Oxidation of o-xylene over vanadium-porous titania glass catalysts

Applied Catalysis, 75 (1991) Elsevier Science

Publishers

237

237247 B.V., Amsterdam

Oxidation of o-xylene over vanadium-porous titania glass catalysts Chee Yee Hong, Ralph P. Cooney and Russell F. Howe* ’ Chemistry Department, University of Auckland, Private Bag, Auckland (New Zealand) (Received 8 August 1990, revised manuscript received 23 April 1991)

Abstract Porous titania glass (PTG) has been loaded with vanadia by several different methods. Nitrogen adsorption-desorption isotherms and X-ray powder diffraction measurements indicate that impregnation methods produce oxovanadium species within the mesopores of the glass. The vanadia-PTG catalysts exhibit similar levels of selectivity and conversion for the oxidation of o-xylene to phthalic anhydride as conventional anatase or rutile supported catalysts, but at significantly lower temperatures. The activity for phthalic anhydride production increases with vanadium loading; the best performance of 50% selectivity to phthalic anhydride at 100% o-xylene conversion was achieved at 280°C with a catalyst containing 5.6% V (w/w).

Keywords: catalyst characterization

(adsorption, XRD), catalyst preparation phthalic anhydride, titania glass, vanadia-titania, o-xylene oxidation.

(wet impregnation),

INTRODUCTION

The partial oxidation of o-xylene to phthalic anhydride is a major industrial process utilizing catalysts containing V,OF, [ 11.The most successful catalyst support is titania, usually in the anatase form, and vanadia-titania catalysts have been extensively studied [2-111. The active phase in these catalysts is generally believed to be a monolayer of vanadia dispersed on the titania surface, and many authors have attempted to characterize the monolayer species with spectroscopic and physical measurements [ 2,5,6,7,9-151. Conventional anatase supports have surface areas of the order of 50 m2 g-l or less. Monolayer coverage was calculated by Roozeboom et al. [ 161 to correspond to a Vz05 loading of 0.145 wt.-% per m’, assuming a single lamella of the vanadia structure laid on the surface. The optimum vanadia content for conventional high-surface-area anatase supported catalysts should therefore be about 5 wt.-%. ‘Present

address: Department

of Physical

Chemistry,

University

of New South Wales, Kensing-

ton, NSW 2033, Australia.

0166-9834/91/$03.50

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Gesser et al. [ 171 and Yong [ 181 have described the preparation of a higher surface area micro- or meso-porous form of titania using sol-gel techniques. The so-called porous titania glasses (PTG ) obtained by controlled hydrolysis of TiCI, and careful dehydration of the resulting gel can have surface areas as high as 300 m2 g-‘, depending on thermal pretreatment [ 181. Such high-surface-area titanias raise the interesting possibility of preparing more active vanadia-titania catalysts for o-xylene oxidation by achieving a higher surface area vanadia monolayer than on conventional supports. In this paper we describe the preparation of vanadia-PTG catalysts and investigation of their activity for the oxidation of o-xylene. Spectroscopic characterization of the vanadium species and their interaction with adsorbed reactants will be reported subsequently. EXPERIMENTAL

Materials used were TiCl, and V,O, from Riedel-de-Ha&, vanadyl (IV) sulphate, ammonium metavanadate and potassium hydroxide from BDH, methanol and oxalic acid from Ajax. All chemicals, except VOS04, were A.R. grade. An anatase sample (P25) was provided by Degussa. PTG was prepared following the procedure of Gesser et al. [ 171 and Yong [ 181 by low-temperature hydrolysis of TiC14, followed by low-pressure distillation to remove most of the HCI, addition of KOH and dialysis against doubly distilled water to remove KCl. The resulting clear gel was then dehydrated by standing at room temperature in a controlled low-humidity environment for approximately two months, and subsequently annealed by heating slowly to 300 oC in air. Two batches of PTG prepared in this way are referred to as PTGA and PTGB respectively. After annealing, the glass was ground to a fine powder, with an average particle size of less than 1 ,um. Vanadium catalysts VPTGA (l-10) and VPTGB (1) were prepared by impregnating PTG with an aqueous solution of a known amount of VZ05 solubilised with oxalic acid. The PTG suspension was stirred continuously at ca. 70 oC to allow the excess water to evaporate slowly to dryness. Vanadium loading of these samples varied from one to ten weight percent V,O, (the number in parentheses refers to the nominal V,O, loading). Catalysts VP25 and VPTGB were prepared by stirring a P25 anatase or PTGB support vigorously in a sealed flask containing ammonium metavanadate solubilised with oxalic acid in methanol at room temperature for five hours. The solid was subsequently filtered, washed with methanol and dried at 110” C overnight. A vanadium doped PTG catalyst was prepared by using a solution of vanadyl(IV) sulphate in place of doubly distilled water during the dialysis step of the PTG preparation (labelled VPTG doped). The vanadium content of each catalyst was determined by atomic absorp-

