dehydrogenation reactions

ELSEVIER

Applied Catalysis A: General 166 (1998) 115-122

Transition metal promoted AlPOd catalyst 2. The catalytic activity of MoeosA10.95P04 for alcohol conversion and cumene cracking/dehydrogenation reactions T. Mishra, K.M. Parida*, S.B. Rao Regional Research Laboratoy, Bhubuneswar-751013. Orissa, India Received 26 February 1997: received in revised form 7 July 1997; accepted 11July 1997

Abstract The 2-propanol and methanol conversion over M 0.05A10,95P04 (M = VO, Cr, Mn, Fe, Co, or Ni) catalysts was studied at different temperatures and flow rates in a fixed bed catalytic reactor while cumene cracking/dehydrogenation reaction was carried out in a micropulse reactor. The low acetone selectivity from 2-propanol in nitrogen and air indicated the presence of an insufficient number of weak redox sites in all samples. Among all the samples, VOAlP04 showed the highest conversion and also selectivity for olefins. Selectivity studies indicated that 2-propanol and methanol dehydration followed a combination of parallel and consecutive pathways. On all the samples, selectivity of cumene cracking product is very low. However, all the samples showed appreciable dehydrogenation activity. Participation of Bransted acid sites in the dehydration as well as cracking reactions is supported by the surface poisoning studies. A good correlation was observed between the acidity and the dehydration activity of all the samples. 0 1998 Elsevier Science B.V. Keywords:

Methanol;

2-Propanol;

Cumene; Cracking; Dehydration;

1. Introduction Amorphous phosphates have found increasing interest as catalysts or catalyst supports in the last three decades [l]. Among these phosphates, aluminium phosphate has been studied extensively due to its high surface area, thermal stability and surface acidbase properties [2-61. To increase its acid-base properties, the effects of anions like F- or SOa- [7] or of cations such as Lit, Na+ I K+ 1 Ni2+ or Fe3+ [8-101

*Corresponding author. Fax: +91 674 481637; E-mail: [email protected] 0926-860x/98/$19.00 II_?1998 Elsevier Science B.V. All rights reserved P/I

SO926-860X(97)00254-8

Dehydrogenation;

Surface acidity

have also been studied. So also the mixed metal oxide systems such as A1203, Ti02, ZrOl, SiOz or ZnO with AlPOd [ 1 l-131 at different ratios have been reported. Out of these A1P04-A1203 system showed enhanced acidity and catalytic activity. Mixed metal phosphates like CrP04-AlPOd have also been reported [ 13,141 as acid catalysts, which showed that the presence of AlP04 increased the acidic property of CrP04 catalyst. Therefore, it seems to be essential to study the effect of other transition metal phosphates on surface properties and catalytic activities of AlPOd. Our earlier study showed that 5% metal phosphate of V, Cr, Mn, Fe, Co or Ni with AlPOd increased the acidic properties of A1P04 [15]. To understand the types of

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Catalysis A: General 166 (1998) 115-122

acid sites present and also to compare the acid-base and redox properties of all the samples, we have studied the alcohol and cumene conversion reactions in this paper. The conversion of aliphatic primary and secondary alcohols to olefins and/or carbonyl compounds on acidic and basic catalysts has been studied extensively [ 16-201. Acidic sites favour the dehydration products, whereas the dehydrogenation is preferred in presence of basic or redox sites. As 2-propanol can undergo both dehydration and dehydrogenation reactions depending on the acid or basic sites, it can be used as a probe reaction to characterise both acidic and basic catalyst [ 171. Usually 2-propanol dehydrates to propene over acidic catalyst and dehydrogenates to acetone over basic catalyst. But according to Lahousse et al. [21], 2-propanol dehydrogenation requires an additional redox ability of the catalyst, since the same reaction does not take place on well-known basic oxides, such as magnesium oxide and magnesium aluminates. The dehydrogenation reaction also depends on the reaction environment (neutral or air/Oz) of the catalyst [22]. On the other hand, Kanazirer et al. [23] found that the selective conversion of methanol to dimethyl ether can be used to estimate the acid sites of a catalyst. So also cumene cracking/ dehydrogenation reaction is used to estimate both Lewis and Bronsted acid sites of a catalyst [24-261 from the product selectivities. The present paper deals with the studies on conversion methanol and cumene 2-propanol, reactions using transition metal-promoted AlPOd catalysts.

