Catalytic activity of copper Y zeolite towards dehydration of tertiary butyl alcohol

T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved

729

Catalytic activity of copper Y zeolite towards dehydration of tertiary butyl alcohol. Sachin B. Kakodkar, Sajo P. Naik, Krishnaswamy Narasimhan and Julio B. Fernandes. Department of Chemistry, Goa university, Taleigao Plateau, Goa 403202, India. ABSTRACT: Reduced and unreduced copper Y zeolites are prepared from sodium Y zeolites (NAY) and their catalytic activity towards dehydration of tertiary butyl alcohol (t-BuOH) is evaluated in relation to pure NaY and HY zeolites. They are characterised by chemical analysis, XRD, TG/DTA and IR spectroscopy. The catalytic activity followed the order NaY < CuY < HY < Cu(R). The activities of the catalyst could be correlated to the corresponding acid strength determined by temperature programmed desorption of ammonia. 1.

INTRODUCTION:

The Y zeolites have been identified as active catalysts for various reactions such as isomerisation, amination, alkylation and deamination [1-4]. Cu +1 containing Y zeolites have been found particularly useful to catalyse Diels - Alder cycloaddition [5-6]. Dehydration of t-BuOH has been suggested as a model reaction for estimating the zeolite acidity [7]. The dehydration of t-BuOH has received particular attention as tertiary species gives the most stable carbonium ion. The dehydration of alcohols by zeolites has been extensively studied [8] and it has been found that even weaker acid sites can promote the dehydration of alcohol

[9]. In the present investigation pure and reduced form of CuY zeolite were prepared, characterised and their catalytic activity towards dehydration reaction of t-BuOH was compared with that of NaY and H-Y zeolite. The dehydration activity is correlated to the Broensted acid density of various catalysts. 2.

EXPERIMENTAL:

NaY zeolite obtained from UCIL (India) was used for the synthesis of CuY zeolite. The NaY zeolite was ion exchanged with various concentrations of aqueous copper sulphate ( C u S O a . 5 H 2 0 ) viz. 0.25 M, 0.5 M, 1.0 M and 1.5 M. The samples were dried in air at 3930 K for 2 hours and the copper content of samples was estimated by EDTA titration. CuY obtained by exchange with 0.5 M Copper sulphate was taken as the representative sample for spectroscopic and catalytic studies and a part of this sample was reduced with hydrogen at 4230 K for 2 hours to get the reduced form i.e. CuY (R). HY was prepared by ion exchange of NaY with NHaNO3 twice, to get the ammonium form of zeolite, the ammonium

730 form was calcined at 8230 K for 12 hours. The CuY sample was characterised by XRD (Rigaku) and TG/DTA (STA 1500 PL Thermal Sciences ). The IR. Spectra of NaY, HY, CuY and CuY(R) were recorded in the range of 4500-400 cm 1 using shimadzu 8101A FTIR. Temperature programmed desorption was carried out in temperature range 313-6730 K. 2.1

CATALYTIC MEASUREMENTS: The t-BuOH reaction was carried out in a vertical continuos flow fixed bed reactor at atmospheric pressure. 1 gm of catalyst was charged into the reactor and activated at 7230 K under an air stream for 4 hours and then brought to reaction temperature of 5230 K. t-BuOH was passed into the reactor at a contact time of 2.75 sec. The rate of formation of the dehydration product isobutene was analysed in an Orsat apparatus and the percentage conversion of t-BuOH was calculated. 3

R E S U L T S AND DISCUSSIONS:

From Table 1, it is seen that the extent of ion exchange does not significantly depend on the concentration of aqueous copper sulphate solution. XRD pattern of CuY is given in Fig. 1. From the TG/DTA pattern of CuY [Fig. 2], it is seen that molecular water is lost in 3 stages. The water loss between room temperature to 383~ is believed to be physisorbed water, between 383 - 473~ could be molecular water evolving out of various zeolite pores as well as some water coming out due to dehydroxylation where as small percentage of water loss in temperature range 673 - 1073~ could be due to the dehydroxylation of the zeolite or loss of some lattice oxygen. The infra red absorption frequencies and their assignments for various catalysts are presented in Table 3. There is no significant difference in the IR. Spectra of NaY and HY. However the absorptions at 570 cm l , 680 cm l and 765 cm 1 in copper exchanged zeolites CuY and CuY(R) are either very weak or missing all together. This could be due to distortion of the hexagonal double ring and change in symmetry around the Cu +2 ion. Similar conclusions have been reached by other workers on the basis of ESR spectra [ 10]. The percentage conversion of t-BuOH over various catalysts and the acidity of catalysts between 513-6730 K is given in Table 4. The catalytic activity follows the order NaY < CuY < HY < CuY(R). The relatively low activity of NaY is because of its very low Broensted acidity due to the presence of strongly basic sodium ions. The acid zeolite HY is obtained by the decomposition of the corresponding ammonium zeolite [ 11 ]. NH4+Y + 3/2 02 = N2 + 3H20 + H+Y The protons thus formed as above are present as surface hydroxyl groups and is a cause of Broensted acidity in zeolites. This accounts for the high acidity and catalytic activity of HY. CuY showed a better activity than NaY. This as probably due to Cu +2 ions which constitute Lewis acid sites generating Broensted acid sites during activation treatment of CuY carried out prior to the dehydration reaction. The precursor for the above protons and hence the new Broensted sites could be the molecular water that gets evolved during the activation treatment as seen from TG/DTA profile. C u +2 + H 2 0 = CuOH + + H + CuY(R) shows better activity as compared to CuY. This confirms to the acid strength of the two catalysts between 513 - 613 o K. CuY when reduced with hydrogen at 423~ undergoes the reaction [ 12].

