of iron oxide. The regenerability of ZnO is restricted by loss of surface area (sintering) a t high regeneration temperatures and formation of zinc sulfate at low regeneration temperatures. Also, during sulfidation, reduction of ZnO followed by vaporization of elemental zinc can take place at temperatures as low as 600 "C. Recently, the two oxides of iron and zinc were combined into the mixed oxide (spinel) compound zinc ferrite, ZnFe204,which has been found to possess H,S removal efficiency comparable to ZnO but somewhat better capacity (Grindley and Steinfeld, 1981). However, the drawbacks of zinc ferrite are also similar to those outlined above for ZnO. 0 1989 American Chemical Society