for 5 h and these were kept in the desiccator for further use. Detailed methods of preparation and characterisation of all the samples were reported earlier [15]. 2.2. Textural properties XRD patterns of all the samples calcined at different temperatures were recorded on a Philips (model 1710) semiautomatic X-ray diffractometer using CuK, radiation with a scanning speed of 2 degree/min. The BET surface area, average pore diameter and pore volume were measured from N2 adsorptiondesorption isotherms obtained at liquid nitrogen temperature using a Quantasorb (Quantachrome, USA). Prior to adsorption-desorption measurements all the samples were degassed at 393 K and 10m4 torr for 5 h. Surface acidity and basicity were measured by adsorption of organic bases such as pyridine (PY, pK, = 5.3), morpholine (MP, pK, = 8.33), piperidine (PP, pK, = 11.1) and acidic substrates like acrylic acid (AA, pK, = 4.2) phenol (PH, pK, = 9.9) spectrophotometric respectively, by methods [ 15,271. Redox properties (one electron donor and one electron acceptor) were determined by the same method using meta dinitrobenzene (MDNB, electron affinity, EA = 2.21 eV) and phenothiazine (PNTZ, ionisation energy, IE = 7.13 eV) as the adsorbates. The detailed procedure was reported earlier [15]. 2.3. Catalytic activity

2. Experimental

2.1. Material preparation Samples (M0.05A10.95P04) were prepared from aqueous solutions of aluminium nitrate, metal nitrate (M = V, Cr, Mn, Fe, Co, Ni) and phosphoric acid in the ratio of 0.95 : 0.05 : 1.O, using 1 : 1 ammonia as the precipitating agent at pH 7.5. Thus the gel obtained in a stirring condition at room temperature was filtered, washed and dispersed in isopropanol for 2 h, followed by filtration. Then the dried (383 K) samples were powdered and calcined at different temperatures

Catalytic activities of M0.u5A1a.9sP04 samples for dehydration/dehydrogenation of 2-propanol and methanol were studied using a fixed bed catalytic reactor (10 mm i.d.) with on-line GC. Prior to the reaction, catalysts (0.5 g) were heated in a nitrogen atmosphere at 773 K for 1 h. The conversion of 2propanol was studied in the temperature range of 423 to 573 K, whereas the methanol reaction was tested within 523 to 773 K. Alcohol was quantitatively supplied to the reactor from a continuous microfeeder (Orion, U.S.A) through a vapouriser using nitrogen or air as the carrier gas (flow rate 70-90 ml/mm). To avoid condensation of alcohol and liquid products in

T. Mishra et al. /Applied Catalysis A: General 166 (1998) 115-122

3. Results and discussion

the apparatus, all the connections from reactor to Gas Chromatograph were heated around 393 K by a heating tape. The reaction products were analysed by means of GC (CIC, India) in FID mode using Porapak T and Q columns. The rates of alcohol conversion were calculated under steady-state conditions by applying the equation: x = Y(W/F)

117

3.1. Textural properties Chemical analysis showed that (Al+M)/P04 ratios in all the samples are nearly 1.0/0.95, which are due to incomplete precipitation of phosphates [ 151. XRD patterns of all the samples suggest the amorphous character of all the samples below 873 K calcination, which converted to crystalline at 1073 K [ 151. The BET surface area, average pore diameter and pore volumes calculated from nitrogen adsorption and desorption isotherms are presented in Table I. Surface area of all the samples remained within a narrow range. However, there is an increase in average pore diameter and total pore volume due to the transition metal promotion. Amounts of surface acid, basic and redox sites measured by spectrophotometric method are presented in Table 2. Among all the promoted samples, surface acidity is highest and lowest on VOAlP04 and AlPOd, respectively. However, all the samples have very few number of redox sites. According to our earlier report [ 151, all the samples promoted by transition metal calcined at 873 K possess high surface area, pore volume and surface acid-base sites. Therefore, in this study only the catalysts calcined at 873 K were used.

