ORIGINAL PAPER

Intrinsic Kinetics of Ethanol Dehydration Over Lewis AcidicOrdered Mesoporous Silicate, Zr-KIT-6

Qing Pan • Anand Ramanathan • W. Kirk Snavely •

Raghunath V. Chaudhari • Bala Subramaniam

� Springer Science+Business Media New York 2014

Abstract Lewis acidic Zr-KIT-6 catalyst was tested for

ethanol dehydration. Under the reaction conditions studied

(T = 300–380 �C, P = 1 atm, Pethanol = 5 % in N2), Zr-

KIT-6 materials showed high ethylene selectivity (*80 %)

with stable activity (60 h). The activation energy for eth-

anol dehydration to ethylene, estimated from intrinsic rate

constants normalized with respect to the Lewis acid sites,

was approximately 79 ± 1 kJ/mol.

Keywords Ethanol dehydration � KIT-6 � Zirconium �Lewis acid � Kinetics

1 Introduction

Ethylene and propylene are considered to be among the

most important raw materials in the petrochemical indus-

try, and ethylene production capacity is one of the indi-

cators used to assess the development of the petrochemical

industry in countries [1, 2]. Currently, *99 % of the global

ethylene is produced by steam cracking of hydrocarbons,

with petroleum crude or natural gas as raw materials [2].

Given the increasing demand for both fuels and chemicals,

fossil fuel resources are being rapidly depleted. Alternative

raw materials for the production of ethylene and propylene

are therefore being actively sought. Presently, an important

technology to convert renewable biomass resources into

liquid fuels is the production of ethanol (the so-called

bioethanol) by fermentation of carbohydrates [3]. Much

work has been done on ethylene production by catalytic

bioethanol dehydration [1, 4, 5]. The high oxygen content

(40–45 wt%) [3] in biomass renders the production of

hydrocarbons, either fuels or chemicals, rather challenging.

Catalytic dehydration is a preferred route for biomass

deoxygenation because it preserves the carbon content in the

biomass and generates only water as product. Based on lit-

erature reports, the rather high temperatures (300–500 �C)

required for dehydration and catalyst deactivation appear to

be the main challenges to overcome [2].

Several studies have been reported for the dehydration

of short chain alcohols on microporous catalysts (such as

various H-form of zeolites and silica-alumina, SAPOs and

alumina based catalysts) with high conversions and olefin

selectivity being reported at relatively mild temperatures

(200–350 �C) [6–10]. However, catalyst deactivation

caused by coke formation hinders commercial application.

In recent years, mesoporous materials have attracted much

attention as catalysts for processing long-chain alcohols

such as dehydration of sugars [11, 12]. Haishi et al.

reported dehydration of ethanol, 1-propanol, 1-butanol, and

2-propanol over Al-MCM-41with nearly 100 % yields of

the corresponding olefins at 430, 400, 350 and 280 �C

respectively [13]. Interestingly, La incorporation into SBA-

15 was also reported to enhance the ethanol dehydration

activity with 65 % ethanol conversion and 40 % ethylene

selectivity at 500 �C [14]. However, most of these studies

were not aimed at obtaining intrinsic kinetic data, essential

to obtain fundamental insights into the reaction

mechanism.

Recently, we reported that exclusively Lewis acidic

Zr-containing large pore cubic Ia3d mesoporous silicates,

Q. Pan � A. Ramanathan � W. Kirk Snavely �R. V. Chaudhari � B. Subramaniam

Center for Environmentally Beneficial Catalysis, The University

of Kansas, Lawrence, KS 66047, USA

Q. Pan � R. V. Chaudhari � B. Subramaniam (&)

Department of Chemical and Petroleum Engineering,

The University of Kansas, Lawrence, KS 66045, USA

e-mail: bsubramaniam@ku.edu

123

Top Catal

DOI 10.1007/s11244-014-0311-7

Zr-KIT-6, are highly active for dehydration of isopropanol

with high ([98.5 %) selectivity to propene [15]. System-

atic kinetic studies performed over Zr-KIT-6 catalysts with

various Zr loadings revealed that the observed rates were

proportional to the measured Lewis acid sites on the cat-

alyst, revealing a moderate activation energy of

*49 ± 1 kJ/mol. In the present work, we investigate the

application of Zr-KIT-6 catalysts for the selective dehy-

dration of ethanol, comparing the intrinsic kinetic param-

eters obtained from experimental data with those reported

in the literature.

