Heteropolyacids As Solid-Acid Catalysts

167

B. 1melik et al, (Editors), Catalysis by Acids and Bases

© 1985 Elsevier Science Publishers B. V., Amsterdam - Printed in The Netherlands

HETEROPOLYACIDS AS SOLID-ACID CATALYSTS Y. ONO, M. TAGUCHI, GERILE, S. SUZUKI, and T. BABA Department of Chemical Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 (Japan)

ABSTRACT Heteropolyacids and their metal salts are active catalyst for methanol conversion. The activity of the silver salt is enhanced by the presence of gaseous hydrogen. Generation of acid sites in the silver salt by its interaction with hydrogen was discussed. Interaction of hydrogen with group VIII metals induces high solid acidity as evidenced by enhancement of the catalytic activity for methanol conversion. Bifunctional catalysis in alkane isomerization is also described. INTRODUCTION Heteropolyacids like dodecatungstophosphoric acid

(H PW12 040 , HTP) 3

and dodeca-

tungstosilicic acid (H4SiW12040' HTS) are highly active for. conversion of methanol into hydrocarbons. It was also found that silver salts of HTP and HTS are more active than the corresponding parent acid. The activity for methanol conversion was greatly enhanced by using heteropolyacid in conjunction with a group VIII metal and gaseous hydrogen. The purpose of the present review is to describe unique feature

of heteropolyacid as solid-acid catalyst, which are

found in catalysis for methanol conversion. Special emphasis will be placed on the role of hydrogen in generating acid sites in the silver salts and also in heteropolyacid-metal systems. Isomerization of alkanes by heteropolyacid-group VIII metal catalyst is also described. CONVERSION OF METHANOL OVER HETEROPOLYACIDS Zeolites, especially ZSM-5, are very effective catalysts for converting methanol into hydrocarbons. Ono and Mori have studied the mechanism of the conversion and have concluded that Bronsted acid sites are sole responsible sites for the conversion (ref.l). This led to the idea that methanol conversion should proceed also over solid substances other than zeolites, if they are highly acidic. In fact, HTP and HTS are found to be very active for the conversion (refs.1-3). The high catalytic activity of HTP seems to be rather unusual, considering the small surface area of HTP (4 m2g-1 ). To determine the effective number of active sites of HTP, the effect of pyridine sorption was examined. The reaction

168 was started by feeding methanol and nitrogen at 573 K. After 2 h, pyridine was added to the feed, which was finally changed back to the starting mixture. The catalytic activity after pyridine sorption decreased linearly with the amount of the sorbed pyridine, the complete loss of the activity being attained when the molar ratio of the sorbed pyridine and HTP used as catalyst was three. The amount of acid thus estimated indicates that all the protons in solid HTP are accessible to pyridine molecules and also to methanol molecules. The stoichiometric absorption of pyridine by heteropolyacids at 573 K was reported earlier (refs.4,5). Since methanol and other polar molecules are known to be adsorbed by HTP even at room temperature (refs.6,7), it is presumed that methanol molecules penetrate into the bulk of solid HTP at the reaction temperature and react in the bulk. This explains the high activity of HTP irrespective of its small surface area. To confirm that the phenomenon is not specific to the conversion of methanol, the number of acid sites effective for the dehydration of I-butanol was determined in a similar manner (ref.8). The amount of acid sites responsible for the dehydration is again three times as large as the amount of HTP used as catalyst. Thus, the dehydration of I-butanol is also presumed to proceed in the bulk of the solid. The "pseudo-l i qiid behavior of heteropolyacids have been well described by Misono and coworkers (ref.9). CONVERSION OF METHANOL OVER METAL SALTS OF HETEROPOLYACIDS Besides heteropolyacids, metal salts of heteropolyacids have activities for many reactions, for which Bronsted acid sites are supposed to be responsible (refs.5,lO). Therefore, the catalytic activities of various metal salts of HTP and HTS for the conversion of methanol were examined at 573 K (refs.ll,12). The activities of the series of metal salts of HTP and HTS are listed as follows. For salts of HTP Ag(98)