239

tion spectroscopy. Surface areas and mesopore volumes of the catalysts were determined before and after the catalytic tests from nitrogen adsorption-desorption isotherms measured at liquid nitrogen temperature on a Micromeritics AccuSorb 2100E instrument. Surface areas were calculated by fitting the adsorption data between P/P ,=0.03 and 0.33 to a BET isotherm. Mesopore volumes were estimated from the difference between the nitrogen uptakes at P/PO = 0.2 (approximately the monolayer point) and P/PO = 0.8 (corresponding to a Kelvin equation pore radius of 4.0 nm). X-ray diffraction (XRD) patterns were recorded from most of the fresh and some of the used samples. The rutile fraction, X,, in the catalysts was calculated from the equation given by Criado and Real [91:

x*=[ 1+&l-’

where I, and I, are the intensities of the (110) and (101) reflections for rutile and anatase, respectively. Oxidation of o-xylene was carried out in a conventional flow reactor at atmospheric pressure. The reactant mixture had an oxygen-to-o-xylene mole ratio of 107 to 1. This was obtained by passing a stream of dry air through a bubbler containing liquid o-xylene at 0°C. The weight hourly space velocity (WHSV) for all experiments except those with vanadium doped PTG was between 0.09 and 0.16 h- ’ with catalyst weights of 0.1-0.19 g and a gas flowrate of 28 or 37 cm3 min- ‘. Vanadium doped PTG catalysts required a lower WHSV (e.g. 0.034 h-l) which was achieved by increasing the amount of catalyst (e.g. 0.44 g). The catalyst was placed in a glass tube reactor (1cm I.D.) and formed a bed approximately 2 mm deep. Reactor temperature was monitored with a thermocouple. Effluent from the reactor was directed through heated tubing (170°C) to a gas chromatograph (Hewlett Packard 5890) for hydrocarbon analysis or through glass tubing held at room temperature to an Edwards EQlOOF mass spectrometer for carbon dioxide analysis. Hydrocarbon products were separated on a 5-m silica macrobore capillary column with methyl silicone stationary phase in a temperature program of 80 oC to 280 *C at 5 oC per minute; retention times of the major products were 0.75,3.46,4.03 and 4.52 min after o-xylene for o-tolualdehyde, phthalic anhydride, o-toluic acid and phthalide, respectively. Prior to each set of tests, the catalyst was calcined in flowing air at 300°C for more than 10 h. The temperature was lowered to the desired reaction temperature, and reactant flowed over the catalyst. The reactor effluent was then sampled between six and ten times in the period of eight hours or more until a constant product distribution was achieved. After the catalytic test, the catalyst was calcined at 400°C for approximately one hour and then at 300°C for more than 10 h overnight. Each catalyst was tested at several temperatures

240

between 185°C and 36O”C, at o-xylene conversions of lo-100%. Blank runs showed the reactor walls and the supports to be inert to the oxidation of oxylene over the temperature range investigated. RESULTS AND DISCUSSION