731 Table 1" Percentage of copper in CuY Zeolites prepared from different concentrations of copper sulphate. Concentration of aqueous copper sulphate (M) 0.25 0.5 1.0 1.5

Percentage of copper in exchanged Samples 4.650 4.520 4.320 4.620

Table 2: TG/DTA results of CuY Temperature range (OK)

TG % weight loss

298-383 383-473 473-673 673-1073

9 7 6 2

DTA peak Temperature(~ 50 85 175 250

Table 3" Frequencies ( cm 1) of the main absorption in the infra red spectra of various zeolite samples. Sample

NAY[14] NaY CuY CuY(R) HY

Assym. Ext.

Stretch Int.

1130 1075 1075 1150 1160

1018 990 1018 1075 1075

Symm. Ext. 787 765 . . . . vvw 790

Stretch Int.

Double Six ring

T-O bend

717 680

577 570 vvw vvw 575

497 465 465 465 465

-730

vvw = very very weak absorotion. Table 4: Conversion of t-BuOH over various catalysts at temperature = 5230 K, contact time = 2.75 sec., TOS = 60 min. and their acidity in meq/g between 513 - 6730 K. _

Catalyst

NaY CuY CuY(R) HY

Percentage conversion of t-BuOH 60 74 90 78

Acidity between 513-7130 K,meq/g 0.05873 0.3070 0.6678 0.6802

732

7 O0

-

>OO 3 5 0 Z IJJ I-Z

I 17.0

I 37.0 2 e

FIG. I .

XRD

I 57.0

(Deg)

PATTERN

OF

CuY

-I0

105 -

-8

TG

I00-

-6 95-4

IZ 90LIJ 0 12:: bJ85n

-2 -0

80-

--2 RESIDUE 77. 14 %

750

,~o

2bo

s6o

~o DEG

FIG. 2 .

TG/DTA

~

~o

7bo

C PATTERN

OF

CuY

860

--4 30

03 h..I 0 > 0 nU :E

733

El

I--

I

i

I

I

4600.0 4000.0

I

I

3000.0

i

WAVE

FIG. 3

IR

!

2000.0

1500.0

NUMBER

SPECTRA

OF

I000.0

400.0

-I

(cm)

VARIOUS

CATALYSTS

1.3(

~

o-----o

Na Y

;

Cu Y

=

CuY(R) \ 0.9~3r

E >- O.7F-a o
0.3-

0.1 ,

(273.0)

313

=

353

,

393

433

473

l

TEMPERATURE K FIG. 4

TEMPERATURE

l

513

553

v

,

593

-

i

633

a

-

673

~-

PROGRAMMED DESORPTION OF NH 3 FOR VARIOUS CATALYSTS

734 C u +2 4- 1/2 H2 = Cu + + H + and hydrogen ion formed is consumed by the reaction H + + ZO l = Z-OH, where Zrepresents zeolite framework; thus generating Broensted sites which could be responsible for enhanced activity. Cu +2 ions in the dehydrated catalysts, believed to be present in S(I) and S(I') sites are considered mobile and may represent active catalyst sites[ 13]. It is therefore conceivable that protons generated upon hydrogen reduction of CuY are more labile or accessible for the dehydration reaction. This could be the reason for the enhanced activity of Cu(R) over HY. . Similar trend in dehydration activity is observed when ethanol (EtOH) was used. However when reaction of EtOH and ammonia vapour was carried out in presence of air at 723 o K the dehydrogenation product acetaldehyde predominated (- 45%) with small amount of dehydration product i.e. ethene (less than 15%). Our preliminary studies with ethanol indicate that the dehydrogenating activity of NaY, CuY and HY was much less under similar experimental conditions. This seems to suggest that NH3 has partially blocked the Broensted acid sites while Cu l+ in CuY acted as dehydrogenating centers. CuY reduced with hydrogen at 673 o K showed very less dehydrogenating activity (- 20%) as at this temperature Cu +2 state[ 12] which was not active for dehydrogenation.

4.

.

.

.

5.

CONCLUSIONS: The degree of ion exchange of NaY is not significantly dependent on the concentration of the aqueous copper sulphate solution. IR. Spectra of NaY and HY are similar. However the adsorption in CuY and CuY(R) in the region 800-500 cm l are either very weak or missing. This is attributed to the distortion of the hexagonal double ring and change in symmetry around the Cu 2+ ion. TG/DTA profile of CuY shows water loss from room temperature to 1073~ Water loss below 573~ is mainly molecular and beyond 673~ due to dehydroxylation of surface hydroxyls. Catalytic activities of the sample follow the order; NaY < CuY < HY < CuY(R). The high catalytic activity of CuY and CuY(R) is due to conversion of Lewis sites to Broensted sites during activation of CuY and due to generation of protons during reduction of Cu +2 by hydrogen respectively. The relatively low activity of NaY is due to very low Broensted acidity. High activity of HY is due to very large value of Broensted acidity.

ACKNOWLEDGMENT: We thank DST (New Delhi) for financial support. REFERENCES: .

2. 3. 4.

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735 ,

,

7. ,

9. 10. 11. 12. 13. 14.

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