>

Cu(60)

>

H(60)

>

Fe(48)

>

AI(36)

>

Pd(26)

>

H(39)

>

Fe(24)

>

AI(15)

>

Zn(7)

>

La(24)

>

Zn(13)

For salts of HTS Ag(79)

>

Cu(61)

>

La(2)

Numbers in parentheses indicate the hydrocarbon yield at 2-6 h of running time. In general, the metal salt of HTP is more active than the corresponding metal salt of HTS. The distributions of hydrocarbons over various metal salts are very similar to that over HTP, indicating that the reaction mechanism is common to parent heteropolyacids and their metal salts. Thus, the active centers for methanol conversion should be common, and they are presumably Bronsted acid sites. It should be noted that silver and copper salts are more active among

169

metal salts and even more active than parent acids. Therefore, the mechanism of acid sites formation of silver dodecatungstophosphate(AgTP) and CuTP were studied in detail. FORMATION OF ACID SITES IN AgTP In the methanol conversion over neat AgTP at 513 K, a long induction time was observed as shown in Fig. 1. Since the induction time is often related to the formation of active centers (H+), the examination of the factors influencing the induction time may give a clue for the mechanism of acid site formation. Effect of hydrogen was examined as a possible source of protons, since small amount of hydrogen was always found in the reaction products (ref.13). The catalyst was kept in a hydrogen stream (4.1 x 10

-2

mol h

-1

) at 523 K for

1 h and then reaction started. As is shown in Fig. 1 the induction time almost disappeared by hydrogen pretreatment. It is clear that hydrogen plays an essential role in the formation of Bronsted acid sites. Protons may be generated by the reaction of silver cations with hydrogen molecules. +

+

(1)

During the methanol conversion, hydrogen molecules (or atoms) may be provided by the decomposition of methanol. Induction time is supposed to be the period which is required for the establishment of the equilibrium of Reaction (1). The induction time was also observed in methanol conversion over CuTP at 523 K, and it disappeared by the pretreatment of CuTP by hydrogen. Thus, the mechanism similar to Reaction (1) is operative also in CuTP (ref.14). Reaction (1) explains why silver(I) and copper(II) salts are the most active among the metal salts of heteropolyacids, since these salts are known to be the one which are most easily reduced by hydrogen (ref.ls). The generation of Bronsted acid sites by the interaction of hydrogen and AgTP or CuTP is confirmed by examining the catalytic activity for the isomerization of o-xylene, which is the reaction catalyzed by Bronsted acid sites (ref. 16) . The reaction was carried out at 573 K by using AgTP or CuTP (30 wt%) on active carbon as catalyst. AgTP showed no activity for o-xylene isomerization, but the activity developed when the catalyst was pretreated in a hydrogen or methanol stream for 2 h at 573 K. These facts show that AgTP, as prepared, has rro Brdrts.ted acid sites, but the acidity is induced by its interaction with hydrogen or methanol. Further evidence of the interaction of AgTP with hydrogen was obtained from infrared spectroscopy of adsorbed pyridine (ref.16). AgTP evacuated at 573 K did not give the bands due to pyridinium ion, while AgTP treated by hydrogen

170

or methanol at 573 K gave them. Similar results are obtained also for CuTP. Thus, the effects of the treatments by hydrogen and methanol on the Bronsted acidity of AgTP as observed by infrared spectra of adsorbed pyridine are in complete conformity with the effects of the pretreatments by the substances on the catalytic activity of o-xylene isomerization. AgTP was exposed to deuterium of 7.5 kPa at 563 K for 1 h and evacuated at