Characterization of catalysts Table 1 summarizes the properties of the catalysts studied. The surface areas of the P25 anatase, PTGA and PTGB supports were respectively 56,105 and 145 m2 g-l. The calculated vanadium contents corresponding to monolayer uptake (following ref. 16) are 8.1,15.2 and 21.0 wt.-% V,O, respectively; all of the catalysts thus contain less than one monolayer equivalent of vanadia. X-ray diffraction measurements showed no evidence of a bulk vanadia phase in any of the catalysts. The P25 anatase was found to consist of 56% anatase and 44% rutile, as determined from the relative intensities of the anatase and rutile reflections. PTG and PTG supported vanadia which had not undergone prolonged thermal treatment above 300 oC showed only the characteristic diffraction peaks of anatase. The microporosity of PTG has been reported previously [ 17,181. Yong [ 181 showed that the pore-size distribution depended critically on the annealing treatment to which the glass had been subjected. Fig. 1 shows nitrogen adsorption-desorption isotherms for PTGA and PTGB. The hysteresis loops between P/P0 = 0.5 and P/P,, = 0.8 are typical of a porous material with pore radii TABLE 1 Catalyst properties Catalyst

Vanadium (wt.-%)

PTGA PTGB P25 VPTGA ( 1)

0.68

VPTGA(5) VPTGA(l0) VPTGB VPTG (doped) VP25

2.85 5.60 0.58 0.28 0.13

Fresh catalyst

Used catalyst

Surface area Mesopore volume (cm3/g) (m*/g)

Surface area Mesopore volume (m*k) (cm3/g)

105 145 56 111

0.119

_

0.139 _

_

0.066

96

104 152 145 56

aCatalyst was calcined at 400°C.

0.043

94 81” 71a 66 117* 117a 60

_ _ 0.061 0.039 0.103

241

.-_ (0) 120..

F g

2

E

. .,j

go--

lh.0

. 0

*Cl

60.-

>O 30 -oo"

&Jo0

0

0.200

0

*

0'0

0.600

0.400

0.800

1.000

p2/ps

Fig. 1. Nitrogen adsorption-desorption isotherms of (a) PTGA and (b) PTGB.

between 1.5 and 4.0 nm (type E in the classification of De Boer [ 191) . The corresponding isotherms for vanadium loaded PTGA catalysts are shown in Fig. 2. The BET surface areas of the vanadium loaded PTG samples after calcination at 300°C are closely similar to those of the PTG supports (Table 1). There is clearly, however, a significant reduction in the mesopore volume as a result of vanadium loading, as seen in the reduced size of the hysteresis loops. Mesopore volumes estimated as described above are listed in Table 1. The decrease in pore volume due to vanadium loading indicates that vanadia species are located within the mesopores of the PTG. Physical blockage of the pores with bulk V205 would also be expected to decrease the BET surface area (monolayer uptake). Since this is not observed, a more likely explanation for the reduced mesopore volume is coating of the walls of the mesopores with a monolayer or two of oxovanadium species which reduces the pore radius to an extent that capillary condensation no longer occurs (i.e. the mesopore is converted to a micropore). Also shown in Fig. 2 are the adsorption-desorption isotherms of the VPTGA catalysts following their use in the o-xylene oxidation reaction. Some decrease in BET surface area is seen (the extent of which depends on the temperature to which the catalyst has been heated during reaction or subsequent regeneration, consistent with the known instability of PTG above 300°C [ 18]), but no further loss in mesopore volume occurs.

242

0, 0.000

0.200

0.400

0.600

0.800

01 0.000

I.000

0.200

0.400

0.600

0.600

1.000

p2/ps

wps 90

(d) cl

F

SO-

.

,”

.

“E

.

.!+ 30.. P

0.400

0.600

0.600

o

0

‘0

*

‘0

o

I

1.000 OO.ooo

p2/ps

.d

0 ooo~”

09000 1 0.200

.

0.200

0.400

0.600

0.800

1. 00

p2/ps

Fig. 2. Nitrogen adsorption-desorption isotherms of (a) fresh VPTGA ( 1) , (b ) fresh VPTGA ( lo), (c) usedVPTGA(1) and (d) usedVPTGA(10).

