A comprehensive review on the fluorinated catalysts is presented with respect to their preparation, catalytic activity, surface structure, and nature of active centers. The survey includes the original publications and the patents appearing during the past 20 years.
introducing HF, e.g., by mixing y-Al203 with NH4F powder or by impregnating it with an aqueous solution of NH4F, have no effect on the final product since the distribution of H F in the sample proceeded by H F adsorption from the gaseous phase. The phase homogeneity of the samples was evaluated by red and violet luminescence in UV light, corresponding to the a and y forms, respectively. Vesely and Indrova (232) studied the HF-treated alumina obtained by precipitation of aluminates by nitric acid. Kirina and Antipina (133) reported the preparation of sodium free fluorinated a-alumina. Gulyaeva and Khyanina (90) have described a procedure for the determination of fluorine contained in fluorinated Pt-A120:1 catalysts.
Catalytic Activity of Fluorinated Catalysts Fluorinated alumina, silica-alumina, and supported noble metal alumina are effective catalysts for carbonium ion type reactions, such as alkylation-dealkylation, cracking, isomerization, polymerization, and reforming. Alkylation and Dealkylation. Kolesnikov and co-workers (136,137) studied the alkylation of benzene with propylene on BF3-treated silica-alumina catalyst and found that the catalyst activity increases with the saturation of silica-alumina catalyst by BF:+The yield of alkylbenzene was proportional to the amount of BF3 in the catalyst. The activity of the catalyst decreased with time after continuous use without regeneration. The alkylation efficiency and catalyst life were found to increase with temperature (136,143). Kolensnikov and Mirgoleev (138) and Mirgoleev and Kolesnikov (170) have studied the effect of heat pretreatment of BF:j-aluminosilicate catalyst on its activity in the alkylation of benzene by propylene and on the residual quantity of BF:j in the catalyst. The activity was approximately constant below 500 "C but dropped markedly at 700 "C as BF3 desorbed. Saturation of the silica-alumina with gaseous BF3 gave a catalyst with the same activity when calcined a t 600,700, or 800 "C. Kozorezov and Novozhilova (144) have prepared 1,2,4,5tetraalkylbenzenes by the alkylation of p -xylene and pseudocumene by propylene in the presence of alumina promoted with BFs (3.5-4 wt % BF3) at 30-100 "C. The best (or optimum) conditions for alkylation of benzene with ethylene on BF:1-treated alumina were found (145,146). Alkylation of benzene with butylenes (216) and amylenes (152) on a BF3 containing catalyst are also reported. In vapor phase alkylation of benzene by propylene, the activity of catalyst was maintained by adding BF3 (0.05-0.2% BF3) to benzene (141).
Recently, Matsuura et al. (166) studied the vapor phase alkylation of toluene with ethylene over BFs-treated alumina and obtained high selectivity in regard to monoalkylation of toluene with ethylene. This activity showed a strong correlation with the overall acid amount determined by butylamine titration of the catalyst. Berge and co-workers (24) and Strand and Kraus (207) reported the dealkylation of aromatic hydrocarbons over fluorinated alumina catalyst. Recently, Giordano et al. (85) have studied hydrodealkylation of cumene on fluorinated alumina containing 1 to 63 wt % F. Highest activities were exhibited by the catalyst having intermediate fluorine content. Comparison of the catalytic activities with the results of bu-
tylamine titration, using both Ho and H R indicators, showed that the cracking conversions were in far better agreement with data obtained with H R indicators, whose acidic form is a carbonium ion. Kraus and co-workers (147,148,199) studied dealkylation or alkyl and dialkylphenols over fluorinated alumina catalyst and the results were discussed in terms of electronic and steric effects of the structure of the reacting substances. They have established that the steric conditions are the most important factors determining the reactivity within a series of homologues and isomers over a particular type of catalyst (148). Several patents have been recorded for the alkylation and dealkylation processes based on fluorinated catalysts (Table