573 K for 30 min; new bands appeared at 2542 and 2641 cm- l, which are ascribed to the streching of 0-0 groups. The sample was then exposed to pyridine vapor at 393 K for 1 h and evacuated at 393 K for 2 h. The 0-0 bands completely disappeared and the band due to deuterated pyridinium ion (C at 1482 cm- l 5H5ND+) appeared. These results clearly demonstrate that hydroxyl groups are formed by the interaction of hydrogen and AgTP and they are acidic. While hydrogen pretreatment eliminates the induction period in the methanol conversion, the presence of gaseous hydrogen enhances the reaction rate (ref.13). The methanol conversion was carried out with AgTP (30 wt%) on active carbon as catalyst with the initial partial pressure of methanol of 51 kPa and with varying partial pressure of hydrogen. The hydrocarbon yield increased as the increase in the partial pressure of hydrogen. Thus, without hydrogen, the hydrocarbon yield was 24%, while it was 43% under the hydrogen partial pressure of 51 kPa. The effect of hydrogen was reversible as is shown in Fig. 2. After carrying out the run under a hydrogen pressure of 51 kPa for 2 h, hydrogen was replaced by nitrogen. The hydrocarbon yield was reduced to the value which would be expected when the reaction was started without gaseous hydrogen. Then, nitrogen was again replaced by hydrogen, the hydrocarbon yield being back to the original value. Thus, it is concluded that Reaction (1) is really operative and reversible under the conversion conditions. Oxygen was found to depress the catalytic activity. Thus, a small amount of

oxygen (9.8 x 10- 5 mol) was pulsed into the feed during the run in the presence and in the absence of hydrogen. The activity was sharply depressed, but gradually returned to that before adding oxygen. The retardation by oxygen may be caused by oxidation of silver metal to the cation. +

+

+

(2)

The recovery of the activity may be due to the reduction of silver cation to the metal by Reaction (1). Effect of hydrogen is not restricted to methanol conversion. The catalytic activities of AgTP for the synthesis of methyl t-butyl ether from isobutene and methanol (ref.17) and the esterification of acetic acid with ethanol are greatly enhanced by hydrogen pretreatment and also by the presence of hydrogen

171

50

30 s-. .....

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Q>

20

Q>

>-

>-

c 0

•

...0

.Q

10

u

...

0

C

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0

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... u

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>-

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'--

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5 on

Stream

0

Time on Stream

100 . - - - - - - - - - - - - ,

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/ .:

80

60

60

a u

40

40

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>-

c

o

.0

o...

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o

°.

0 0-

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3

Time on Stream

4 I h

Fig. 3. Effect of cofeeding gas on C

+

Z

yield in methanol conversion

over PdTP/SiO z' 573 K, (a) HZ' (b) NZ' (c) NZ~ HZ' (d) HZ~ methanol: 51 kPa.

NZ'

5

I h

Fig. 2. Effect of cofeeding gas on hydrocarbon yield in methanol conversion over AgTP/C at 573 K. Cofeed gas: hydrogen (0), nitrogen (0). The gas was changed from hydrogen to nitrogen (~) and from nitrogen to hydrogen (1-). 573 K, methanol: 51 kPa, W/F = 50 g.h.mo1- l.

80

"U

10

5

0

I h

Fig. 1. Change in hydrocarbon yield with time on stream in methanol conversion over AgTP with (.) or without (0) hydrogen pretreatment. 513 K, methanol: 30.4 kPa, W/F = 57 g.h.mol- l.

<,

• __ - 0

o

00/

Time

25

.D

0

"U

/

N2 f _ _0 -0-0-0-

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.\.-.I

"0

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.

H2 J.

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.....

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10

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Hydrogen

Pressure

I kPa

Fig. 4. Effect of hydrogen pressure on product distribution in methanol conversion over PdTP. 573 K, methanol: 51 kPa.

