J . Chem. Tech. Biotechnol. 1993, 56, 265-270

Kinetic Studies on Dehydrogenation of Butanol to Butyraldehyde Using Zinc Oxide as Catalyst V. K. Raizada, V. S. Tripathi, Darshan Lal, G. S. Singh, C. D. Dwivedi & A. K. Sen Defence Materials and Stores Research & Development Establishment (DMSRDE), PO GT Road, Kanpur 208 0 13, India

(Received 17 February 1992; revised version received 4 August 1992; accepted 2 October 1992)

Abstract: Butyraldehyde is an important chemical for many industrial applications, especially in the production of polyvinylbutyral. A systematic study of its synthesis by catalytic dehydrogenation of butanol, using zinc oxide as catalyst has been carried out. The effect of the method of preparation of zinc oxide on its catalytic activity for the title reaction has also been studied. The optimum conditions for maximum yield have been worked out and on the basis of secondary electron micrograms the reasons for higher activity/selectivity in the case of zinc oxide calcined from zinc hydroxide is attributed to the presence of hexagonal morphology. A kinetic study for the best zinc oxide catalyst has been carried out and the rate equation has been determined.

Key words : dehydrogenation, kinetics, order of reaction, energy of activation, optimum conditions.

NOTATION

Concentration of butanol (mol dm-3) Initial concentration of butanol (mol dm-3) Feed rate (mole min-l) Gas constant (g cal g mol-' K) Rate of reaction (mol min-l g cat.-') Temperature (K) Weight of catalyst (g) Fractional conversion of butanol Weight-time (g cat. min dm-3)

1 INTRODUCTION

been found to give predominantly dehydration products while non-reducible basic oxides are good dehydro- genation catalysts.'*' Rare earth oxides fall between these

The review of literature reveals that no systematic study has been reported for the synthesis of butyraldehyde using zinc oxide or other oxides of a basic nature. In the present work, studies have been made to synthesize butyraldehyde in a single step by dehydro- genation of butanol, using zinc oxide as catalyst in a fixed bed glass flow reactor. The effect of method of preparation of zinc oxide on its activity/selectivity has also been studied. The kinetics of the reaction using zinc oxide catalyst showing maximum yield of butyraldehyde have been studied. An attempt has also been made to find

Butyraldehyde is an important chemical, used in the synthesis of polyvinylbutyral, having many important applications in the polymer industry. Metal oxides have been extensively used for dehydration/dehydrogenation of alcohols' and the multipathway dehydration/ dehydrogenation of alcohols has been employed as a 2 EXPERIMENTAL model reaction for studying catalytic properties of metal oxides. A correlation between acid/base properties and 2.1 Reaction apparatus and procedure catalytic activity, selectivity2 and mechanism3 has emerged as a result of these studies. Acid o ~ i d e s ~ - ~ have A block diagram of the experimental set up is given in

J . Chem. Tech. Biotechnol. 0268-2575/93/$06.00 0 1993 SCI. Printed in Great Britain

out the factors responsible for maximum activity of zinc oxide calcined from zinc hydroxide by studying its morphology using scanning electron microscopy (SEM).

265

266

L CJ C

2 40- s v

% 2 0

V. K. Raizada et al.

/

-

Heating element 1 - .

Chilled methyl alcohol out

alcohol in

- . - . - . . . 5 '-- .----: B .- - - _ _

Product (Butyraldehyde+ n-butyl alcohol 1

Fig. 1. Flow diagram. (1) n-Butyl alcohol storage vessel. ( 2 ) Pre-heater. (3) Reactor. (4) Vertical condenser. vessel. (6) Capillary flow meter. (7, 8) Needle valves. (9, 10) On-off valves.

( 5 ) Jacketed product

adopting the area normalization procedure. The gas in the exit stream was analysed separately in the same conditions as above using a gas-tight syringe.

2.2 Preparation of catalyst

-1 I L

0 200 400 600 800 1000 1200 w/FA, (g catalyst mole-' min)

Fig. 2. Effect of temperature and flow rate: ZnO calcined from zinc hydroxide precipitated by NH,OH.

