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J. CHEM. SOC. FARADAY TRANS., 1994, 90(16), 2413-2415

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Conversion of lsopropyl Alcohol to Acetone Catalysed by Cr,O, at 473 K: Role of Molecular Oxygen Published on 01 January 1994. Downloaded by State University of New York at Stony Brook on 25/10/2014 19:03:55.

Mohammad Ilyas,* Subhanullah Shah, Rizwana Nigat and Hizbullah Khan National Centre of Excellence in Physical Chemistry, University of Pesha war, Pesha war, Pakistan

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The conversion of isopropyl alcohol to acetone has been studied over Cr,O, powder at 473 K in a flowing microcatalytic reactor. Appreciable steady-state catalytic activity was achieved only when oxygen was present at 473 K on the initial activity, and the in the reactant gas flow. The effect of mild pretreatment of Cr,O, in 0,-N, observed dependence of the process on 0, pressure has been related to the oxidation-reduction cycle of the surface chromium species. Cr(3+n)+on the surface of Cr203 have been considered to be the active species, wheren = 1-3.

The conversion of isopropyl alcohol to carbonyl products catalysed by oxides of chromium or its complexes has been a subject of interest for a long time.'-' In the liquid-phase oxidation of isopropyl alcohol CrO, or other chromium compounds are used as catalysts and Cr", CrV and Cr'" are considered to be the active species.' *2 In heterogeneous systems, Cr,O, (both supported and unsupported) is used as the c a t a l y ~ t ~and - ~ isolated Cr3+ ions and/or Cr3+ ion pairs are considered to be the active species. In both these cases the chromium species involved are believed to be reduced to the corresponding lower oxidation state by one- and twoelectron reduction. These observations did not envisage any links between the homogeneous and heterogeneous systems. In our previous ~ t u d i e s using ~ ~ . ~ Cr,O, as a catalyst at 473 K, it was observed that (i) isopropyl alcohol was converted to acetone with more than 99% selectivity, (ii) an initial higher activity level decreased to an insignificantly low level with the passage of time in the deoxygenated atmosphere, and (iii) sustained catalytic activity of a significant nature was observed only in the oxygenated atmosphere. Combining these observations with some recent reports that (i) CrO, supported on SiO, was an efficient catalyst for conversion of isopropyl alcohol to acetone' at 483-543 K, (ii) exposure of Cr,O, surface to low oxygen pressure at lower temperatures can oxidise surface Cr"' to produce species of higher oxidation states. l o The presence of chromium species with a higher oxidation state has also been reported for mixed and supported chromium oxide (iii) Reduction of Cr"' (as in Cr203)at 473 K is difficult even in the presence of hydrogen,14 and (iv) formation of a CrIV-02--type species on the surface of C r 2 0 3as a result of oxygen adsorption,' combined with the report that a similar species (CrO,,+) in aqueous media and in the presence of excess oxygen catalyses the oxidation of alcohols.2 These reports have renewed our interest in the isopropyl alcohol/Cr,O, system. Therefore, we have reinvestigated this system in order to find some links between the homogeneous and heterogeneous catalysis of isopropyl alcohol by C r 2 0 3 .

Experimental Cr,O, (Merck, 2484) was used as supplied. All other materials, e.g. isopropyl alcohol, acetone, were supplied either by BDH or Merck and were used as supplied or purified by distillation, crystallization, etc. Oxygen and nitrogen were supplied by POL (Pakistan Oxygen Limited). Air was produced by a greaseless air compressor. All gases were passed through filters to remove moisture, organic impurities and traces of impurity gases. X-ray diffraction analysis confirmed the C r 2 0 3 to be cc-Cr,O,. The surface area, determined by

N, adsorption, was found to be 2.62 m2 g-'. Catalytic studies were carried out in a glass microcatalytic r e a ~ t o r in ,~ a continuous reactant mode at atmospheric pressure. The temperature of the reactor was maintained at 473 f 1 K. The pressure of isopropyl alcohol was maintained at 40 Torr and N, was used as a carrier gas for the alcohol vapour. Total flow of N, with or without 0, was kept at 40 cm3 min-'. The reaction products were analysed by gas chromatography (BAIF model SQ 206) with a FID, using a six-port gassampling valve. Diffuse reflectance spectra were recorded by using a Perkin-Elmer PC 16 FTIR spectrophotometer and a DMS 200 UV-VIS spectrophotometer equipped with diffuse reflectance accessories. CrV' present in Cr,O, was extracted by mixing Cr,O, with (i) hot water,16 (ii) 0.1 mol dm-, HCl or (iii) refluxing with H,SO, l 1 for 6 h. The amount of CrV'in the filtrate was determined by UV-VIS spectrophotometry using K,CrO, solution as standard or by atomic absorption spectrophotometry. An iodometric titration procedure' 7 , 1 8 was also employed for this purpose. l 7 9 l 8