172

in the gas phase. Reduction of metal cations is not only way of acid site formation. For example, in the case of the Al salt, the mechanism of the acid site generation is entirely different (ref.16). Hydrogen has no effect on the catalytic activity for o-xylene isomerization. The catalytic activity and the capacity for pyridinium ion formation· are enhanced by the pretreatment with water. The plausible mechanism for proton formation may be associated with dissociation of water, as suggested by Niiyama (ref.10). +

------~)

[A1(OH))2+

+

HYDROGEN SPILLOVER IN METAL-HETEROPOLYACID SYSTEM When methanol conversion was carried out over palladium salt of heteropo1yacid supported on silica as catalyst, the great effect of hydrogen was observed. As shown in Fig . .3, the yield of hydrocarbons with carbon numbers more than one (C2+ yield) was about 70% when palladium dodecatungstophosphate(PdTP) was pretreated with hydrogen at 570 K, and the reaction was carried out by cofeeding hydrogen (51 kPa) (Curve a). The C yield was about 10% when PdTP was pre2+ treated under nitrogen and the reaction was carried out by cofeeding nitrogen (Curve b). When the cofeed-gas was changed from nitrogen to hydrogen (Curve c) or hydrogen to nitrogen (Curve d), the C yield gradually changes to the value 2+ which was supposed to be obtained if the reaction was carried by cofeeding the second gas from the beginning. The effect of hydrogen is reversible. The activity of PdTP in the presence of hydrogen is much higher than HTP or AgTP in the presence of hydrogen. Because of high

hydrogenation

activity of Pd metal, no olefinic products

were observed in the presence of hydrogen, all the hydrocarbon products being alkanes. Decomposition of methanol into carbon monoxide and hydrogen, and hydrogenation of methanol into methane and water also occurred. Fig. 4 shows the effect of the partial pressure of hydrogen on the product yield. The C yield increases almost linearly with hydrogen partial pressure. 2+ On the other hand, the yield of carbon monoxide did not depend on the partial pressure of hydrogen. A plausible mechanism for the enhancement of the acidity by hydrogen may be as following. Palladium cations are completely reduced to the metal by the pretreatment with hydrogen. Hydrogen molecules from the gas phase may dissociate into hydrogen atoms over the metal, and hydrogen atoms thus formed may spillover and interact with surrounding heteropo1yanions converted into protons. The processes is reversible. 2 H (over Pd metal)

to be

173

H

+

+

If palladium metal is the center for hydrogen dissociation and not the direct active site

for methanol conversion, the activity was expected to be not neces-

sarily proportional to the number of Pd content in the catalyst. Therefore, the catalytic activity of PdxH3_2xPW12040 supported on silica was examined. The results is given in Fig. 5. As is shown in Fig. 5, the C + yield greatly in2 creased with addition of small amount of palladium (x = 1/16) to HTP. Only a slight increase in the C + yield was attained by further increase of x. The 2 yield of carbon monoxide increased with increasing content of palladium, confirming that the active centers for the decomposition of methanol is metallic palladium. Now, it is clear that the catalyst is not necessarily prepared from metal salts of heteropolyacid. Therefore, a mixture of HTP and chloroplatinic acid was supported on silica. By the pretreatment of the catalyst by hydrogen, platinum metal is expected to be formed and to act as center for hydrogen dissociation. Thus, methanol conversion over 30 wt% HTP together with 0.07% Pt supported on silica gave the C

2

+

yield of 50% with the negligible formation of

carbon monoxide at 570 K. This type of the catalyst preparation may open up a novel method for obtaining highly acidic catalyst. HETEROPOLYACID AS A COMPONENT OF BIFUNCTIONAL CATALYST Isomerization of alkanes is an industrial process, which uses platinum in combination with acidic carriers such as fluorinated alumina and zeolites. As for the reaction mechanism, the dual functionality is generally accepted. The isomerization of alkanes was attempted by using palladium dodecatungstophosphate [Pd3(PW12040)2' PdTP] supported on silica-gel (ref.17). Prior to the reaction, the salt was heated in a hydrogen stream at the reaction temperature (443-523 K). By this treatment, Pd(II) cations are reduced to metal and protons are created by the reaction. Pd(II)

