Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 325
Improved Hydrodechlorination Catalysis: Chloroform over Platinum- AIumina with SpeciaI Treatment s Charles J. Noelke and Howard F. Rase" Deparfment of Chemical Engineering, The University of Texas, Austin, Texas 78772
Improved activity and activity maintenance have been demonstrated in the hydrodechlorination of chloroform to methylene chloride over commercial Pt/A1203 reforming catalysts. Continuous addition of water more than doubled the reaction rate by removing chloride from the alumina, thereby preventing excessive acidity and associated rapid coking. Two Pt/Al2O3catalysts were tested of different metal content. Pt-Re/A1203 was also studied and, atthough not as active, deserves further consideration at other Pt-to-Re ratios.
The hydrodechlorination of chloroalkanes to products of lower chlorine content is a potentially important reaction, but it has been plagued with poor selectivity, rapidly declining catalyst activity, and impractically short reactor operating cycles. Recently, Dodson and Rase (1978)have found the current generation of Pt/A1203and Pt-Re/A1203 reforming catalysts to have promising characteristics that suggest the possibility of an economically viable process. In a temperature range of 166-208 "C chloroform was converted with good selectivity to methylene chloride, a valuable product because of its varied uses as a propellant, drying agent, food and spice extractant, foam-blowing agent, and cleaner. Rapid growth in this particular chlorohydrocarbon is expected since it has been shown to be neither mutagenic nor carcinogenic (Chem. Eng. News, 1978) and was also shown to be nontoxic in test animals up to 3500 ppm (Chem. Eng. News, 1977). In addition to good selectivity, the reforming catalysts exhibited lower but more stable activity with increasing hydrogen partial pressure below 200 "C. Above this temperature, coking occurred at a rapid rate and the activity declined precipitously. The deactivated catalyst could be regenerated and a major portion of the activity restored. Higher reaction rates and resistance to coke formation are essential for further development of reforming catalysts in this service. It is clear that increased hydrogen pressure attained by higher operating pressure should reduce coke formation and increase conversion, but this route is energy intensive. The purpose of the present study was to discover and develop means for further improving the activity and coke inhibiting character of Pt/A1203 and Pt--Re/ A1203reforming catalysts in hydrodechlorination service by means that could prove attractive on a commercial scale. Nature of Dechlorination Reaction on Pt/A1203 Weiss and Kreiger (1966)and Weiss et al. (1971)have studied hydrodechlorination of CCla over Pt/A1203with the goal of understanding the probable reaction sequences. A free-radical mechanism was proposed by Weiss et al. (1971)in which both H2 and CCla dissociate on Pt sites, and two parallel reactions follow. The CC13. radical in the gas phase can either react with H2 gas or adsorbed or gaseous Ha radicals to produce chloroform. Alternatively, CC13-radical can readsorb on a Pt site and not desorb until it is converted to CH3. radical and 3 moles of HC1. The methyl radicals can either react with hydrogen molecules or atoms to produce methane. In the temperature range studied (12-123 " C ) the products were indeed confined to CHI, CHC13, and HC1. Chloroform was found to be unreactive even when added as a reactant. Dodson and Rase (1978) observed reaction of chloroform to methylene 0019-7890/79/1218-0325$01.00/0
Scheme I
CHCl,CH,Cl-
CCl,=CH,
chloride beginning at 177 "C. At even higher temperature, selectivity to methylene chloride was excellent. There is no conflict in these findings since Weiss et al. (1971)did not study the higher temperature region. Basset et al. (1970)have studied chlorided aluminas by treating alumina at various temperatures with carbon tetrachloride. They concluded that a t temperatures above 300 "C, CCll exchanges four chlorine atoms against two surface oxygen atoms. At lower temperatures, chlorination of the alumina proceeds through the exchange of OH groups by chlorine atoms. Dehydrochlorination of various chloroalkanes over A1203 has been studied in some detail by Mochida et al. (1968, 1971,1976, 1978). The products for chloroalkanes with two or more carbon atoms are the corresponding monoalkene and water. In the case of a single-carbon chloroalkane such as methylene chloride, CO and methyl chloride were the observed products. These studies were made by a pulse technique and after a few pulses the alumina activity disappeared, obviously because the chloride that remained associated with the aluminum atoms produced a strongly acidic surface which was no longer effective in dehydrochlorination. In