2 Experimental Section

2.1 Catalyst Synthesis and Characterization

Solvents including dehydrated ethanol (99.5 %), acetoni-

trile (99.9 %), diethyl ether (99.9 %), and acetone (99.5 %)

were purchased from Fisher Scientific and used as

received. Zirconia nano-powder was purchased from

Sigma-Aldrich. Helium (ultra-pure grade), nitrogen

(industry grade) and air (industry grade) were purchased

from Matheson Linweld.

The Zr-KIT-6 material used in this study was synthe-

sized and characterized as reported elsewhere [15, 16].

Three samples with different Si/Zr ratios (100, 40 and 20)

were investigated for ethanol dehydration activity. Table 1

shows the properties of the samples. It was found previ-

ously that the catalysts contained predominantly Lewis

acidic sites [15].

2.2 Catalytic Dehydration Studies

A detailed schematic of the reactor setup was described in

our earlier paper [15]. Briefly, the ethanol dehydration

experiments were carried out in a continuous fixed-bed cat-

alytic reactor over Zr-KIT-6 catalysts (1.5 g, 250–700 nm)

in the 300–380 �C range at atmospheric pressure. A solution

of ethanol and acetonitrile (ACN, internal standard) with a

molar ratio of approximately 10:1 was fed to the reactor by

means of a HPLC pump at a typical flow rate of 0.1 cm3/min.

The pumped liquid mixture was vaporized by preheating to

150 �C and then mixed with flowing N2 (200–800 standard

cm3/min) in an in-line mixer. Downstream from the reactor,

the effluent stream containing the unreacted reactants and

products was maintained in the vapor phase (*160 �C) by

means of a heating cord and was sampled online for analysis

in a Hewlett-Packard 5890 Series II gas chromatograph. The

products were analyzed with a Phenomenex Zebron Phase

ZB-WAX capillary column (30 m 9 0.25 mm 9 0.25 lm)

and a flame ionization detector (Fig. 1).

3 Results and Discussion

3.1 Catalyst Performance

Steady conversion and selectivity values were observed in

approximately 2–4 h at each temperature. As shown in

Fig. 2, the steady-state conversion increased only slightly

(from 15 to 30 %) in the temperature range over all Zr-KIT-6

materials. The Zr-KIT-6 catalysts are predominantly Lewis

acidic (Fig. 3) and their acidities increase with Zr loading

Table 1 Properties of the Zr-KIT-6 catalyst samples

Zr-KIT-6 (Si/Zr)a Si/Zrb Zrb SBETc Vp, BJH

d dP, BJHe Total acidity

wt% m2/g cc/g nm NH3 mmol/g

100 92 1.6 980 1.65 9.3 0.19

40 39 3.8 881 1.42 9.3 0.40

20 23 6.2 810 1.07 9.3 0.49

a numbers in parenthesis represent Si/Zr ratio in synthesis gelb ICP-OES analysisc SBET = Specific surface aread VP,BJH = Total Pore Volume measured at 0.995 P/Po

e dP,BJH = BJH adsorption Pore Diameter

Fig. 1 Sample chromatogram of the effluent stream during ethanol

dehydration over Zr-KIT-6 (20), T = 260 �C, GHSV = 7,200 h-1,

p = 1 atm

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resulting in increased ethanol conversion. Further, the eth-

ylene conversion observed over commercial ZrO2 was not

significantly lower. However, Zr-KIT-6 materials showed a

higher ethylene selectivity (in the range of 60–80 %)

compared to commercial ZrO2 (around 40 %). This is

attributed to the enhanced Lewis acidity of the Zr-KIT-6

samples (0.19–0.49 mmol NH3/g) [15] compared to ZrO2

(0.14 mmol NH3/g) [16]. Clearly, the total acidity (based on

NH3-TPD) of ZrO2 is lower than Zr-KIT-6(100), which has

the lowest Zr loading (See Fig. 3a). This finding is also

confirmed from the FTIR spectra of adsorbed pyridine

(Fig. 3b), which not only show lower number of Lewis acid

sites on ZrO2 but also the presence of more weakly acidic

H-bonded pyridine species (*1570 and*1650 cm-1) and a

shoulder near 1550 cm-1 corresponding to Brønsted acid

sites or vibrations of surface water molecules [17] on ZrO2.