+

--)~

Pd(O)

+

It is noted that PdTP is highly active for acid-catalyzed reaction such as esterification of acetic acid with ethanol and MTBE synthesis even without hydrogen pretreatment, indicating that there is another way of acid-site formation. Moreover, as discussed in a previous section, the activity of PdTP as solid-acid is greatly enhanced by the presence of hydrogen in the gas phase. Therefore, PdTP after the reduction is expected to be a potential catalyst for alkane isomerization, since it would contain both metal (Pd) and strong acid

174

.

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-.-.- ......... ,

100 80

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60

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1j

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;;'!

80

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0

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500

550

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Temperature / K

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0

a L-_ _---' 0.5 a

40

?

100

80 ;;'!

40

10

\,

~ >

hydrogen: 71 kPa, W/F = 100 g·h·mol -1

i-

0

60

;;'!

Fig. 6. Effect of reaction temperature on the activity and the selectivity in hexane isomer over PdTP(50 wt%)/SiO Z' hexane: 30 kPa,

40/"••_ _ _, - - - - - , _____ 1100

c

80

0--0 -0-0

0

Reaction

Fig. 5. Catalytic activities of PdxH3_ZxPWlZ040 for methanol conversion. 573 K, methanol: 51 kPa, W/F = 50 g.h.mol- l.

;;'!

....... --

'-......,

0


~

100

0

1.5

.~

>

c

8

· 0

o/--~

SO;;'! ..... 60~

40 20

0/0 /

o L . . - -........._ - - ' -_ _'--_-'--_....u o 450 475 525 SOD 550

Reaction

Fig. 7. Catalytic activities of PdxH3_ZxPWlZ040 for isomerization of hexane. 443 K, hexane: 30 kPa, -1 hydrogen: 71 kPa, W/F = 100 g·h.mol

100

~-;

Temperature

/ K

575

Fig. 8. Effect of reaction temperature on isomerization of hexane over HTP supported on Pd/C. hexane: 30 kPa, hydrogen: 71 kPa, W/F = 100 g.h·mol

-1

>

li:

Q;

U'l

175

centers (H+). The reaction was carried out with a continuous flow reactor operating at atmospheric pressure. Table

shows the effect of hydrogen on the conversion

of hexane and the selectivity to hexane isomers together with detailed product distribution. As shown in Table 1, both the activity and the selectivity depend

very strongly on hydrogen pressure. Besides hexane isomers, methylcyclo-

pentane and cyclohexane were also found in the products. Formation of aromatic compounds was not observed. The effect of hydrogen is reversible; elimination of hydrogen from the gas-phase depressed the conversion sharply. Fig. 6 shows the effect of the reaction temperature on the conversion and the selectivity in isomerization of n-hexane. The conversion increases with reaction temperature up to 500 K, but it decreases at higher temperature. The decrease in the activity at higher temperatures may be due to loss of protons as water. The selectivity is constant (94%) below 450 K, but decreases at higher temperatures. The similar trend was observed in isomerization of pentane. Thus, at 453 K, the selectivity of 97% was obtained at the pentane conversion of 40%. Under the same reaction conditions, the selectivity of 92% and the conversion of 58% were obtained at 473 K. Isomerization of heptane is more difficult than that of pentane or hexane. Thus, the selectivity to hexane isomers was 70% at the conversion of 20% at 423 K. Since the presence of two components (Pd metal and H+) are essential for the ~an optimum ratio of Pdo and H+ for the catalytic

reaction, there must be

TABLE 1 Effect of hydrogen partial pressure on the conversion of hexane and the product distribution. Partial pressure of H2 / kPa Conversion / % Selectivity / % Product distribution / % Ethane Propane Butanes Pentanes 2,2-Dimethylbutane {2,3-DimethYlbutane 2-Methylpentane 3-Methylpentane Methylcyclopentane Cyclohexane