a recent study Mochida et al. (19781,added water pulses prior to chloroalkane pulses and between pulses of chloroalkane. The result was higher activity of the alumina. They proposed the reaction Scheme I for 1,1,2-trichloroethane in which the water restores the original alumina surface by removing the chloride. Mochida et al. (1978)postulate that water may be adsorbed on the aluminum ions, thereby decreasing the Lewis acidity. Exchange of chloride for hydroxyl groups by the water produces weak Bronsted sites in place of the strongly acidic chloride sites. Oxide ions exposed on the surface may behave as basic sites which are thought to be important in chemisorption of chloralkanes and chloride removal. The proper amount of water provides the optimum strength and distribution of basic and acidic sites, but excess adsorbed water could block basic sites and prevent favorable reaction paths. Relevant Characteristics of Pt/A120, in Reforming Service Since commercial catalytic reforming catalysts were used in the research to be described, some brief comments based
0 1979 American Chemical Society
326 Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
on the vast literature on reforming are in order. The dual functional character, as represented by the hydrodehydrogenation activity of the metallic component and the isomerization and cracking activity of the acidic alumina, is essential to the reforming reaction scheme. Coke forms on both Pt and acidic alumina sites (Myers et al., 1961) and, in large enough quantities, seriously inhibits the reforming reaction. Hayes et al. (1974) have found that sulfided Pt/alumina cokes at about one-half the rate of unsulfided catalyst by apparently reducing the tendency of Pt to catalyze the formation of coke precursors. An unsulfided catalyst quickly reaches an activity level somewhat below the stable level of the sulfided sample (Sivasanker and Ramaswamy, 1975). Maximum activity and selectivity in reforming catalysts is enhanced by careful adjustment of the acid strength of the alumina using chloride treatment with HC1 or a chlorinated hydrocarbon from which HC1 is formed on the catalyst by hydrodechlorination (Ciapetta and Wallace, 1971; Sinfelt, 1964; Sinfelt and Rohrer, 1961). Excessive chloride treatment, however, produces strong acid sites and causes increased coke production. The chloriding reaction which produces H 2 0 as a gaseous product is reversible (Peri, 1966). Experimental Goals Because of the reported favorable effects of water vapor in dehydrochlorination over alumina and in the reforming reaction along with the benefits of presulfiding, tests were designed to elucidate any advantages to similar treatments for the dehydrochlorination system. Although this system is quite different in operating conditions and products, possible similarities in portions of the activation-deactivation process are intriguing. Experimental Details Reactor System. The microreactor system as developed by Harrison et al. (1965) and altered by Dodson (1977) for hydrodechlorination studies was used. A 6 in. i d . X 12 in. long tank was used as a water saturation system for studies with continuous water addition. Hydrogen feed gas was sparged through water. Concentration of water in the gas was varied by changing the tank pressure. Analysis of Product. Products were analyzed by vapor phase chromatography using a 6-ft stainless-steel column (0.085 in. i.d.) packed with 50/80 mesh Porapak Q. The column was operated at 144 "C at a helium flowrate of 35 cm3/min and an operating pressure of 9.0 psig. Thermal conductivity detection was used, and total elution time was 10 min. Reactor Feed Materials. Chloroform was a technical grade supplied by Dow Chemical Co. having minor amounts of other chlorinated alkanes. Both hydrogen and nitrogen were prepurified grades supplied by Scientific Gas Products (99.99-99.999%). Hydrogen sulfide was supplied as a premixed H2-H2S mixture by Matheson; it contained 475 ppm of H2S. Catalysts Tested. Three reforming catalysts all manufactured and supplied by American Cyanamid were tested in addition to the blank alumina extrudate. All samples were extrudates with 0.18 cm diameter by 0.54 cm average length. Total surface area was 200 cm2/g. The pore volumes and bulk densities were all in the range 0.60-0.63 cm3/g and 0.64-0.68 g/cm3, respectively. The catalyst designations are given according to metallic content as Aeroform PHF, 0.1% Pt; Aeroform PHF, 0.3% Pt; and Aeroform PR-6, 0.3% Pt/0.3% Re. Procedure. Catalyst samples in the range of 0.5 g were used in all tests. Each run was preceded by a 90-min preheat period during which N2 was passed over the cata-
t
h 2 0 HOLE FRbCT
R12 R31 A R29 +RU I
1
120
I
240 RIICIION TIMI
'
0.000 0.0110 0.0194 0.0254 I
I
360
MINUTIS
Figure 1. Effect of water addition on 0.3% Pt/A1203at 165 "C.