I). Cracking. Paushkin and Lipatov (184) studied cracking of diesel fuel on activated carbon and aluminosilicates treated with BF3. In cracking on activated carbon, addition of BF3 lowered the yield of gasoline by almost a factor of 2, lowered its bromine number considerably, and also lowered the yield of gas and the content of unsaturated compounds. In contrast to this, treatment of BF3 on aluminosilicate catalyst yielded a higher octane gasoline than aluminosilicate alone. Masakatsu et al. (159) have observed that catalytic activity for cumene cracking was proportional to the acidity of fluorinated silica-alumina catalyst and both showed maxima a t 0.5-1.0% F. Miyake et al. ( I 72) have found by a microstatic method that alumina containing fluorine had higher activity for hexane cracking than a common silica-alumina or an untreated alumina catalyst. The maximum activity was given by alumina containing HBF4 5-7% (4.3-6.1 wt % F), H F 5-7% (4.8-6.7 wt % F), and NH4HF22-4% (1.3-2.7 wt % F). Chernova and Antipina (51,52)have also observed a sharp increase in cumene cracking activity of alumina treated with BFa. Further, Antipina and Shchukina (11)have studied the effect of surface properties and structural differences of the modification of A1203on the activity of BF3 promoted aluminas. The most active cracking catalysts were obtained by fluorination of alumina produced by decomposition of bayerite at 900 "C and of hydrothermal boehmite at 700 "C. Yushchenko and Antipina (243) have found that the cumene cracking activity and acidity of two series of catalysts, Houdry alumina and zeolite-Hy, differing in chemical composition and structure, increase with increasing concentration of fluorine in these catalysts. Antipina and co-workers (13) observed a linear relationship between the specific activity for cumene cracking and the number of proton sites on the fluorinated alumina. A pulse microcatalytic study of cumene cracking over F-alumina was also reported (242). Many patents have been claimed for the cracking of high boiling hydrocarbon oils over fluorine promoted catalysts. The catalysts are listed in Table 11. Isomerization. Berge (25) reported isomerization of olefins containing 5-8 carbon atoms on HF-treated alumina a t 300-400 "C. Kraus et al. (149) studied the relation between the catalytic activity and properties of Al(BF4)S deposited on activated alumina for the isomerization of cresols. The catalyst containing 2096 Al(BF4)3had maximum isomerization activity, while the acid strength was maximum at 5% Al(BF4):I.The high activity of the catalyst was explained by the formation of a new compound on the surface of alumina after it was soaked in the borate solution. Tikhomirova and co-workers (214) found that fluorinated aluminas are highly active and Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
13
Table I. Alkylation-Dealkylation Processes Based on Fluorinated Catalysts Reaction Alkylation of aromatic hydrocarbons with olefins
Fluorine promoter
Catalyst base A1203 A1203
A1203
Si02-Al203 SiOz-MgO ZrOz Zr02-Si02-Al203 Ag-Si02-Al203 A1203 A1203
Alkylation of benzene by propane Alkylation of paraffins
Pt-Al203 A1203 A1203
SiOZ-Al203 Crz03-Si02-Al203 Alkylation of phenols with olefins
Dealkylation of aromatic hydrocarbons
A1203
Si02-Al203 KUZ [H form of cation exchange resin] Pt-Al20:I
Temp, "C
F BF:I
250-415
BF3 and S BF3 or F BF3 BF3 BF3 F AlF3 SiF4 F and C1 BF3 and HCl BF3 and fluoborate F BF3 F F BF3
-
0-300
150-300 20-250 -
300
125 -
0-100 480 -
_-
150-300 -
> 265
F
Reference 187,221,222 71,112, I 13,153, 204,220 114,115 68,116,127 117 118 119 201 64 140 I76 212 134 197 84 70 65 233
31,32
Table 11. Fluorinated Catalysts for Hydrocarbon Cracking Processes Catalyst base
Fluorine promoter SiF4 BeF2 or MgFz F F F SiF4 MgFz MgFz F CaFz,MgFz,SrFz or InF? MgF2 and A1F:j CrF2 F F or BF3
stable catalysts for the isomerization of 2-methyl-1-pentene to 2-methyl-2-pentene. Vereshchagin (231) carried out the isomerization of n pentane to isopentane over platinum on fluorinated alumina. A constant conversion (52-53%) was observed for one year on the above catalyst. Orkin (181) studied the isomerization of CI1to CI4normal paraffins on fluorided Pt-alumina catalyst under hydrogen pressure and found that a t 300 psig, a 5% fluoride catalyst was more selective than either the 10%or the fluorine free catalyst. Recently, Choudhary and Doraiswamy ( 5 5 ) have applied the group screening method for the selection of the best catalysts among 46 solid acid catalysts for the isomerization of n-butene to isobutene. Fluorinated 7-alumina containing 1.0 wt % F was found to be the best catalyst with highest activity (33.5%conversion of n-butene to isobutene) and good selectivity (87.1% for isobutene). After regeneration (by burning coke in air at 480 "C) the catalyst almost attained its initial activity (33.2%conversion of n-butene to isobutene). Pis'man 14