Results and Discussion Earlier r e ~ u l t sfor ~ ~a~ high-purity ~ Cr,O, sample (Spex, 99.99%) in contact with deoxygenated isopropyl alcohol vapour at 473 K, showed it to possess only moderate and rapidly declining activity (but high selectivity, > 9OOh) for dehydrogenation (- H,). Dehydrogenation was also found to be the dominant pathway in the present studies, with a selectivity of almost loo%, with both deoxygenated and oxygenated isopropyl alcohol/Cr,O, interfaces. There was no clear evidence of any products other than acetone. A typical time profile for the decay of -H, activity from an appreciable initial level (i.e. after a contact time of CQ. 5 min with deoxygenated isopropyl alcohol vapour at 473 K) towards very low residual activity at long contact times is shown in Fig. l(a). The sample was pretreated at 473 K for 2 h in the flow of an oxygen and nitrogen mixture (1 : 1). On the other hand, a sample pretreated in the same way, when brought in contact with oxygenated isopropyl alcohol vapour [Fig. l(b)] shows a comparatively small decline in -H, activity, owing to an appreciable steady-state activity at longer contact times. The origin of the declining -H, activity in the deoxygenated atmosphere and the appreciable steady-state activity under oxygenated conditions was further investigated. Fig. 2 shows the result obtained with Cr203 samples pretreated in a deoxygenated atmosphere (N, flow for 2 h at 473 K). The initial activity of the sample was very low, both with oxygenated and deoxygenated isopropyl alcohol

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J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90

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atmospheres, as well as by N, at 473 K, while the second type of site is reduced by isopropyl alcohol vapour in the absence of only oxygen and could be the sites responsible for the high steady-state activity in oxygenated systems. These sites can easily be regenerated on Cr,O, surfaces whenever oxygen is reintroduced into a deoxygenated system (Fig. 2 and 3).

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I

E

-$3

16

Effect of Oxygen Partial Pressure

a -

g

12

(D r

$ 8 c

?!?

4

40

80 time/min

120

Fig. 1 Activity profiles of - H, over samples of Cr,O, , pretreated in 0, at 473 K for 2 h; (a) deoxygenated system (0) and (b) oxygenated system).(

vapour; however, with the passage of time the oxygenated isopropyl alcohol vapour shows an increase in -H, activity, reaching the same steady-state level as observed in Fig. 1. A change from an oxygenated to a deoxygenated atmosphere or vice versa, leads to the restoration of the steady-state activities to the corresponding level observed in Fig. 1. Furthermore, a sample (Fig. 3) pretreated in the flow of air (at 473 K for 2 h) shows similar behaviour to that observed in Fig. 1 and 2. These results show that the -H, activity of Cr,O, depends upon the pretreatment (at least in the initial stages) as well as on the presence of oxygen in the vapour phase. Therefore, it is possible that two types of surface sites are responsible for the -H, activity. The first is responsible for the initial high activity and is reducible in the presence of isopropyl alcohol in both oxygenated and deoxygenated

The manner in which gaseous oxygen admixed at increasing pressures into the reactant flow, caused an enhancement of the steady-state rates of production of acetone is illustrated in Fig. 4. The form of this plot with a relatively abrupt threshold for the enhancing effect of oxygen between Po, of 50 and 100 Torr, did not correspond to what would be expected from competition between oxygen and alcohol for surface sites. Transformation of the data to linear plots by a standard expression and applying various assumptions, e.g. molecular or dissociative adsorption of oxygen, was not successful. Published results of 0, adsorption on Cr203 10*12*13*1sdid not rule out the possibility of the formation of higher chromium species having an oxidation state greater than three. The formation of polychromate surface species of CrVand/or polychromate species of mixed valence (CrV and Cr") has also been ~uggested."~'~ Even oxygen adsorption (with 0, pressure as low as 1 x lo-' Torr) on C r 2 0 3 to produce CrV has been reported at room temperature." Therefore, the presence of chromium species with a higher oxidation state on the Cr,O, surface cannot be ruled out. Effect of Temperature The activation energy of the reaction was 48.84 & 4.12 kJ mol-' with In A = 51.77 +_ 0.46 molecules rn-' s - l in the temperature range 443-493 K. The value of the activation energy is comparable with that reported by Richter and Ohlmang for the conversion of isopropyl alcohol to acetone, catalysed by CrO,/SiO, . Estimation of Cr"'