(1)

where Y is the rate of reaction, W the weight of the catalyst, X the percentage of conversion and F the flow rate of the reactant. The extent of conversion was maintained within 20 mol% for calculation of reaction rates. Rates were calculated from the slopes of the straight lines obtained by plotting X vs. F-’ and the reaction was found to follow first-order kinetic. Cumene cracking/dehydrogenation reactions were carried out in a micro pulse reactor (Sigma, India) using nitrogen as the carrier gas (30-45 ml/min) in the temperature range of 673 to 873 K. All the catalysts (0.2 g) were activated at 773 K for 1 h in nitrogen stream before reaction. The volume of one cumene pulse was maintained at 1 pl (7.2 mmol). All the products were analysed by the GC on line with the micro reactor using 10 ft SS column with 10% TCEP. Reaction rates were calculated using the BassettHabgood equation for first-order kinetic processes

LW. ln[l/(l

-X,)1

= RTr(W/F)

3.2. Catalytic activities

(2)

where X, is the total conversion, r the rate of reaction, W the weight of the catalyst and F the flow rate of the carrier gas.

Table 1 Textural parameters

of M0.05A10.95P04 catalysts

calcined

3.2.1. Alcohol conversion The 2-propanol conversion produces propene as the major product while acetone (< 3 mol%) and

at 873 K

Sample No.

Catalysts

P/(Al+M)

Al/M

%ET

“, a

Ra\h

1 2 3 4 5 6 7

AlPOd VOAlPOz, CrAlP04 MnAlP04 FeAlP04 CoAlPOz, NiA1P04

0.95 0.94 0.96 0.95 0.94 0.96 0.96

19.06 19.31 19.11 19.17 19.08 19.21

203 193 198 203 187 191 209

0.267 0.294 0.326 0.344 0.343 0.364 0.307

26.3 30.4 32.9 33.9 36.7 38.0 29.3

* Total pore volume. ’ Average pore diameter

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Table 2 Surface acidic, basic, one electron donor and acceptor

properties

Samples

Acid sites

Basic sites

AlPOd VOAIP04 CrAIPOj MnAlP04 FeAlP04 CoAIPOj NiAlP04

of Mc,,,,5A1095P04 calcined

at 873 K Redox sites

AA ” (pmol/m’)

PHh (pmol/m’)

PP’ (pmol/m’)

MPd (pmol/m’)

PY’ (pmol/m’)

MDNB ’ (pmol/m’)

PNTZ I (pmol/m’)

1.72 2.44 2.32 2.22 2.94 2.83 2.23

0.96 1.41 1.66 1.43 1.61 1.62 1.46

2.43 3.52 3.33 3.22 2.68 2.67 3.01

2.07 3.06 2.95 2.86 2.94 2.88 2.65

1.07 1.38 2.35 2.22 2.29 2.25 2.13

0.044 0.077 0.066 0.064 0.064 0.068 0.062

0.054 0.145 0.101 0.123 0.117 0.109 1.11

a Acrylic acid, pK, = 4.2. h Phenol, pK, = 9.9. ’ Piperidine, pK, = 11.1. dMorpholine, p& = 8.33. ’ Pyridine, p& = 5.3. f Meta dinitrobenzene, electron affinity=2.21 eV. 8 Phenothiazine, ionisation energy=7.13 eV.

isopropyl ether (< 4 mol%) are found as the minor products. Although all the samples posses substantial amounts of basic sites, acetone selectivity is very low. Acetone selectivity increases only up to 15 mol% when the same reaction is performed in presence of air. Probably this is due to the unavailability of a sufficient number of redox sites. From the Table 3, it is clear that the rate of acetone formation increases due to the presence of transition metal phosphates with pure AlPOd. Among all the catalysts, acetone selectivity is the lowest over AlPOd. Although the rate of acetone formation varies from catalyst to catalyst, activation energy remains within a small range