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
Other promoters -
Pt Fe group metals
Reference 81 124 91 15,16
155,206,215 82 230 17 234 66
230 230 239 223 213
and co-workers (189,190)have also showed that fluorinated alumina is most suitable for the above reaction and that its catalytic activity is retained after several regenerations. The influence of the acidity of fluorinated alumina on its isomerization ability was studied (190)by poisoning the catalyst with LiOH, NaOH, and pyridine and it has been shown that the selectivity of the catalyst increases with increase of the poison but the isomerization ability decreases especially in relation to the formation of isobutene. The processes for isomerization of n-butene to isobutene over fluorinated alumina catalysts are recently reviewed by Choudhary ( 5 4 ) . Several patents have been obtained for the use of fluorinated catalysts in the isomerization processes (Table 111). Polymerization. Karzhev and co-workers (131) studied the polymerization of propylene to motor fuel over HF-treated alumina a t 70 atm pressure and a t temperatures of 215 to 365 "C. Topchiev and Alyavdina ( 2 1 7 )found that BF3 adsorbed on carbon, silica gel, and alumina gives effective catalysts for
Table 111. Fluorine Promoted Catalysts for Isomerization Processes Reaction
Fluorine promoter
Catalyst base
Isomerization of paraffins
Isomerization of cycloparaffins Isomerization of n-pentane to isopentane Isomerization of neohexane to diisopropyl Isomerization of olefins
Isomerization of 1-butene to 2-butene Isomerization of butene to isobutene
Isomerization of pentene to isopentene Isomerization of 3-methyl-2pentene to isoprene Isomerization of Cs-hydrocarbons Isomerization of xylenes Isomerization of pseudo cumene to mesitylene Isomerization of aromatic and cyclic hydrocarbons Isomerization of cresols
Other promoters
Temp, "C
Reference
F
Pt
150-475
A1F3 AlF3 NH4BF4 F or C1 HBF4 F F F F F
Pt Pt Pt Pt and Sn Pt Pd or Pt Pd Pt Pt Pt
359 359 350 180-400 350 70
F
Pt
400
28
F AlF3 BF3 BF3 BF3 F AlF3 or A1 [BFl4 BF3 BF3 F
-
-
Pt
140
-
-
260-510
IO9,I 26 205 33,36,37,39,
SiFs NH4BF4 AlF3
-
450 450 315-650
34,38 35 174
F
cue
-
193
AlF3
-
-
88
F CeF3 F ThF4 BF3
-
Pt
482 370 470 400 250
40 202 60,123 203 142
Pt
-
106
-
-
135
HF or NH4F F
-
450 360 270-320
1,21,209,210224,225 75 75 167 192 78 211 77 80,226 80,226 200
-
150
173 62 111
-
-
110
ZrOz
-
0-425
Crz03 or W03
550-600
150
108 107 79
180,188
Table IV. Fluorinated Catalysts for Polymerization Processes Reaction Polymerization of olefins
Polymerization of conjugated dienes Polymerization of vinyl alkyl ethers Dimmerization of a-olefins Copolymerization of benzene with ethylene or propylene
polymerization of isobutene. In the case of silica-alumina catalyst, the activity for the oligomerization of propylene on the catalyst surface was found to be decreased by fluorination (198).
Catalyst base
Fluorine promoter
Other promoters
Reference 83 227 237 139 186 194 154 158 73 76 45 87 67
The fluorinated catalysts reported in the patents for the polymerization processes are listed in Table IV. Reforming. Minachev and co-workers (168,169)reported a study of reforming of the gasoline fraction from Volga-Urd Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
15
crude oil over palladium alumina treated with hydrogen fluoride and hydrogen sulfide gases a t 470-80 "C and 20 atm pressure. Chang and Kalechits (46,48) studied the relative performance of Pt-Al203, F-Al203, and Pt-F-A120:3 catalysts for their activity in reforming. Fluorinated platinum alumina was found to be the most active catalyst among the above catalysts. Its high activity is attributed to the formation of a bifunctional catalyst by an addition of platinum to fluorinated alumina (or by an addition of fluorine to Pt-Al203); Pt-alumina mainly catalyzed th'e dehydrogenatibn reaction, F-alumina the isomerization, and Pt-F-alumina effectively catalyzed both the reactions. They concluded that the active centers of platinum and fluorine on Pt-F-Al203 catalyst are joined together to form a complex active center and dehydrogenation-isomerization occur on the complex. They have also studied the poisoning effect of sulfur and nitrogen compounds on dehydrogenation and isomerization in reforming over the above catalyst ( 4 7 ) . Fluorinated noble metal-supported catalysts are widely used in reforming processes and several patents have been recorded. The catalysts reported in the patents are given in Table V.