lyst to purge the sample and the reactor system. At the completion of the preheat and purging period H2 was passed over the catalyst a t the same temperature and flowrate as that to be used in the test. Hydrogen pretreatment was used to activate the catalyst. After completion of the hydrogen pretreatment, the CH3C1feed was introduced at the rate of 8.04 liquid cm3/h and sampling of the reactor effluent was begun 15 min later. When H2-H20 was to be used, H2 was rerouted through the saturator and then to the reactor 2 min prior to the start of the run. Pretreatment by H2S, when used, was begun after purging the sample with nitrogen at 100 standard cm3/min while the system was being heated to operating temperature. Then H2was passed over the catalyst at 40 standard cm3/min for 30 min after which the H2-H2S mixture was introduced for the desired time period followed by an N2 purge at a low rate while the reactor was allowed to cool overnight. The sample was transferred to a different reactor for the reaction study to avoid biased results which might be caused by desorption of H2Sfrom the reactor wall during the test. All reaction tests were made at relatively low conversions to assure isothermality and at atmospheric pressure to accentuate deactivation and avoid excessively long tests. The reactor system was periodically tested for activity by operating without catalyst. No conversion was detected in the temperature range of 165-220 "C, 1 atm pressure, and at the same V / F as for the catalytic test, 11.6 cm3 h/g-mol.
Results Effect of Water. The addition of water to the feed significantly increased the reaction rate and decreased fouling of the catalyst. Figures 1, 2, and 3 present results for 0.3% Pt/A1203a t three temperatures and three different water partial pressures compared to runs without added water. As found by Dodson and Rase (1978), an optimum temperature range ( N 190-195 "C) was observed for methylene chloride production and a steady activity (Figure 2). This optimum persisted in the water-addition runs as well. The activity is also a strong function of water concentration as can be seen at 195 and 220 "C (Figures 2 and 3). Runs a t 0.0196 mole fraction water exhibited consistently higher activity. At this concentration and 195 "C the activity was more than doubled over that observed without water addition. Selectivity to methylene chloride was 0.99. The runs at 220 "C (Figure 3) were purposely made at a temperature known to cause excessively rapid coking.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18,No. 4, 1979
71
327
I
Temp: 195'C
mt
I
I
0 1
A R24
0,0196
*R26
0.0251
I20
-
I-t 240
01
360
I
1
I
1
120
0
I
I
I
360
240
RUCIIOY IlWi . WlYUItS
RtlCTlON TIM . WlllUIIS
Figure 2. Effect of water addition on 0.3% Pt/A1203at 195 "C.
Figure 4. Effect of water addition on 0.1% Pt/A1203at 195 "C.
Temp: zmQc ~/CHCIJ. 3.0 V 2 J MOLE FRb:T
0 R15
R22 A
Rll RI9
0.OW 0.0130 0.0194 0.0252
I
0 240
I20 R I l C l l O Y TlWt
'
Again a mole fraction of water of 0.0194 appeared to give best performance. The deactivation was still excessive, but it appeared to be leveling off a t the maximum run length for the feed system. The curve for 0.0252 mole fraction of water could ultimately produce the more stable activity. Improved activity at higher pressure coupled with the improvements indicated by Figure 3 could make higher temperature operation feasible. That the water had affected coking characteristics was unmistakable when the used catalysts were visually compared. Catalysts used in runs a t less than 200 "C with water were much lighter in color, indicating less coke. There were small dark regions suggesting that portions of the catalyst did promote coke formation. The catalyst used without water was darker in color. All catalysts except those tested a t 220 OC exhibited lighter internal cores at the end of the run when broken, indicating significant internal diffusional resistances as shown by Dodson and Rase (1978). A 0.1% Pt/Al2O3 and a 0.3% Pt-0.3%Re/Alz03 catalyst were also tested. Again an optimum water composition prevailed as shown in Figures 4 and 5. Higher water concentrations seem more optimal for lower metals content perhaps because of the increasing dominance of the Alz03. The larger amount of surface covered by Pt and Re in the Pt-Re/A120B catalysts makes them less sensitive to the water concentration, as indicated by the results in Figure 5. Comparison of the three catalysts with or without water at 195 O C is interesting. Pt-Re/A1,03 with the highest total metal content (0.6% total) had the highest initial
1
1
I
240
I
160
RIlCIlON TIME. MlUUIiS
MINUTES
Figure 3. Effect of water addition on 0.3% Pt/A1203at 220 "C.
I
I20
360
Figure 5. Effect of water addition on 0.3% Pt4.370 Re/A1203at 195 "C. 30
Temp i
0 WCL'
8 -
a
OR12
mRI3
ZZO'C IRAI(
H2 CHCl, 4 I 45
0000
n2s yes no
328 Ind. Eng. Chern. Prod. Res. Dev., Vol. 18, No. 4, 1979 Scheme
I1 H,
*RZO
I
0
I
I20
0.000
yes
0.000
no
4RIB
0.0194
y8s
*Rl7
0.0194
"Q
I
210
I
I
360
REACTION TIME. MlNUTtS
Figure 7. Comparison of H2S pretreatment, water treatment, a n d combination treatment o n 0.3% Pt/A1203a t 220 "C.
