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Applied Catalysis A: General 340 (2008) 1–6

Review

1-Pentene isomerization over SAPO-11, BEA and AlMCM-41

molecular sieves

Carmen M. Lopez *, Leyda Ramırez, Virginia Sazo, Vanessa Escobar

Universidad Central de Venezuela, Facultad de Ciencias, Escuela de Quımica, Centro de Catalisis, Petroleo y Petroquımica,

Apartado 47102, Caracas 1020-A, Venezuela

Received 14 August 2007; received in revised form 18 January 2008; accepted 8 February 2008

Available online 16 February 2008

Abstract

The skeletal isomerization of 1-pentene at atmospheric pressure and 89 mm Hg 1-pentene partial pressure was investigated over SAPO-11,

BEA and AlMCM-41 molecular sieves. The effect of the reaction temperature between 250 and 400 8C was studied. Acidity of the catalysts seems

to play a more important role than the pore size in the reaction conditions used.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Skeletal isomerization; 1-Pentene; Molecular sieves; Zeolites

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1. Introduction

The reduction in sulphur and olefin content in gasoline may

be one of the best ways to improve automobile emissions. The

components of the gasoline pool in a refinery vary from case to

case. A post-treating process becomes a desirable schedule for

achieving sulphur and olefin reduction in the pool, however, the

olefin reduction will lead to octane loss. In this sense, skeletal

isomerization can be an alternative reaction to upgrade refinery

and petrochemical feed streams. Especially, the isomerization

of 1-butene to isobutene has been the subject of many

investigations [1–5], because isobutene is the alkene precursor

in the synthesis of methyl tertbutyl ether (MTBE). Among the

higher alkenes, n-pentenes are an interesting feedstock. The

* Corresponding author. Tel.: +58 212 6935981.

E-mail address: cmlopez@ciens.ucv.ve (C.M. Lopez).

0926-860X/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.02.005

skeletal isomerization of n-pentenes to isopentenes is a topic of

increasing interest, because of the demand for isoolefins to ter

amyl methyl ether (TAME) synthesis and for alkylation

reactions [6–10].

Experimental studies in skeletal isomerization of olefins

indicated that this reaction proceeds via either monomolecular

or bimolecular (dimerization-cracking) mechanisms. Mono-

molecular skeletal isomerization was found to be more

common [11–14]. Indeed, n-pentene and higher hydrocarbons

can isomerize on zeolites via a monomolecular path, which

includes an alkoxy intermediate and a cyclopropane ring

[15,16]. Other side reactions such as dimerization–oligomer-

ization, cracking, hydride transfer and coking can occur. It is

generally accepted that the acid strength required for these

reactions decreases in the order: cracking � oligomeriza-

tion > skeletal isomerization� double bond isomerization

[17]. According to this, moderate acid sites will be more

selective towards skeletal isomerization.

C.M. Lopez et al. / Applied Catalysis A: General 340 (2008) 1–62

Catalyst pore topology, and acid site strength, density and

location are factors that can control the skeletal isomerization

of alkenes. It is often considered that the pore structure plays a

more important role than acidity. The most suitable zeolites are

the 10-membered ring molecular sieves with pore diameters

between 4 and 6 A [18–20]. However, Seo et al. [21] have

reported a high selectivity for isobutene with mesoporous

proton and metal containing MCM-41. On other side,

Trombetta et al. [22] have investigated the catalytic activity

of amorphous silica-alumina, HZSM-5 and H-ferrierite (FER)

zeolites in the skeletal isomerization of 1-butene. The Bronsted

surface acidity correlates well with the catalytic activity in 1-

butene isomerization. The selectivity to isobutene follows

nearly an inverse trend except for FER. According to what has

been mentioned above, it can be considered that the relationship

between acidity and catalyst pore topology with the activity and

selectivity is not clear and is matter of debate.

The main objective of this work is to compare the activity

and selectivity to iso-pentenes of microporous and mesoporous

molecular sieves in the isomerization of 1-pentene. The

influence of acidity and pore size was investigated.