Reactivation of Fluorinated Catalyst A process for reactivation of spent F-Pt-A120:3 catalyst has been described in the British patents (41,42).The reactivation is done by impregnation with an aqueous solution of fluorine compound after removal of carbonaceous material on the catalyst to replace a major portion of the halogen lost during use. Surface Properties and Active Centers of Fluorinated Catalysts Fluorinated Alumina. Effect of Fluorination on Surface Structure of Alumina. According to Antipina and coworkers (3,8),a chemical compound is formed on the surface of alumina by treating the oxide with BF:+ The compound formed was very active in the cumene cracking and was decomposed by ethanol dehydration products and steam a t 150 "C.Their data show that the activity of this catalyst increases with increase in the concentration of OH groups on the alumina surface (the surface concentration of OH groups depends on the temperature to which the original specimen of alumina is heated) and with an increase in the temperature a t which BFa absorbed, and also depends on the fluorine concentration in the catalyst. They explained an increase in acidity of the alumina by the adsorption of BF:j as due to weakening of the 0-H bond and increase in the mobility of the H+ ion. The similarity in the promoting action of BF:j and H F indicated that the catalyst activation involved the introduction of fluorine atoms into the catalyst (4,52). An adsorption of benzene from the vapor phase on alumina and on BF:j-treated alumina a t 2! "C indicated that adsorption properties and structural characteristics of alumina, containing irreversibly chemisorbed BF:r (up to 1%F ) on its
surface, do not differ significantly from those of the original alumina ( 4 9 ) .Ballous et al. ( 1 8 ) studied the effect of water vapor adsorption on the results of Hammett indicator (acidity) tests on fluorinated and nonfluorinated aluminas and found that acid surfaces of both samples could be poisoned by a trace amount of water vapor, but fluorination affected the sensitivity and retention of water vapor on alumina. Antipina and Chernov ( 4 ) have studied chemisorption and total adsorption of methanol in the unimolecular region and found it to increase considerably after activation of alumina by BF:3 in the presence of hydrogen. They associated this effect with a decrease in the OH group concentration on alumina during the promotion process. They found that the promotion of alumina by BF3 and H F did not result in an increase in the strength of acid centers, but was accompanied by the formation of new acid centers of comparatively low strength. Based on the results, they hypothesized that promotion of alumina with BF3 and H F takes place by the same mechanism and is associated with the formation of an A1-F compound on the catalyst surface. According to Gerberich and co-workers ( 8 6 ) ,the fluorination of alumina involves the following substitution reactions. (1)In aqueous media the surface hydroxyl group exchanges with F according to H30+
+ F- + OH(s)
+
2Hr0
+ F(s)
(2) A t the same time, according to Peri ( I 8 5 ) ,reaction with the dual acid sites of alumina should proceed according to 0,
0-
AI+ 'AI/
0
/
\o/
0
\o
+
HF
-
\ AI/ /
0
F OH \ / O Al
\o/
\o
(3) The new hydroxyl groups are then replaced according to eq 1. (4) Further dehydration will occur by condensation of adjacent OH groups during pretreatment. The overall result is a lowering of the catalyst hydrogen content. Gerberich and co-workers (86) have calculated the surface concentrations for the model of Peri (185)using their hydrogen and fluorine data. Their results are reproduced in Figures 1 and 2. As shown in Figure 2, both the catalyst OH and the concentration of dual acid-base sites decreased with fluorine concentration. The substitution of 2F for 0 (or F for OH) would be expected to enhance the acidity of the residual sites. The interaction of these factors affords a quantitative explanation of their data. At a low percentage of F, the enhancement in strength dominated, while a t high concentrations the sparsity of sites limited the rate. Antipina et al. ( 5 )examined the IR spectra of alumina and fluorinated alumina a t 3500-4000 cm-'. The spectra on alumina showed absorption peaks a t 3603,3683, and 3739 cm-l due to surface hydroxyl groups and the spectra obtained after the absorption of HzO a t room temperature and 10 Torr indicated an interaction of the surface OH groups with adsorbed
Table V. Fluorinated Catalysts for Hydrocarbon Reforming Processes Catalyst base
Fluorine promoter
Other promoters
A1203
F
Pt
A1203
F and C1 F or C1 F F AlF3
Pt Pt MOO:],FenO3, and Ti02 Pt
A1203 A1203
Si02-Al203 Si02-Al203 16
-
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 1, 1977
(2)
Reference 26,29,59,69,89, 93-95,97-103,160,175,178,196,228,229,235,241 104,179 96,183,171,240 177 19 74
0
4.5 G. SAMPLES 1.1 G. SAMPLES
-
-
I.
4
0
8
450 "C. The introduction of F into alumina the lattice was accompanied by F replacemnt of the surface OH groups. Based on these results, they concluded that the catalytically active structure of the F-promoted alumina is associated with the possible substitution of the surface OH group by a halide. Topchiev and Ballod (218) studied the adsorption-desorption isotherms of BF:, on several oxides and found that the adsorption of BF3 is partially irreversible and that the resulting surface compounds are thermally stable. Bubushin (14) performed an infrared spectroscopic study of the adsorbed BF3 on alumina and observed both chemisorbed and physically adsorbed BF3; the spectra indicated that the chemisorbed species contain B-0 and 0-F bonds. Recently, Rhee and Basila (195)have studied the chemisorption of BFs on silica, silica-alumina, and alumina catalysts by an infrared spectroscopic technique in order to study the surface chemistry of BF3 adsorption and the nature of the chemisorbed species. They have found that a chemical reaction occurs between the BF:3 and basic sites such as surface hydroxyl groups or the oxygen atoms in surface M-0-M' linkages (where M and M' are Si or Al). Part of the chemisorbed BF3 immediately reacts with surface OH groups to form MOBF2 and MF according to the reaction H F BF, MOH MO=BF2 MOBF, HF ( 3 )
-
+
-
+
As the temperature is raised, the MOBF2 groups react further according to the reactions F MOBF, MOH MOBOM HF (4)
-
+
+
or N
P
-I
v z
I
'I,
MOBF,
+ M'OM
z
F MOBOM
6
P 4
2
0
0
1
2
3 WT. % F
4
5
6
Figure 2. Dependence of surface composition of fluorided aluminas on fluorine content. Calculated for 1-g sample (86).