(29.1-25.2 kcal/mol). This indicates that the different transition metal promoters only increase or decrease the number of available reactive sites on the catalysts. It is seen from the Arrhenius plots that the dehydration activity of 2-propanol increases over all the promoted samples, though the increase is very low on FeAlP04 and CoAlPO+ Among all the catalysts, VOAlP04 shows highest rate of propene formation (Table 4). The increase in the rate of propene formation can be correlated to the increase in the number of active sites of the catalysts. Probably iron phosphate and cobalt phosphate are not able to increase the number of acid sites when mixed with pure AlPOd.

Table 3 Rate (r) and activation energy (I?.,) for the 2-propanol acetone over M. osAlo slsP04

Table 4 Rate (r) and activation energy (E,) for the conversion of 2-propanol to propene over M,, 05AlclqsPOJ

Samples

AlPOd VOAIPOj CrAlP04 MnAlP04 FeAlP04 CoAlP04 NiAIPOj

lo6 r (mol/g s) at reaction temperatures of

conversion

to

E, (kcal/mol)

473

498

523 K

at 473 K

36.6 308.1 174.9 104.0 83.9 122.6 106.0

60.0 393.9 242.1 160.0 121.1 178.5 161.9

89.3 489.8 313.1 207.9 174.1 245.5 225.9

29.1 ho.09 25.1*0.07 25.6&O. 12 25.3&O. 1 28.6&0.06 27.0+0.08 26.31tO.09

Samples

AlPOd VOAIPO? CrAlP03 MnAIPOl FeAlP04 CoAlPO, NiAlP04

10h r (mol/g s) at reaction temperatures of

.%I (kcal/mol)

448

473

498 K

at 413 K

190.1 360.2 309.2 296.5 198.4 213.8 293.3

264.4 517.2 440.1 359.7 269.3 292.5 396.1

340.2 670.3 579.6 471.7 344.5 348.1 502.3

23.710.1 18.3&0.05 20.7hO.08 21.1~0.09 23.9&0.05 23.6f0.08 22.8fO.15

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330 300

Catalysis

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11’)

1. Alp04 2. VOAlPO4 3. CrAlP04 4. MnAlP04 5. FeAIP04 6. CoAIP04 7. NiAIPO4

270

S B

240

& % ‘u d

210 60 180 151

1

)

500

550

600

650

7

0

1

2

3

4

5

6

I

50

samples Fig. 1. Correlation plot between total surface acidity and the rate of propene formation at 448 K.

A linear relation between the rate of propene formation and the total acidity of the catalyst is also observed in Fig. 1. It is well observed that the activation energy for propene formation remains nearly the same for all the catalysts, which again would justify that the addition of transition metal phosphate to the pure AlPOd does not affect the nature of the active sites available on the pure AlPOd. Rather it only increases the number of active sites, thus increasing the dehydration activity. Generally alcohols dehydrate on acid sites to give ether and olefins. Of course, the selectivity to a particular product is determined by alcohol structure (primary, secondary), reaction temperature and surface acidity of the catalyst. As the basicity of 2propanol is higher than the primary alcohols, so it can give olefin as a major product even on weak acid sites, whereas methanol dehydrates to olefins only on strong acid sites. Therefore, to differentiate between the weak and strong acid sites of the catalyst, methanol conversion was carried out on all the samples. Methanol dehydration gave three types of products (Fig. 2). DME and Cl olefins are the dehydration products, whereas CO and CH4 are the decomposition product of DME [29]. It is evident that VOA1P04 produces the highest decomposition product (CO + CH4), whereas pure A1P04 shows lowest in the series. It was reported earlier that the coordinately unsaturated Al atoms in

Fig. 2. Selectivity of different products from methanol conversion reaction at 723 K over M. “>Alo 95P04 catalysts. Number refers to samples (cf. Table 1).