+ = I B

J

1

50

100

150 ti me/m i n

200

250

Fig. 2 Activity profiles of -H, over samples of C r 2 0 3 , pretreated in N, at 473 K for 2 h. (0) Deoxygenated system in (a) and oxygenated system in (b);).( oxygenated system in (a) and deoxygenated system in (b).

Results from the extraction and estimation of the amount of CrV' present in Cr,O, using the various methods described earlier, show the amount of CrV1to be in the range 6.218.38 x 10l8 molecules (g CrZO3)-', The method used by Weller and co-workers7y8shows an amount of 1.89 x 1019 molecules of CrV' (g Cr203)-'. This difference could be attributed to the non-specific nature of the iodometric method used to determine the total amount of oxidizable material.

I

v)

N

'E

12

7

Iin

(D c

0 F

-c.'2 4 9 i

0

d 100

200

300

400

500

P,2/Torr

time/min Fig. 3 As for Fig. 2, but Cr,O, was pretreated in air for 2 h at 473 K

Fig. 4 Oxygen pressure dependence of pseudo-steady-state rate of conversion of isopropyl alcohol to acetone at a constant alcohol pressure of 40 Torr

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J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90

Published on 01 January 1994. Downloaded by State University of New York at Stony Brook on 25/10/2014 19:03:55.

Diffuse Reflectance Spectra of Cr,O, The diffuse reflectance FTIR spectrum of Cr,03 is shown in Fig. 5; comparison of the spectrum with those reported earlier11-1 3,15 indicates the presence of CrV' and CrV on the surface of Cr,O,. However, C r 2 0 3 treated with isopropyl alcohol and N, or isopropyl alcohol and 0,, at 473 K and cooled to room temperature in a flow of N, or 0, did not show any appreciable difference from the fresh sample, probably because of the unavoidable exposure of the sample to air whilst recording the spectra. The UV-VIS diffuse reflectance spectra revealed only CrIn. CrV' is probably confined to the surface and as stated by Davydov," determination of the oxidation state of surface cations by direct methods (e.g. diffuse reflectance) is difficult as they are masked by absorption of the bulk. However, the IR spectrum of Cr,O, (Fig. 5 ) is similar to that reported by Davydov, which was outgassed at 873 K and treated with 0, at 673 K, and was shown to have surface chromium species with higher oxidation states.

The possibility that Cr" may be present at the surface as a result of oxygen adsorption (during calcination at high temperature) cannot be ignored either.' In view of recent reports' that CrIV(in the form of CrOZ2+)catalyses isopropyl alcohol conversion to acetone [reaction (4)] in homogeneous systems in the presence of excess oxygen, CrIV can be considered as an active species at the surface. However, this would require formation of Cr" on the surface.

+

CrIV (CH,),CHOH

-+Cr"

+ (CH,),C==O

(4)

Formation of Cr" at the surface has been reported for mixed and supported chromia catalyst^.'^-^^ At the same time it is important to note that reoxidation of Cr" (in the presence of H,O-isopropyl alcohol at <623 K)'9*20to Cr"' is more favoured than its reoxidation to CrIv by 0,. This would explain the observed behaviour of Cr,03 for the conversion of isopropyl alcohol to acetone at 473 K. In the absence of any C-C bond cleavage products it is certain that only two-electron reduction-oxidation takes place.

Conclusion The demonstrated ability of the Cr,03 surface to catalyse the conversion of isopropyl alcohol to acetone, and the effect of pretreatment in N, and 0, at 473 K to have low or high initial activity would suggest that the active surface species is most probably an oxidized entity at the surface of Cr,03. This species could be easily reduced or oxidized at 473 K, which effectively excludes Cr" which is difficult to reduce at 473 K even in the presence of hydrogen. Therefore, this leaves CrV' and CrV(which are produced as a result of surface oxidation in air or during calcination at high temperatures after its preparation) as the possible active species available at the surface to initiate the reaction. The ability of 0, to sustain the catalytic activity of C r 2 0 3 could be partly attributed to the regeneration of the surface CrV species as suggested by Foord and Lambert." However, CrV', which is reduced during the process to CrtV,could not be regenerated by 0, at such a low temperature and this could be the reason for the small decline in activity. On the basis of these experimental results and the literature reports of 0, adsorption on Cr,03, the following sequence of steps have been proposed.