H20 molecules, while desorption at 400 "C restored the original spectrum almost entirely, a complete restoration being achieved at 650 "C. The fluorinated samples did not show the peaks associated with surface hydroxyl groups, but adsorption of a small amount of H20 a t room temperature on a sample calcined at 650 "C resulted in a peak at 3686 cm-' which corresponds to the middle peak in nonpromoted A1203. The other two peaks were observed in an F-promoted sample only after heating the sample with adsorbed H20 under vacuum a t 250 "C. They further extended their spectroscopic study of surface properties and examined the IR spectra of aluminum hydroxyl fluorides (43) exposed to D20 vapors. The structural OH groups (3680 cm-') of hydroxy fluorides were inaccessible to the adsorbed molecules and they did not change on being exposed to D20 vapors. The concentration of'OH groups was found to be decreased with fluorine content. On hydroxy fluoride surfaces, OH groups were present, which
MO~OM
+
M'F
(5)
and/or
0
U
l7 --t
+
MOH
-
M' 0 MOBOM
+
HF
(6)
until all of the surface OH groups are destroyed, producing either surface MOB(F)OM' or borate groups. The products of the reactions are dependent on temperature, BF:j concentration, and contact time. Antipina et al. ( 6 )reported x-ray analysis of alumina samples activated by BF3. They found that adsorption of BF:(on alumina causes creation of a new crystal phase with interplanar distances of 3-14 and 1.776 kX. It was confirmed that a t 450 "C, in addition to chemisorption, BF3 reacts with alumina to form AlF3, the amount of which increases with temperature. Comparison of x-ray and kinetic data showed that the presence of AlF3 does not affect the catalytic activity of alumina in the cumene cracking reaction. From the study of surface acidity of fluorinated alumina by the indicator method, Antipina and Vinokurov ( 7 )arrived at the conclusion that the original structure of alumina is retained during its treatment with BF3 and HF solutions, so long as the F content of the sample does not exceed 4% by weight and the F that combines with the alumina occurs only on the surface of the catalyst. Nature of Active Centers on Fluorinated Alumina. Attempts have been made to correlate the activity of fluorinated alumina with the variation in surface acidity by studying adsorption of ammonia on the catalyst surface, by the indicator method, and by spectroscopic examination of the adsorbed species on the catalyst. Adsorption of Ammonia. Webb (238) found that the amount of ammonia chemisorbed is independent of fluorine Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 1, 1977
17
content, but its removal is increasingly difficult as fluorination proceeds. His results indicate that fluorination increases the acid strength of alumina without affecting the number of active centers. Holm and Clark (121)determined the isosteric heats of adsorption of ammonia on fluorinated alumina and concluded, in contradiction to Webb's results (238),that the addition of fluorine reduces the strength of acid sites of alumina. Gerberich and co-workers (86) have reexamined this problem and have established a relationship between acidity and quantity of adsorbed ammonia, assuming that a t 500 OC ammonia is chemisorbed mainly as NH2 OH on the dual acid-base sites in a way analogous to H20 or H F (eq 2). They found that the amount of NH3 chemisorbed as NH2 OH passed through a maximum near 1.7% F. Their data suggested that the heat of adsorption measured by Holm and Clark (121) (using the Clausius-Clapeyron equation and assuming equilibrium between the gas and adsorbed phases) is not related to the catalytic activity, because this correlates with the irreversible portion. Gerberich et al. (86) compared the chemisorption of ammonia a t 175 OC with that a t 500 "C and found a bigger change a t higher percent F than a t low, indicating a lower heat for the reversible portion, in agreement with Holm and Clark. They correlated catalytic activity for cracking of 2,3-dimethyl butane and isomerization of cyclopropane with ammonia adsorption on the stronger sites. Both the catalytic activity and high temperature ammonia adsorption passed through maxima as the fluorine concentration in the catalyst increased. Choudhary and Doraiswamy (55) have also observed a maximum for the conversion of n-butene to isobutene near 0.86% F. Such a relationship was absent for the reaction studied by Holm and Clark (122)and hence the determination of heat of adsorption of ammonia (121) enabled them to suppose that the catalytic activity of fluorinated alumina is due to the presence of acidic sites of medium strength on the surface. Gerberich et al. (86)have shown that the dual acidbase sites which vary in strength and number, as the alumina is fluorinated, are probably responsible for the catalysis and the residual surface hydroxyl groups may act as co-catalyst in cyclopropane isomerization. Surface Acidity by Indicator Methods. Covini and coworkers (57) have studied the effect of fluorination on the acidity of alumina by performing nonaqueous butylamine titrations, using both Ho (Hammett) (22,23,105,129,236)and H K (61,120)indicators, similar to an earlier study of Hirschler (120).The main experimental results of Hirschler and Covini et al. are in agreement that strong acidic H R sites were present in fluorinated alumina but not in pure alumina and that these sites are needed for high cracking activity. However, they disagreed about the nature of the strong H R sites. Hirschler (120)suggested that they are protonic (Bronsted) acids. Covini et al. (57)concluded that H R indicator measures aprotonic (Lewis) acidity. Covini et al. (58) further studied the variation in surface acidity of fluorinated alumina and their catalytic activity in carbonium ion type reactions by varying their fluorine content or their pretreatment temperature, by adding water or by doping with NaOH and showed that the same sites are involved both in activity determination, and in acidity titration. Antipina and Bulgakov (12) found that an increase in F content in aluminum hydroxy fluoride catalyst from 2-3 to 56.5%was accompanied by the increase in surface acidity ( H K sites) and activity for cumene cracking. When the acidity was determined by high temperature adsorption of organic bases, then a difference in acidity between initial and fluorinated alumina was observed (50); however in titrating n -butylamine, using the Hammett indicator, it was found that the concentration of acidic sites of medium strength increases with fluorine content in catalysts