Results with H2Spresulfiding and reaction in the presence of water vapor are compared in Figure 7. Unfortunately, because of limits of the feed system, runs longer than 6 h were not possible and complete trends were not established. It appears in the case of presulfiding alone that the presulfided activity may continue to line out at a higher activity than the unsulfided case. When water treatment is also used, convincing trends did not develop in the first 6 h. Longer term life studies are definitely required. An interesting observation which also deserves further investigation was made by examining a number of particles that were broken perpendicular to the diameter for purposes of viewing with a stereomicroscope. Catalyst which had only been presulfided appeared uniformly black throughout while presulfided catalyst used in the presence of water revealed a white or gray central core. Since the observed rates were comparable in each case, intraparticle transport gradients may have become large due to a higher intrinsic rate produced by water vapor treatment. Conclusions Continuous water addition during reaction improved activity and activity maintenance. The highest sustained activity with the catalysts tested was observed at 195 "C on 0.3% Pt/A1203when water was used continuously with feed. Further favorable development of this particular catalyst seems a most promising prospect. The Pt-Re/ A1203catalyst should not be abandoned. Its high initial activity might be prolonged using catalysts of different Pt/Re ratios and possibly lower total metals content. Lower initial activity of the presulfided catalyst was not unexpected. It is well known that Pt loses activity when
A1,0,
+
2[Pt] 2H.[Pt] CHCI, _ C H C l z . [ a l u m i n a ]
H O
2 ,A l ( O H ) ,
mf,
--$ A1(OH),Clmf
HC1H,O CHC1, . [ a l u m i n a ] t H,[Pt] CH,Cl, H[Pt] t A I ( O H ) , mfCl e H C l ( g )
+ H,O
+ [Pt] + [ a l u m i n a ] + A l ( O H ) , suf
sulfided. The H2Smay also react with the Al and produce sites no longer useful in the chloride extraction step. Convincing evidence of H2S pretreatment will require long-term life studies. By borrowing concepts proposed by Weiss et al. (1971) for Pt at conditions where the A1203is inactive and by Mochida et al. (1978) for A120, alone in dehydrochlorination, reaction Scheme I1 is proposed, where the suffix "suf' designates a surface compound. The word [alumina] in this scheme refers to basic sites on the alumina carrier. This scheme is helpful in explaining the strong influence of water addition. Without water the alumina would be inert or rapidly become so and the reaction would depend on the Pt alone to remove the chloride. In fact, even in runs made without water addition, one must assume that water is present in the feed liquid since the technical grade chloroform was not dried, nor would it be in an operating plant. Under such conditions the reaction would proceed as suggested above but without the optimum amount of water.
Literature Cited Basset, J., Figueras, F., Mathieu, M. V., Prettro, M., J . Catal., 16, 53 (1970). Chem. Eng. News, 55, 6 (May 9, 1977). Chem. Eng. News, 56, 7 (July 24, 1978). Ciapetta, F. G., Wallace, 0. N., Catal. Rev., 5 , 67 (1971). Dodson, D. A., Rase, H. F., I d . Eng. Chem. Rod. Res. Dev., 17, 236 (1978). Dodson, D. A,, M. S.Thesis, The University of Texas, Austin, Texas, 1977. Harrison, D. P., Hall, J. W., Rase, H. F., Ind. Eng. Chem., 57, 18 (1965). Hayes, J. L.. Mitsche, R. T., Politzer, E. L., Homeier, E. H., Prep. Am. Chem. SOC., Div. Pet. Chem., 19, 334 (1974). Meyers, C. G., Lang, W. H., Weisz, P. B., Ind. Eng. Cbem., 53, 299 (1961). Mochida, I., Anju, Y., Yarnzrnoto, A. K., Seiyama, T., Bull. Chem. SOC.Jpn., 44, 3305 (1971). Mochida, I., Uchino, A.. Fujitsu, H., Takeshita, K., J. Catal., 43, 264 (1976). Mochida, I., Uchino, A,, FujRsu, H., Takeshita, K., J . Catal., 51, 72 (1978). Mochida, I., Take, J., Saito, Y., Yoneda, Y., Bull. Chem. Soc. Jpn., 41, 65 (1968). Peri J. B., J . Phys. Chem., 70, 1482 (1968). Sinfelt, J. H., Adv. Chem. Eng., 5 , 37 (1964). Sinfelt, J. H., Rohrer. J. C., J . Phys. Chem., 65, 978 (1961). Sivasanker, S.,Ramaswamy, A. V., J. Catal., 37, 553 (1975). Weiss, A. H., Gambhir, B. S..Leon, R . B., J . Catal., 22, 245 (1971). Weiss, A. H.. Kreiger, K. A,, J. Catal., 6, 167 (1966).
Received for review May 18, 1979 Accepted August 6, 1979