2. Experimental

The acid catalysts used consisted of HDBEA, SAPO-11 and

AlMCM-41 molecular sieves. The starting BEA zeolite was a

commercial sample. First, the starting zeolite was ion-exchanged

with a 3 M ammonium acetate solution for 3 h, under continuous

stirring at 60 8C using 20 ml of solution per gram of zeolite. Then

the exchanged zeolite was calcined in flowing air at 550 8C for

4 h to obtain the hydrogen form (HBEA). HBEA was then

refluxed with 1 M HCl at room temperature for 8 h, using 10 ml

of solution/g of HBEA. After the acid treatment, the sample was

washed free of chloride, dried overnight at 100 8C and calcined

statically under ambient atmosphere at 550 8C, for 15 h. This

product was named HDBEA.

The synthesis procedure for SAPO-11 has been reported

elsewhere [23]. The gel molar composition was Al2O3:P2O5:D-

PA:0.3SiO2:50H2O, the crystallization temperature was 200 8Cand crystallization time was 4 h.

The hydrothermal synthesis of AlMCM-41 mesoporous

material was carried out at 90 8C using a gel with a

molar composition of SiO2:0.26Na2O:0.026 NaAlO2:0.124

CTMABr:135 H2O. Cethyltrimethylammonium bromide

(CTMABr, from Aldrich) was used as template, tetraethylortho-

silicate (TEOS, from Aldrich) as silica source, and sodium

aluminate (made in the laboratory, 49 wt% Al2O3) was added to

the reactant mixture as aluminium source. Sulphuric acid was

added by drops under stirring until the pH became 11.0. The

synthesis mixture was heated at 90 8C for 12 h and then cooled to

room temperature. The precipitated solid was filtered, washed

and dried at 60 8C for 24 h. To remove surfactant, the dried solid

was first heated under nitrogen flow at 250 8C for 3h, and then

calcined under air flow at 550 8C for 8 h. Sodium ions were

exchanged with a 0.5 M ammonium acetate solution at 60 8Cunder agitation for 3 h. H-form AlMCM-41 (HAlMCM-41) was

obtained by calcination in air stream at 500 8C for 8 h.

X-ray diffractograms (XRD) of the as prepared solids were

recorded with a Philips diffractometer PW 1730 Phillips using

Cu Ka radiation operated at 30 kV and 20 mA, and scanning

speed of 28 2u/min. Chemical analysis for Al, P and Si of the

calcined samples were performed using atomic emission

spectroscopy with a source of plasma inductively coupled.

Samples were previously fused with lithium metaborate and

dissolved in dilute nitric acid before analysis. N2-specific

surface areas (SSA) were obtained on Micrometrics 2200

equipment at liquid nitrogen temperature. All the samples were

pre-treated at 350 8C under vacuum conditions overnight.

The total acid site density and the acid strength distribution

of the catalysts were measured by temperature programmed

desorption of ammonia (TPDA), using a Micromeritics TPD/

TPR 2900 analyzer. The total acidity was obtained by

integration of the area under the curve. This curve was fitted

using three peaks, which were classified as weak, moderate and

strong acidity depending on the desorption temperature. It was

a convenient way to categorize the acid strength distribution

obtained by this method.

1-Pentene transformation was carried out under atmospheric

pressure at temperatures ranging from 250 to 400 8C in a

continuous fixed-bed reactor. 1-Pentene was introduced in the

reactor by the gas flow saturation method. The feeding gas

mixture consisted of N2 (15 ml/min) with a partial pressure of

1-pentene of 0.12 atm. To reach this pressure, N2 was passed

through a glass vessel with 1-pentene kept to�19 8C in an ice–

salt mixture. The mass of catalyst was about 0.1 g (weight

hourly space velocity WHSV = 3.5 h�1). Before the reaction

test, the catalyst was activated with a nitrogen flow (30 ml/min)

at 500 8C for 2 h. The product analysis was done after 30 min

on-stream by on-line chromatography using a Hewlett-Packard

5890A. A fused silica KCl/Al2O3 column was used for

separation purposes.