+
+
18
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
prepared by the interaction of alumina with BF2. The methods described in the above literature determine the aggregate acidity of protonic (Bronsted) and aprotonic (Lewis) sites. Antipina and co-workers (2,7) employed n butylamine titration using aryl methanol as an indicator which permits the determination of the magnitude of protonic acidity and calculated the general and the protonic acidity individually. They have shown that development of increased catalytic acivity of alumina by fluorine promotion is attributed to the formation of powerful protonic centers on the surface of the catalyst. Spectroscopic Study of Adsorbed Species. The presence of strong protonic acid centers on the surface of fluorinated alumina has been confirmed by Antipina and co-workers (2,121 by examining the IR spectra of adsorbed pyridine on the initial and fluorinated alumina samples. Bulgakov et al. (44)found that both the proton and aproton acid centers exist on the surface of the catalyst. The ratio of Lewis to proton centers depends on the concentration of F in the catalyst. The proton acidity does not depend upon the surface or structural OH groups. Antipina and co-workers ( 9 )studied the state of adsorbed H20 and pyridine on fluorinated alumina by IR spectroscopy and observed that roasting of the catalyst a t 6500 "C resulted in the appearance of 1610 and 3540-cm-' bands. The energy of H20 bound at the surface a t 500 "C was much higher than the energy of the H bond. They explained this as due to the coordination nature of the bond between the adsorbed H20 and the alumina surface. The IR spectra of pyridine on the alumina heated a t 600 "C and then treated with HzO vapor showed that the adsorbed H20 molecules forming coordination bonds with the catalyst surface are the donor of acidic protons. Antipina and co-workers (10) pointed out that determination of the protonic acidity by a visual observation of the variation of coloration by aryl methanol is erroneous. They studied (9,101 the ultraviolet spectra of di- and triphenyl methanols, anthracene, p-dimethylamino-azobenzene and anthraquinone adsorbed on fluorinated alumina, aluminum hydroxyl fluoride and y-alumina of industrial production (the bases are used as indicators in the determination of catalyst acidity by titration with n -butylamine).The spectra indicated the presence of acid centers on surfaces of the samples causing ionization of the indicators and an increase in the number of acidic sites by fluorination. From a comparison of the UV spectra of diphenyl methanol in acid solutions with those on the surface of fluorinated alumina, they found (10) that both Bronsted and Lewis centers exist on the surface. Itoh et al. (128) and Matsuura and co-workers (164,165) have studied the surface acid properties of a series of BF:jtreated aluminas by spectrophotometric and ESR studies of adsorbed propylene and the results were correlated to the butylamine acidity. The Lewis acidity was measured by using spectrophotometry of a chemisorbed species, and the total acidity was measured by n-butylamine titration; the Bronsted acidity was calculated as the difference of both acidities (128). They conclusively suggested that the remaining hydroxide groups of alumina become strong Bronsted centers by promotion with BF:+ By following the method of Parry (182) which determines, by quantitative IR spectroscopy of adsorbed pyridine species, the types (whether protonic or aprotonic) and concentrations of the acid sites, Hughes et al. (125) have established that both types of sites (i.e., protonic and aprotonic) are present in alumina fluorinated with hydrogen fluoride vapor or aqueous solution of hydrogen fluoride or ammonium fluoride. Quantitative measurements have shown that many of the protonic sites were removed a t elevated temperatures. However, an appreciable concentration was retained after dehydration a t
538 "C. Exposure to water vapor increased the concentration of protonic sites and decreased that of Lewis sites. Thus the acidic sites of F-alumina appear to undergo a reversible dehydration-rehydration analogous to the interconversion of protonic and aprotonic sites in silica-alumina. Ammonia Blocking and D-tracer Technique. Recently, Finch and Clark (72) have investigated the reaction of 1butene with fluorinated alumina containing 1.0,7.5, and 12.1% F using NHS-blocking and D-tracer techniques. They found that on all catalysts there was an initial formation of a polymeric complex. Ammonia-blocking studies indicated that the complex formation is confined to the higher energy sites than is the same reaction on silica-alumina and the isomerization activity is associated with the presence of the complex. Their products obtained by passing pulses of 1-butene over perdeuteriobutene-treated catalyst contain significantly more deuterium than does the starting material. From these results, they have concluded that the complex participates in the isomerization reaction and it probably furnishes the protons to form carbonium ions necessary for reaction. They also found that the complex formation depends a t least in part on fluorine content and isomerization depends on both the fluorine content and the presence of the complex. Choudhary and Doraiswamy (56) could explain the bond and cis/trans isomerizations in n-butenes at the comparatively lower temperature (-150 "C) than that (>300 "C) necessary for skeletal isomerization of n-butene by carbonium ion therory assuming the protonic active centers are on the fluorinated q-alumina. Effect of Temperature on Properties of F-Alumina. Chernov and Antipina (49) found that a change in the adsorption of BF3 on alumina from 200 to 400 "C does not alter the structural characteristics of the resulting catalyst significantly. Kanjiro and co-workers (130)have studied surface acidity of BF-treated alumina obtained from bayerite by heating a t 300-1000 "C. They found that the catalysts are strong solid acids with strong Bronsted acid sites and those from alumina obtained at 400-500 "C show the maximum acidity for each pK, value in the range -5.6 to 3.3. The catalytic activity for cumene cracking and o-xylene isomerization showed two maxima a t the precalcination temperatures of alumina of 400 and 800 "C and a minimum at 600 "C. This behavior was not in accordance with the preparation temperature dependence of the acidity of the fluorinated catalyst but agrees with that of the acidity of untreated alumina. Matsuura et al. (163)and Itoh et al. ( 1 2 8 ) have also reported the influence of precalcination temperature of alumina on the surface acidic properties of BF3-treated alumina. They found that the strong Bronsted sites exist mainly on the surface precalcined a t relatively low temperatures (up to around 600 "C), but when precalcined above 700 "C, the Lewis acid sites became an important portion in the total acidity of the catalyst. The concentration of hydroxyl groups and the Lewis sites as aluminum ions partially exposed a t a few special sites in the surface were considered as important factors contributing to the existence of the two different types of acid sites. Kuklina and co-workers (151)performed a phase transition study of fluorinated alumina and found that in the presence of >I% HF, a-alumina appeared a t 800 "C, whereas without the mineralizer (Le., HF) it appeared a t >lo50 "C. They also observed that increasing of the dosage of H F above 2% had parctically no additional effect and the specific surface of the catalyst decreased in the presence of H F even at Br > I > F. They further found that the OH groups on the silica surface are partially replaced by OMe groups by treatment with methanol a t 260 "C, and the total acidity and catalytic acitivity for isomerization of 1-pentene decrease linearly with increase of these OMe groups. From these results they discussed the mechanism of enhancement of the acidity in silica gel by halogenation. Toshio and co-workers (219) compared the performance of silica-alumina catalysts ion-exchanged with various halogen atoms (F, C1, Br, and I) for cracking of cumene and found that the activity of the catalysts increased in the order of I- (20% increase) > Br- > C1- > F- silica-alumina catalyst; on the other hand, in the isomerization of o-xylene the order of activity increase was F- (70% increase) > I- > C1- > Br- silica-alumina catalyst. The activities of the halogenated catalysts were proportional to the acidities per unit surface area of the catalyst in both reactions. They concluded that the acidity of the silica-alumina catalysts ion-exchanged with halogen atoms cannot be due to the inductive effect of halogen atoms since the order of activity of these catalysts did not coincide with the electronegativities of the exchanged halogen atoms. Barthoment et al. (20)studied the effect of lead fluoride on the acidity of silica-alumina catalyst by differential thermal analysis and obtained a correlation between acidity and surface reactions. The distribution of acid strength in the impregnated catalyst was very compact (8.3 2 pK, 2 5.7) which gave a catalytic selectivity superior to that of nonimpregnated sample. According to them, lead fluoride seemed to affect only Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,
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1977
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the Al, not the Si part of the catalyst; probably new Lewis sites (A1F:J were formed. Sano and co-workers (198) studied the state of catalyst surface and attempted a correlation between catalytic activity and surface acidity of fluorinated silica-alumina. Catalytic activity for cumene cracking was proportional to the acidity and both showed a maximum a t 0.5-1.0% F. They found that 70% of the F exchanged with OH and 30%of the F combined with A1 on the catalyst surface. Effect of Temperature on Properties of F-SilicaAlumina. Kolesnikov and Mirgoleev (138) found that the amount of BF:1 chemisorbed on silica-alumina was independent of the precalcination temperature between 500 and 800 "C and was 17.5% (by wt). They studied the effect of the heat pretreatment of BF:j-aluminosilicate catalyst on its activity in the alkylation of benzene with propylene and found that the activity was approximately constant below 500 "C but dropped markedly a t 700 "C as BF.7 desorbed. Effect of Irradiation of F-Silica-Alumina. Masakotsu and co-workers have studied the cracking of cumene (161,162) and isomerization of o -xylene (162)over fluorine-exchanged silica-alumina catalysts (containing 0-2 wt % F) irradiated by y-rays (with 1-10 X IOTy ) and found that, in both the reactions, the activity of the catalyst increased because of irradiation. The irradiation had no effect on the acid content, surface area, and pore volume of the halogenated catalyst. There was a good correlation between the increase in activity of the halogenated catalyst after irradiation and the electronegativity of the exchanged halogen atom. This fact and the ESR spectra of the irradiated catalyst (the strength of spectra was maximum a t 5 X lo5y and the activity of the catalyst was also maximum a t 5 X lo5 y ) indicate that the active sites on the catalyst depend on the lattice defects produced by irradiation; the larger the electronegativity of the exchanged halogen atom, the greater the production of the lattice defects.