+ CrV+ (CH,),CHOH

CrV1 (CH,),CHOH

-

(CH,),C=O (CH3),C=0

+ Cr'" + Cr"'

(1) (2)

(3)

1100

1000 900 wavenum ber/cm - '

800

Fig. 5 Diffuse reflectance FTIR spectrum of Cr,O, temperature

Financial assistance and leave for study from the Department of Education, Government of NWFP (S.S.) and the University Grants Commission (R.N. and H.K.) are gratefully acknowledged.

References 1 D. Benson, Mechanism of Oxidation by Metal Ions, Elsevier, Amsterdam, 1976,pp. 149-215. 2 S. L. Scott, A. Bakac and J. H. Espenson, J . Am. Chem. SOC., 1992,114,4205. 3 (a) L. Nondek and J. Sedlacek, J . Catal. 1975, 40, 34; (b) L. Nondek and M. Kraus, J . Catal., 1975,40,40;(c) L. Nondek, D. Mihajlova, A. Andreev, A. Palazov, M. Kraus and D. Shopov, J . Catal., 1975,40,46. 4 F. Pepe and F. S. Stone, Proc. 5th In?. Cong. Catal., ed. J. W. Hightower, North-Holland, Amsterdam, 1973, vol. 1, p. 127; M. S.Scurrell, Annu. Rep. Prog. Chem., Sect. A, 1973,70,87. 5 (a) B. Jover, J. Juhaz and Z. G. Szabo, Z . Phys. Chem., 1978,HI, 239; (b) D. A. Dowden, Annu. Rep. Prog. Chem. Sect. C., 1979, 76, 19. 6 E. Iodumah and J. C. Vickerman, J . Catal., 1980,62,195. 7 (a)J. Cunningham, B. K. Hodentt, M. Ilyas, J. Tobin and E. M. Leahy, Faraday Discuss. Chem. SOC., 1981, 73, 283; (b) D. A. Dowden, Faraday Discuss. Chem. SOC., 1981, 72, 313; (c) M. Ilyas, Ph.D. Thesis, National University of Ireland, 1983. 8 E. M. Ezzo, N. A. Yousaf and H. S. Mazhar, Egypt. J . Chem., 1984,27,35;Chem. Abstr. 1986,104,1858OOt. 9 M.Richter and G. Ohlman, React. Kinet. Catal. Lett., 1985,29, 21 1. 10 J. S. Foord and R. M. Lambert, Surf:Sci., 1986,169,327. 11 C. Cristiani, P. Forzatti and M. Belloto, J . Chem. SOC., Faraday Trans. I , 1989,85,895. 12 A. Cimino, D. Cordischi, S. Febbraro, D. Gazzoli, V. Indovina, M. Occhiuzzi, M. Valigi, F. Boccuzzi, A. Chiorino and G. Ghiotti, J . Mol. Catal., 1989,55,23. 13 V. Indovina, D. Cordischi, S . D. Rossi, G. Ferraris, G. Ghiotti and A. Chiorino, J. Mol. Catal., 1991,68,53. 14 H.W.Chem and J. M. Lin, J . Phys. Chem., 1992,%, 10353. 15 A. A. Davydov, J. Chem. SOC.,Faraday Trans., 1991,87,913. 16 F. D. Snell, Photometric and Fhorometric Methods of Analysis, Metals, Wiley, New York, 1978,part 1, pp. 708-714. 17 S.W. Weller and S. E. Voltz, J . Am. Chem. SOC., 1954,76,4695. 18 S . W.Weller, Acc. Chem. Res., 1983,16,101. 19 G. Ghiotti, A. Chiorino and F. Boccuzzi, Surf: Sci., 1991,251-2, 1 loo. 20 D. A. Dowden, Annu. Rep. Prog. Chem. Sect., C , 1982,79,128. 21 B. Rebenstorf and S . L. T. Anderson, J. Chem. SOC., Faraday Trans., 1990,86,2783. 22 B. M.Weckhuysen, L. M. DeRidder and R. A. Schoonheydt, J . Phys. Chem., 1993,W,4756.

at room

Paper 4/01087F; Received 22nd February, 1994