0 2.1 41.9

30 7.2 82.5

29.8 89.6

0.0 3.3 4.8 3.3 trace

0.0 1.1 4.2 2.1 1.0

trace 1.5 4.5 2.4 3.1

28.1

57.2

59.7

13.8 46.7 0.0

24.3 8.0 2.1

26.9 1.0 0.9

71

-1

Reaction temperature 483 K, W/F = 47.7 g.h.mol , Z' Hexane pressure 30 kPa, The data are average of 1-5 h of the process time. Catalyst 50 wt% PdTP/SiO

176 activity. Therefore, the catalytic activity of PdxH3_2xPW12040 supported on silica for hexane isomerization was examined as a function of x. The result is shown in Fig. 7. The conversion of hexane over HTP was 5%. The increase in Pd(II) in the starting catalyst causes the enhancement of the activity up to x = 0.75. The further increase in x did not affect the catalytic activity. The selectivity did not depend on the content of palladium. In order to confirm that palladium metal plays an important role in alkane isomerization, HTP was supported over Pd(5%) on carbon which was obtained from a commercial source. The result is shown in Fig. 8 which shows the effect of the reaction temperature on the conversion and the selectivity in hexane isomerization. The comparison of Fig. 6 with Fig. 8 shows that HTP supported on

Pd/C gives better performance. The higher selectivity was attained up to 532 K together with higher activity. Thus, the selectivity of 97% was obtained at hexane conversion of 78% at 523 K. AgTP on Pd/C also gave the high activity.

REFERENCES 1 Y. Ono and T. Mori, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 2209. 2 Y. Ono, T. Mori and T. Keii, Proe. 7th Intern. Congress. Catal., Kodansha, Tokyo, 1981, 1006 pp. 3 T. Baba, J. Sakai, H. Watanabe and Y. Ono, Bull. Chem. Soc. Jpn., 55 (1982) 2555. 4 M. Furuta, K. Sakata, M. Misono and Y. Yoneda, Chem. Lett. (1979) 31. 5 N. Hayakawa, T. Okuhara, M. Misono and Y. Yoneda, Nippon Kagaku Kaishi (1982) 356. 6 T. Okuhara, A. Kasai, N. Hayakawa, M. Misono and Y. Yoneda, Chem. Lett. (1981) 391. 7 T. Okuhara, A. Kasai, N. Hayakawa, M. Misono and Y. Yoneda, Bull. Chem. Soc. Jpn., 55 (1982) 400. 8 T. Baba and Y. Ono, J. Phys. Chem., 87 (1983) 2406. 9 M. Misono, Proe. Climax 4th Int. Conf. on Chemistry and the Uses of Molybdenum, Climax Molybdenum Company, p. 289. 10 H. Niiyama, Y. Saito, S. Yoshida and E. Echigoya, Nippon Kagaku Kaishi (1982) 569. 11 Y. Ono, T. Baba, J. Sakai and T. Keii, J. Chem. Soc., Chem. Comm. (1981) 400. 12 T. Baba, J. Sakai and Y. Ono, Bull. Chem. Soc. Jpn., 55 (1982) 2657. 13 Y. Ono, M. Kogai and T. Baba, Proc. Pan-Pacific Synfuel Conference Vol. 1, p. 115, 1982, Tokyo, Japan Petroleum Institute. 14 S. Yoshida, H. Niiyama and E. Echigoya, J. Phys. Chem., 86 (1982) 3150. 15 T. Baba and Y. Ono, J. App1. Catal., 8 (1983) 315. 16 T. Baba and Y. Ono, J. Phys. Chem., 87 (1983) 2406. 17 Y. Ono and T. Baba, Proc. 8th Intern. Congr. Cata1., 1984, Vol. 15, p. 405, Verlag Chemie. 18 S. Suzuki, K. Kogai and Y. ana, Chem. Lett. (1984) 699.