In the case of the doped VPTG catalyst, previous EPR and XPS measurements have shown that vanadium is incorporated completely into the titania matrix, with a negligible concentration at the surface [ 201. The surface structure and chemistry of this catalyst is therefore expected to closely resemble that of the PTG support. o-Xylene oxidation C, hydrocarbon products detected during o-xylene oxidation were mainly phthalic anhydride, o-tolualdehyde and o-toluic acid. Fig. 3 shows the conversion of o-xylene to these products and to carbon dioxide as a function of timeon-stream for the VPTGA (5) catalyst at 240 oC. An initial high conversion to phthalic anhydride declined after several hours on stream to a steady-state value of about 20%. The conversion to o-toluic acid varied in a similar manner with time-on-stream, while conversion to carbon dioxide and o-tolualdehyde held approximately constant. For all of the catalysts studied, steady-state conversions were achieved after six to eight hours on stream, although the initial conversion versus time behaviour varied. All of the conversions and selectivities quoted below are the steady-state values. Accurate carbon balances are difficult to achieve in reactions such as this involving relatively involatile reactants and products. The total conversions of

243

1

E -80 1? s z 60 k F E 40 E T) s ,$ 20 P P 5 0

0 0

2

4 Time-on-stream

6 (Hour)

8

10

Fig. 3. Product distribution with time-on-stream in the oxidation of o-xylene over VPTGA (5) catalyst at 240’ C. ( 0 ) Conversion to phthalic anhydride (% ); (A ) conversion to carbon dioxide (% ); ( A ) conversion to o-tolualdehyde (% ) and (n ) conversion to o-toluic acid ( % ).

0.120

4,

0.000

7

185

225

345

385

I

Fig. 4. Weight-hour activities of various catalysts at different reactor temperatures. (0 ) VPTG (doped); (0) VP25; (A) VPTGB; (A) VPTGB(l); (0) VPTGA(1); (m) VPTGA(5) and (V) VPTGA(lO).

o-xylene to all products detected were generally less than the measured loss of o-xylene from the reactant stream at reactant temperatures below 280°C. We attribute these discrepancies to adsorption of involatile products on the catalyst support. Calcination of used catalysts in flowing oxygen at 400’ C released carbon dioxide, but the accompanying irreversible loss of surface area prevented quantitative estimation of adsorbed carbon. At reaction temperatures of 280’ C or higher, approximate carbon balances were achieved. Fig. 4 plots the steady-state activity for phthalic anhydride production (grams

244 TABLE 2 Steady-state conversion and selectivities Catalyst

Vanadium (wt.-%)

Temperature (“C)

Phthalic anhydride Selectivity

Yield (%Ja

Absolute

Organic products

VP25

0.13

260 295 320

12 20 18

27 33 36

88 94 88

VPTGA ( 1)

0.68

240 275 285

10 25 26

25 63 65

90 95 95

VPTGA(5)

2.85

240

17

48

86

VPTGA( 10)

5.60

185 210 240 280

6 10 26 50

24 18 43 50

85 90 88 100

VPTGB

0.58

255 270 295 315 360

10 10 25 22 5

17 20 36 31 6

92 90 94 92 85

VPTGB ( 1)

0.68

260 300 345

11 25 17

18 36 20

92 94 92

VPTG (doped)

0.28

295 340 385

12 13 12

20 18 15

92 90 85

&Conversion of o-xylene to phthalic anhydride = total conversion

x

selectivity.

of phthalic anhydride per gram of catalyst per hour) for the various catalysts studied as a function of reaction temperature. All of the catalysts produced phthalic anhydride above 250 ’ C. The least active catalyst was the doped VPTG sample; its activity may be considered as a baseline for the PTG support. All of the PTG supported vanadia catalysts were more active than the P25 anatase supported catalyst at any given temperature. The most active catalyst for phthalic anhydride production was VPTGA (10) containing 5.6 wt.-% vanadium. This catalyst produced significant yields of phthalic anhydride at temperatures as low as 180°C (apparent steady-state conversion of ca. 5% ). The data plotted in Fig. 4 for this catalyst at lower temperatures will be a lower limit to the true rate of production of phthalic

245 TABLE 3 Selected conversion and selectivity for conventional vanadia-titania catalysts Catalyst

Reference

Temperature (“Cl

Phthalic anhydride Yield (% )”

Selectivity ( % )

320 300 280 270

77.5 50 20 2

79

VzO,-anatase (1.3%,9.7 m*/g)