octahedral coordination in A1203 phase can serve as the active site for decomposition of DME [30]. Of course, in this case the role of a particular transition metal ion cannot be ruled out, as CO + CHq selectivity is higher for all the promoted samples in comparison to the pure AlP04. But at this stage it is difficult to conclude the exact role played by transition metal ions in the decomposition reactions. It is seen from Fig. 2, that in all the cases dimethyl ether (DME) is the predominant product. Among all the samples, VOA1P04 shows highest selectivity towards Cl hydrocarbons (16 mol%). But as a whole, all the samples show low selectivity towards the Cz hydrocarbons which indicates that the strengths of the major acid sites are not enough for the conversion of methanol to hydrocarbons. Selectivity of the products in both the reactions are represented by their corresponding optimum performance envelope (OPE) curves. These are obtained by plotting the fractional conversion (X) of a particular product against the total conversion (X,). This is obtained by varying catalyst to alcohol ratio (w/w) as described by Ko and Wojciechowski [31]. The OPE curves represent the conversion selectivity behaviour of active sites present on a catalyst and whose slope at the origin represents the initial selectivity for those products. For obtaining the product distribution as a

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that both of these are primary dehydration products. The downward deviation in the case of DME plot indicates the instability of the product. It means that DME further dehydrates to Cz hydrocarbons in a secondary reaction. All other catalysts also show nearly similar behaviour. Thus the pathway for the formation of Cl hydrocarbons on M0,0sA10.YsP04 catalysts is a combination of parallel and consecutive (primary and secondary) reactions.

x

3.2.2. Cumene cracking/deh_ydrogenution Generally, cumene conversion gives cracking as well as dehydrogenation products on acidic catalysts. Benzene and propene are the cracking products, whereas o-methylstyrene is a dehydrogenation product. Table 5 shows that ru-methylstyrene is the major product over all the samples whereas benzene selectivity is very low. Among all the samples, VOAlP04 and CrAlP04 show nearly 8-9% cracking products at 873 K reaction temperature. However, on pure AlP04, cracking activity is nearly zero. This low cracking activity is may be due to the unavailability of a sufficient number of Bronsted acid sites or owing to the unavailability of Lewis basic sites [32]. On all the catalysts, dehydrogenation activity increases with the reaction temperature. This means that all the transition metal-promoted catalysts possess higher number of Lewis sites. Among all the promoted samples, VOAlP04 showed the highest rate of reaction while FeAlP04 shows the lowest (Table 5). Probably the increase in acidities in case of M0.0sA10.95P0~ catalysts is mainly due to the increase in number of Lewis acid sites which facilitates the dehydrogenation reac-

(mom)

Fig. 3. OPE curve for methanol conversion (DME) and CT hydrocarbons at 573 K.

to dimethyl

ether

function of the conversion, we have included experiments performed by varying the weight of the catalyst, so that conversion as high as 40-50 mol% can be achieved. However, for the kinetic analysis only the conversion data between 2 to 20 mol% were used. The product profiles for 2-propanol dehydration on VOA1P04 show only one product (propene) but for methanol reaction (Fig. 3) on the same catalyst show two types of products (DME and Cl hydrocarbons). But within 4045 mol% conversion the decomposition product is negligible. Both DME and Ct are present from the onset of the reaction, which indicates

Table 5 Cumene conversion Catalysts

AIP04 VOAIPOl CrAIPOj MnAlPO? FeAIP04 CoAlPO, NiAIP04 * cu-Methylstyrene. h Benzene.

over M0.05Alc, ssPO,l at various reaction temperatures IO6 r (mol/g

s) of n-MS ” formation

at

E, at 173 K

Selectivity

(c/r) at 873 K

673

773

873 K

(kcal/mol)

BEN ’

tu-MS ”

IO.5 53.5 53.5 31.7 10.5 21.0 42.5

46.6 144.3 133.8 93.6 6 I .o 75.0 118.2

121.2 236.6 210.2 179.4 138.2 147.2 194.6

23.3H.8 16.8f0.5 16.750.3 21.5f0.2 19.OztO.2 18.410.4 18.210.3

0 8.9 8.0 4.8 I .2 1.6 6.5

100 91.0 92.0 95.0 98.7 98.4 93.5

Table 6 Variation of rate of reaction Catalysts

in presence of different poisons for 2-propanol

10” Rate of reaction in presence 2.Propanol

AIPOJ VOAIPOJ CrAIPOl MnAIPO, FeAIPOJ CoAIPO, NiAIPO,

-

of poisons tmol/g

to propene and cumene to tv-methylstyrene

s) Cumene -

propene

conversion

o-methylstyrene

WP :’

PY

DMPY

WP .’