Summary and Conclusions The present review describes the preparation, catalytic activity, and surface properties of fluorinated commercial catalysts. Fluorination of a catalyst is done either by chemisorption of HF, BF?, organofluoro, or fluorosulfo compounds from the vapor phase a t elevated temperature in the presence of a reducing agent or by impregnating the catalyst with HF, NH4F, NH4BF4,NH4SiFs, etc. from their aqueous solutions a t room temperature, and heating the resulting catalyst a t an elevated temperature. Fluorination enhances the catalytic activity of alumina and silica-alumina catalysts for alkylationdealkylation, cracking, isomerization, and polymerization reactions, and of Pt-alumina and Pt-silica-alumina catalysts for reforming reaction. The initial surface structure (chemical) of alumina, silica, and silica-alumina catalysts changes due to fluorination through the exchange (or substitution) of surface hydroxyl groups by fluorine atoms. The structure of the bulk alumina remains unaltered up to a fluorine content of 4 wt %. Both protonic (Bronsted) and aprotonic (Lewis) acid sites exist on the surface of fluorinated alumina; the ratio of protonic to aprotonic sites on a particular alumina depends upon the fluorine content in the catalyst and the temperature of calcination (before and after fluorination) of the catalyst. The surface acidity and activity of fluorinated alumina containing the same amount of fluorine and subjected to similar heat treatment vary appreciably, depending upon the method and the preparation conditions of the initial alumina. In the case of silica gel, fluorination creates protonic acid sites on the catalyst surface which are responsible for the en20
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
hanced activity of fluorinated silica in the isomerization reactions, while medium strength Lewis sites are probably created on the surface of silica-alumina catalysts due to fluorination. Irradiation of fluorinated silica-alumina catalyst with y-rays does not affect the surface acidity, surface area, or pore volume of the catalyst, but causes lattice defects which are responsible for increased activity of fluorinated silicaalumina in the cracking and isomerization reactions.
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Receiued for reuieiu March 27, 1974 Resubmitted August 17,1976 Accepted September 7 , 1 9 7 6
Selective Oxidation of Hydrogen in Carbon Monoxide/Air Streams. Application to Environmental Monitoring J. R. Setter' and K.
F. Blurton
Energetics Science, Incorporated, Elmsford, New York 10523
In order to use an electrochemical analyzer for monitoring carbon monoxide concentrations (0-200 ppm) in air streams containing high hydrogen concentrations (0-2%), it was necessary to filter the hydrogen selectively prior to the gas analyzer. Selective oxidation of hydrogen was achieved by passing the gas mixture through a high temperature (982 K) "uncatalyzed" Vycor tube. Under these conditions approximately 92% of the hydrogen but only 28% of the carbon monoxide was oxidized and this separation efficiency was sufficiently high for successful operation and application of the carbon monoxide the analyzer.
Introduction
A continuous problem in air pollution monitoring is the quantitative analysis of trace constituents in an air stream containing a variety of gaseous substances which interfere with the analytical technique. One way of dealing with this situation is to prefilter the interferents selectively and this is most commonly achieved with selective gas sorbents. 22
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 1, 1977
In the development of a portable analyzer for continuously monitoring carbon monoxide (0-200 ppm) in environments containing high hydrogen concentrations (20 OOO ppm), it was necessary to prefilter hydrogen selectively since the electrochemical analyzer used had an interferent signal due to Hz (100 ppm of H2 gave a signal equivalent to 1 ppm of CO (Blurton and Bay, 1974)). This separation was achieved by the selective catalytic oxidation of hydrogen in the gas stream. It