These peaks, however, are not clearly evident on the Zr-KIT-

6 materials. This could explain why, even though the ethanol

conversions are similar on both Zr-KIT-6 and ZrO2, the

selectivity profiles are quite different on ZrO2.

Figure 4 shows a 70 h extended run with Zr-KIT-6(100)

at 380 �C. The conversion and selectivity were steady for

the first 30 h. Between 40 and 60 h, the conversion slightly

increased, while the selectivity decreased. After 60 h, both

the conversion and selectivity dropped significantly. The

reason for this deactivation is unclear and is currently under

investigation. However, when the catalyst was calcined

after the 70 h run in flowing air at 400 �C overnight, it fully

regained its original activity suggesting that the deactiva-

tion may be due to formation of coke that was completely

removed by oxidation during the calcination process.

3.2 Kinetic Analysis

The stoichiometries for the dehydration to ethylene

(Reaction 1) and dehydration to diethylether (Reaction 2)

are as follows.

C2H5OH Að Þ ! C2H4 Bð Þ þ H2O Cð Þ ðReaction1Þ2 C2H5OH Að Þ ! C2H5�O�C2H5 Dð Þ þ H2O Cð Þ

ðReaction2Þ

GC analysis confirmed mass balance closure consider-

ing only these two reactions with no other products

0

5

10

15

20

25

30

35

290 300 310 320 330 340 350 360 370

Con

vers

ion,

%

Temperature,

35

45

55

65

75

85

95

290 300 310 320 330 340 350 360 370

Sel

ectiv

ity, %

Temperature, C

(a)

(b)

o

C o

Fig. 2 Effect of temperature on (a) ethanol conversion; (b) ethylene

selectivity.Symbols: diamond: Zr-KIT-6 (20); triangle: Zr-KIT-6

(40); circle: Zr-KIT-6 (100); square: commercial ZrO2. EtOH in

feed = 5 mol% in N2; catalyst loading = 1.5 g; GHSV = 7,200 h-1,

p = 1 atm

Fig. 3 (a) NH3-TPD profiles, and (b) FTIR spectra of adsorbed

pyridine, for Zr-KIT-6 catalysts compared with commercial ZrO2

Fig. 4 70 h stability test on Zr-KIT-6(100) at 380 �C EtOH in

feed = 5 mol% in N2; Catalyst loading = 1.5 g; GHSV = 7,200 h-1,

p = 1 atm

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detected. For developing the kinetic model, the two parallel

reactions were assumed to be first-order in substrate con-

centration. The steady state material balance equations are

derived as follows.

� dFA

dVc

þ rA ¼ 0 ð1Þ

where, � dFA

dVc¼ CA0

vgdXA

dVc

rA ¼ �ðke1CA þ ke2CAÞCA ¼ CA0

ð1� XAÞ

Substituting these expressions into Eq. 1 yields Eq. 2.

vg

dXA

dVc

¼ ke1ð1� XAÞ þ ke2ð1� XAÞ ð2Þ

For Reaction 1, vgdX1A

dVc¼ ke1ð1� X1A � X2AÞ, where,

X1A ¼ S1XA

For Reaction 2, vgdX2A

dVc¼ ke1ð1� X1A � X2AÞ, where,

X2A ¼ S2XA

Combining Eqs. 1 and 2 and integrating, the rate con-

stants for Reaction 1 and 2 are given by

ke1 ¼ �vgS1

Vc

lnð1� XAÞ ð3Þ

ke2 ¼ �vgS2

Vc

lnð1� XAÞ ð4Þ

where, FA = molar flow rate of ethanol (A), rA = rate of

formation of A, ke1 = effective rate constant for Reaction 1

(min-1), ke2 = effective rate constant for Reaction 2

(min-1), vg = volumetric flow rate at reactor P and T

(standard cm3/min), Vc = packed volume of catalyst

(cm3), XA = observed ethanol conversion at steady state,

S1 = selectivity toward ethylene, S2 = selectivity toward

diethyl ether.