Fig. 1. A 9 in. long heated ceramic tube was used to house the reactor. The catalyst bed length was 10 cm, in which the fine zinc oxide powder was packed with glass wool. The design of the reactor was such that it facilitated uniform heat and mass transfer between vapour and the catalyst particles. No more than f 1C temperature variations were observed during the run after steady state was attained and the samples were collected after attaining steady state. The feed was delivered to the reactor via a preheater by means of a calibrated fine capillary flow meter applying pneumatic pressure in a closed system. The product was collected through a heat exchanger (4ft long condenser) cooled to -15C by circulating chilled methyl alcohol. The collection was easy and efficient as no diluent was used with feed. The liquid products were analysed by Aimil Nucon 5700 dual column GC, using TCD detector. The column was a 2 m x 3 mm i.d. stainless steel tube packed with 10% carbowax 20 m on DMCS-coated chromosorb WAW. The flow rate of carrier gas was 50 cm3 min-l and the column temperature was 100C. Quantitative estimation was done by an integrator (Hewlett Packard, USA)

Zinc oxide for the study was prepared using different methods by precipitation of (a) zinc carbonate from an aqueous solution of ZnS0,.7H20 and sodium car- bonate, (b) zinc oxalate from an aqueous solution of ZnS0,.7H20 and ammonium oxalate, and (c) zinc hydroxide from an aqueous solution of ZnSO, .7H,O and ammonium hydroxide/potassium hydroxide. All the chemicals used were AR/GR grade and were added in equimolar ratio in case of (a) and (b) and in the ratio of 1: 1.6 mole in the case of (c). The precipitated gel was aged for 6 days under identical conditions in all cases and was collected and washed by repeated redispersion- filtration cycles till free from anions. The gel was dried at 140C to 150C for 2 h and was calcined at the temperature of 550C for 5 h.

3 RESULTS AND DISCUSSION

3.1 Synthesis of butyraldehyde from butanol

The synthesis of butyraldehyde from butanol by catalytic dehydrogenation, using zinc oxide as catalyst is a single step reaction. A detailed study of the effect of various operating parameters such as method of preparation of zinc oxide, temperature and weight-time has been studied and optimum conditions determined.

The butanol used was 99% pure. The liquid product collected was analysed by GC and contained only butanol and butyraldehyde. The other products found were below 1 % and hence were not identified. The gaseous product consisted of butene and hydrogen. The percentage of butene in the gaseous product was below

Dehydrogenation of butanol to butyraldehyde

f 40- v

x

2 0

1001

-

267

450C 801 425OC

L I I 0 200 4 0 0 600 800 1000

w/FA, ( g catalyst mole- min)

Fig. 3. Effect of temperature and flow rate: ZnO calcined from zinc hydroxide precipitated by KOH.

8o t o--- 4 5 0 C -

425C

z x 20 v

I I I 1 1

0 200 400 600 800 1000 w / F ~ , (g catalyst mole- min)

Fig. 4. Effect of temperature and flow rate: ZnO calcined from zinc carbonate.

2 % on the basis of alcohol feed. The material balance in the results was 93 k 2 YO.

2ot--.-- 0 200 4 0 0 600 800 10- w / F ~ , (g catalyst mole- min)

Fig. 5. Effect of temperature and flow rate; ZnO calcined from zinc oxalate.

showed very good activity as well as selectivity giving yields from 67 to 90 YO.

3.3 Effect of temperature

The effect of temperature is given in Figs 2-5. A series of experiments was conducted in the temperature range 350C to 450C. Conversion in different grades of zinc oxide increased with increase in temperature in the range studied. In the case of zinc oxide prepared by de- composition of carbonate and oxalate, though the conversion kept on increasing with increase in tem- perature, at higher temperatures beyond 450C a lot of carbonization took place making the catalyst lose its activity. The material balance dropped sharply and percent conversion dropped to 55.2% in the case of carbonate and 62.5% in the case of oxalate at a temperature of 475C and at a flow rate of 965.11 g cat. mol-I min.

3.4 Effect of flow rate 3.2 Effect of method of preparation of catalyst

The effect of zinc oxide prepared from three different methods as detailed in Section 2.2 is given in Figs 2-5. All types of zinc oxide prepared by different precursors

Weight-time (z) was studied in the range 3.4 to 19.6 g cat. min dm-3 and it was found that conversion increased rapidly with weight-time increase and became steady at higher W/FA,,.