2

V20,-anatase (2.6%, 49.8 m*/g)

2

295

50

65

V,O,-anatase (lo%, 5.4 m*/g)

3

310

14

70

V,O,-anatase (7%, 9 m*/g)

4

330

67

75

V,O,-anatase (7.7%, 9.6 m’/g)

11

300 290 280

70 50 28

75 70 70

V20,-rutile (7.7%, 8.9 m*/g)

11

300 290 280

60 40 25

66 66 60

75 50 10

*Yield= o-xylene conversion X selectivity.

anhydride, given the involatility of this product (melting point= 131.6”C). The PTG supported catalysts containing lower loadings of vanadium are correspondingly less active for phthalic anhydride production; these catalysts also show a characteristic down turn in activity for phthalic anhydride production at higher reaction temperatures. Selectivity data for the catalysts tested here are summarized in Table 2. Total selectivities for phthalic anhydride (conversion of o-xylene to phthalic anhydride divided by conversion of o-xylene to all organic products and to carbon dioxide) of 50% or higher were achieved with all of the PTGA supported catalysts. The two PTGB supported catalysts gave lower phthalic anhydride selectivities (36% ), possibly because of the higher surface area of exposed titania support. In all cases the selectivity to phthalic anhydride amongst the organic products was greater than 85%. The best overall catalyst performance was achieved with the VPTGA (10) catalyst, which gave 100% conversion of o-xylene at 280’ C with ca. 50% selectivity to phthalic anhydride. Table 3 shows some phthalic anhydride conversion and selectivity data taken from the literature for conventional vanadia-titania catalysts. Conversions of 50-70% and selectivities approaching 80% have been reported for reaction

246

temperatures above 300 oC (these figures can be improved upon by adding promoters). There are however few data available at lower temperatures for comparison with the PTG supported catalysts. Bond and Briickman [2] found a steep decline in both conversion and phthalic anhydride selectivity below 300 oC for their monolayer vanadia-anatase catalysts, and a similar observation has been reported more recently by Centi et al. [ 111. Comparison of the literature data with those for the PTG supported catalysts suggests that on PTG supports a given level of conversion and selectivity can be achieved at lower temperatures than for conventional catalysts. The vanadium loading on the PTG support is a key variable. We suppose that the higher surface area of the PTG permits a monolayer dispersion of the active vanadia phase to be maintained at higher vanadia loadings than on conventional supports, thereby generating a larger number of active sites per unit weight of catalyst. The best PTG supported catalyst tested does not equal the selectivity to phthalic anhydride of the best conventional catalysts, but does approach these values at significantly lower temperatures. Further investigation of these novel catalytic materials is in progress. In particular, the extent to which monolayer vanadia coverage is achieved is being checked by laser Raman and XPS measurements. We note here that the enhanced performance of PTG supports for o-xylene oxidation may well find parallels in other types of vanadium catalyzed reactions. ACKNOWLEDGEMENT

Chee Yee Hong acknowledges the award of a University Grants Committee Post Graduate Scholarship. We thank Professor P.M. Black (Geology Department) for use of the X-ray powder diffractometer, and Professor B.J. Welch (Chemical and Materials Engineering) for access to the gas adsorption instrument. REFERENCES

8 9 10 11 12

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G.C. Bond and P. Kanig, J. Catal., 77 (1982) 309. A.J. van Hengstum, J.G. van Ommen, H. Bosch and P.J. Gellings, Appl. Catal., 8 ( 1983) 369. R. Kozlowski, R.F. Pettifer and J.M. Thomas, J. Phys. Chem., 87 (1983) 5176. F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J.Medema, V.H.J. de Beer and P.J. Gellings, J. Phys. Chem., 84 (1980) 2783. H.D. Gesser, L. Kruczynski, C.W. Turner and E.A. Speers, Nature (London), 291 (1981) 399. Y.S. Yong, M. SC. Thesis, University of Manitoba, 1984. J.H. de Boer, in D.H. Everett and F.S. Stone (Editors), The Structure and Properties of Porous Materials, Butterworths, London, 1958, pp. 68-94. P. van der Heide and R.F. Howe, in preparation.