Equation 3 was applied to calculate the rate constant for

ethanol dehydration. In order to assess the effect of

external mass transfer limitations, a series of experiments

were conducted over the most acidic catalyst sample, Zr-

KIT-6(20), at 360 and 380 �C, and at different GHSV

values at each temperature. As shown in Fig. 5, over Zr-

KIT-6(20), whereas the effective rate constants increased

with GHSV values at 380 �C in the tested range, they

reached a plateau at 360 �C at GHSV values beyond 7,000

h-1. Furthermore, the calculated internal effectiveness

factors [15] for the major reaction were [0.99 within the

tested GHSV range and temperatures (Fig. 6). Therefore, it

is concluded that on all the Zr-KIT-6 materials tested, both

external mass transfer limitations as well as intraparticle

Fig. 5 The dependence of effective rate constant (ke1) on GHSV

values for Zr-KIT-6(20) at atmospheric pressure

Fig. 6 Estimation of internal effectiveness factors (g) assuming

ethanol dehydration to ethylene as the major reaction over Zr-KIT-

6(20) at atmospheric pressure

2

2.5

3

3.5

4

4.5

5

5.5

1.55 1.6 1.65 1.7 1.75

lnk

e1/ m

in-1

(103/T), K-1

2.5

3

3.5

4

4.5

5

5.5

6

1.55 1.6 1.65 1.7 1.75

lnk

e1' /

min

-1

(103/T), K-1

(a)

(b)

Fig. 7 Estimation of activation energy of EtOH dehydration to form

ethylene from (a) intrinsic rate constants (ke1) based on catalyst packing

volume, and (b) intrinsic rate constants (k0e1) based on catalyst acidity

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123

diffusion limitations are eliminated above GHSV values of

7,000 h-1 at temperatures below 360 �C.

3.3 Estimation of Intrinsic Kinetic Energy

Ethanol dehydration reactions were conducted on the three

Zr-KIT-6 samples at temperatures ranging from 300 to

360 �C, employing a GHSV of 7,200 h-1 to eliminate mass

transfer limitations. As shown in Fig. 7a, the Zr-KIT-6

samples with higher Zr content yielded higher effective

rate constants (ke1) when such rate constants are normal-

ized with respect to the volume of the catalyst packing

(Eq. 3). The activation energy in each case was approxi-

mately 65–85 kJ/mol. When the rate constants were nor-

malized with respect to the total acidity of the respective

Zr-KIT-6 materials (Eq. 5), the rate constants at the various

temperatures virtually overlapped for all the catalysts

(Fig. 7b). The intrinsic activation energy based on acidity-

normalized rate constants was estimated to be approxi-

mately 79 ± 1 kJ/mol which is observed to be moderate

and comparable to the reported activation energies for

catalytic dehydration of ethanol over Al2O3 (53–78 kJ/

mol) [18] and microporous Fe-ZSM-5 (137.7–271.1 kJ/

mol) [19].

k0e1 ¼ �vgS1

AcWc

lnð1� XAÞ ð5Þ

where, k0e = intrinsic kinetic rate constant (min-1);

Wc = weight of catalyst used (g); Ac = total acidity of

catalyst [(cm3 NH3 at standard conditions)/g catalyst]

Similarly, the intrinsic activation energy for the unde-

sired reaction was based on acidity-normalized rate con-

stants (Eq. 6) was estimated to be approximately

46 ± 1 kJ/mol (Fig. 8).

k0e2 ¼ �vgS2

AcWc

lnð1� XAÞ ð6Þ

where S2 = selectivity toward diethyl ether at steady state.

4 Conclusions

Zr-KIT-6 materials displayed similar ethanol conversions

compared to commercial ZrO2 but higher selectivity to eth-

ylene due to enhanced Lewis acidity of the catalysts and the

near absence of Brønsted acid sites, compared to ZrO2. Also

Zr-KIT-6 showed a stable activity of ethylene formation for a

period of 60 h with significant deactivation occurring after

approximately 70 h. Taking into account the parallel reac-

tion involving dehydration to form diethyl ether, the intrinsic

rate constants on all the catalysts overlap when normalized

with the acid sites in the catalyst samples. The corresponding

intrinsic activation energy for ethanol dehydration to form

ethylene was found to be 79 ± 1 kJ/mol which is similar to

those either observed experimentally or computationally

predicted for Al2O3 and Fe-ZSM-5 catalysts.

Acknowledgments This research was supported in part with funds

from the U. S. Department of Agriculture/National Institute of Food

and Agriculture (USDA/NIFA) Award 2011-10006-30362.

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2

2.5

3

3.5

4

1.55 1.6 1.65 1.7 1.75

lnk

e2'/m

in- 1

(103/T), K-1

Fig. 8 Estimation of activation energy for EtOH dehydration to form

diethyl ether from acidity-normalized intrinsic rate constants (k0e2)

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123