TABLE 1 Optimum Conditions for Maximum Yield of Butyraldehyde

ZnO Surface Temperature Weight-time Percent calcined from area (C) (g cat. rnin dm-3) conversion

(m2 8-l) optimum WIF,,CAO

(a) Zinc carbonate 36.19 450 6.24 67.00 (b) Zinc oxalate 44.10 450 6.00 85.40 (c) Zinc hydroxide precipitated from 44.12 400 16.65 89.84

(d) Zinc hydroxide precipitated from - 400 15.90 88.00 ammonium hydroxide

potassium hydroxide

268 V. K . Raizada et al.

( 4 Fig. 6. SEM of ZnO from (a) carbonal

(4 te, (b) oxalate, (c) (d) hydroxide.

10 - 275T I I I I I I I I I I I

0 20 40 60 80 100 120 140 160 180 200 220 240 W/f,, (g catalyst mole-' min)

Fig. 7. Effect of flow rate and temperature.

3.5 Optimum conditions

Zinc oxide prepared from the different methods detailed in Section 2.2 showed maximum conversion at different temperatures and different flow rates as given in Table 1. The results in Table 1 as well as Figs 2-5 show that zinc oxide prepared by calcination of zinc hydroxide precipitated by ammonium hydroxide gave maximum

activity/selectivity giving 90 YO yield of butyraldehyde at a temperature of 400C and weight-time of 16.65 g cat. min dm-3 in spite of the fact that the surface area did not differ much. A general mechanism for de- hydration and dehydrogenation of alcohols on acidic and basic oxides has been proposed by Van Reyen and Sachtler." According to these authors, in the case of dehydrogenation large cation-basic oxides behave differently, in that the cation interacts with hydrogen followed by the interaction of the anion with the -OH group of the alcohol. As such the dehydrogenation will be more facile on the catalyst surface, having a greater abundance of metal cations on the surface that is the 0001 plane of the zinc oxide. We have tried to substantiate this observation by SEM studies. The secondary electron micrograms of zinc oxide catalyst prepared via different routes are given in Fig. 6(a)-(d)." The electron micrograms of zinc oxide calcined from zinc carbonate and zinc oxalate shown in Fig. 6 ((a) and (b)) do not show any hexagonal structure even at magnifications of x 30000 and x 40000 while the zinc oxide calcined from zinc hydroxide shown in Fig. 8 ((c) and (d)) shows clear hexagonal platelet morphology at x 20000. It seems that, during the process of calcination of the catalysts obtained via carbonate and oxalate

Dehydrogenation of butanol to butyraldehyde 269

routes, decomposition occurs by condensation of COi- groups with two protons from the nearest OH- group resulting in the release of CO, + H,O along channels parallel to the c axis and leaving behind an open network of zinc and oxygen while, during thermal decomposition via the hydroxide route, zinc oxide is forced to grow in a hexagonal platelet morphology rather than with prism- like habit. Herman et al." have reported two ranges of zinc oxide morphologies while studying catalytic syn- thesis of methanol from CO and H, using Cu/ZnO/ M,O, catalyst. In one range zinc oxide appears to form a network of crystallites with their six-fold crystal axis parallel to major dimensions and in the second zinc oxide crystallites are hexagonal platelets with their six-fold crystal axis perpendicular to major dimensions. It was observed in the SEM picture for zinc oxide prepared by calcination of zinc hydroxide, that the hexagonal platelets are more distinct and in greater abundance than in the other zinc oxides.

In the present work although no cationic dopant was present, it is possible that Zn2+ form hydroxy complexes which age during the process of precipitation and washing, and on subsequent drying and calcination they may be acting as seeds for crystallization of nascent zinc oxide which is forced to grow in hexagonal platelet morphology.

3.6 Determination of rate data

The kinetics of zinc oxide catalyst, i.e. zinc oxide prepared by calcination of zinc hydroxide precipitated by ammonium hydroxide, were studied between 275C and 375C at weight-time ranging between 04-50 g cat. min drn-,. Differential method of analysis has

Fig. 8. Arrhenius plot. In k, = 9.8 x 10-1 = 0.98

k, = 267

= 8.67 x 103 K d(ln k ) - 1.3 x 10-1 d(l/T) 0.15 x

El R = slope of the curve = - - E = 17.22 x lo3 cal g mol-'

k = k , exp (- E / R T ) k = 2.67 x exp (- 17.22 x 103/RT)

TABLE 2 Variation of Reaction Rate Constant with Temperature

Temperature k (dm3 min-' g cat.?) ( K )

548 598 623 648

0.4 17 0.560 1.180 2.000

been used to determined the rate equation. The catalyst quantity was varied between 1 and 5 g keeping W/

270 V. K. Raizada et al.

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