PY

225.7 486. I 3X I .9 295.1 229. I 236. I 3125

104.2 219.5 173.0 136.0 104.9 106.0 142.8

16.2 169.X 133.0 104.9 80.6 X.5.6 105.9

46.6 1475 136.9 95.6 61 .O 75.7 121.3

60.3 54.6 3X.9 25.9 72.3 19.x

19.8

DMPY

45.9 147.0 140.7 97.5 63.X 69.X

I IS.0

“Denotes without poison.

tion. Further it is observed that the activation energy remains within a small range irrespective of the variation in the rate of a-methylstyrene formation. This indicates that the nature of active sites of all the samples are nearly the same. In the OPE plot both the lines (corresponding to benzene and cu-methylstyrene) are straight without any deviation, which indicates the formation of both the products from the on set of the reaction. Therefore, both of these are stable primary products.

dehydration mainly occurs on Bransted acid sites, thus strengthening the carbenium ion mechanism. On the other hand, cumene cracking activities of all the catalysts became nearly zero due to DMPY However, adsorption. dehydrogenation activity remains the same for DMPY adsorption. But the decrease in the rate of dehydrogenation is up to 60% due to pyridine adsorption. This decrease is probably due to the poisoning of Lewis acid sites.

3.2.3. Poisoning studies The poisoning of the active sites of Mo.tJ5A10.~5P04 catalysts in the 2-propanol conversion reaction was performed by presaturation of the acid sites with pyridine (PY) and 2,6_dimethylpyridine (DMPY). The catalysts were saturated with the probe reactant in the nitrogen stream. Then the catalyst bed was flushed with nitrogen at 523 K for 45 min to remove the unreacted base from the catalyst surface. In this way pyridine is bonded to both Br#nsted and Lewis acid sites [33,34] whereas DMPY is only bonded to the Bronsted acid sites [33-351. It is seen from Table 6 that on all the catalysts the decrease in the rate of propene formation is up to 55% due to PY adsorption, while the decrease is about 65% by DMPY adsorption. This difference in the poisoning effect of PY and DMPY is due to the differences in their pk, values. But the dehydrogenation activity practically does not change due to the poisoning effect. As DMPY preferentially adsorbs on Bronsted acid sites, so the decrease in dehydration activity shows that 2-propanol

4. Conclusions Methanol, 2-propanol and cumene conversions on M0,0sA10.95P04 catalysts increases as a function of surface acidity. Among all the samples. VOAIPOl showed the highest alcohol conversion as well as selectivity towards hydrocarbon formation. 2-Propano1 conversion to acetone in air confirms the presence of weak redox sites in all the metal-promoted catalysts. All the promoted catalysts show better selectivity for rr-methylstyrene in cumene conversion reaction. The selectivity study showed that alcohol dehydration on MAlP04 is a combination of parallel and consecutive reactions. Further poisoning study showed that mostly alcohol dehydration and cumene cracking takes place on the Bronsted acid sites. Though the acidity of the AlPOd increased in the presence of transition metal phosphates, it is not enough to dehydrate methanol preferentially to hydrocarbons and also for cumene cracking reaction. Therefore, the increase in acidity of all the

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samples is probably acid sites.

due to the increase

Catalysis A: General 166 (1998) 115-122

in Lewis

Acknowledgements The authors are thankful to Prof. H.S. Ray, Director, Regional Research Laboratory, Bhubaneswar, for his permission to publish this article. One of the authors, T. Mishra, is thankful to CSIR, New Delhi, for providing a fellowship.

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