The total conversion (X) of 1-pentene was calculated

according to Eq. (1)

X ¼P

Ai � ðA1-pentene þ Acis-2-pentene þ Atrans-2-penteneÞPAi

� 100

(1)

where Ai is the corrected chromatographic area for a particular

compound, used to express the conversion and selectivity as

molar percentages.

Since the double bond isomerization is much faster than

skeletal isomerization the three isomers of linear pentenes

(1-pentene, cis-2-pentene and trans-2-pentene) were merged to

one component, and the three isomers of branched pentenes (3-

methyl-1-butene, 2-methyl-2-butene and 2-methyl-1-butene)

were also grouped to one component, in agreement with the

reaction network proposed by Sandelin et al. [24].

For a reaction product, or a set of products, the selectivity (S)

was defined according Eq. (2)

Sp ¼ApP

Ai � ðA1-pentene þ Acis-2-pentene þ Atrans-2-penteneÞ

� 100: (2)

C.M. Lopez et al. / Applied Catalysis A: General 340 (2008) 1–6 3

The products were grouped following Hochtl et al. [17] sug-

gestion:

� S

keletal isomers: adding up the corresponding corrected

areas of (Ssk) 3-methyl-1-butene (3M1B), 2-methyl-2-butene

(2M2B) and 2-methyl-1-butene (2M1B).

� L

ower C5 (S<C5) products: adding up the corresponding

corrected areas of the C1–C4 products.

� S

aturates C5 (SC5sat): adding up the corresponding areas of

iso-pentane and n-pentane.

� H

Fig. 3. XRD diffractograms for BEA, HBEA and HDBEA zeolites.

igher C5 reaction products (S>C5): sum of the corrected

areas of C6–C8 products.

3. Results and discussion

Figs. 1 and 2 show the XRD patterns of the synthesized

SAPO-11 and calcined AlMCM-41, respectively. The diffrac-

tion patterns, agreed well with those reported previously

[17,21]. Fig. 3 shows the XRD diffractograms of commercial

BEA, HBEA and HDBEA zeolites. It can be observed, that

there are not appreciable changes in the XRD patterns. The

nitrogen adsorption isotherm of calcined AlMCM-41 was type

IV in the IUPAC classification. A narrow pore size distribution

with a mean pore size of 28 Awas determined from the nitrogen

isotherm using the BJH model. When combining these results

with the XRD results (unit cell parameter of 44 A), the pore

wall thickness for AlMCM-41 was 16 A.

Table 1 shows the main characteristics of the solids after

calcination under a flow of air. The SSA of the solids lie within

Fig. 1. XRD diffractogram for SAPO-11.

Fig. 2. XRD diffractogram for AlMCM-41.

the range previously generally reported in the literature [17,21].

The acid treatment of HBEA caused 60% dealumination: Si/Al

molar ratio increases from 15 for HBeta to 41 in HDBEA,

retaining more than 90% crystallinity.

The amount and strength of the acid sites for the different

molecular sieves determined by TPD of ammonia are shown in

Table 2. The ammonia desorption peak temperature was used to

characterize acid strength. Deconvolution of TPDA profiles

resulted in three distinct types of acid sites in the ranges 250–

280, 320–370 and 470–510 8C. These peaks have been

described by Sakthivel et al. [25] as follows: the first and

second peaks referred as type (ii) and (iii), are attributed to

moderate and strong structural Bronsted acid sites, respectively.

The high-temperature peak, designated as type (v) is caused by

ammonia desorbed from strong acid Lewis sites, according to

Kosslick et al. [26]. The order of total acid amounts was

HDBEA > AlMCM-41 � SAPO-11. The order of moderate

Bronsted acid amount was AlMCM-41 � SAPO-11 >HDBEA

whereas the order of strong Bronsted acid amount was

HDBEA > AlMCM-41 � SAPO-11. Finally the order of

high-temperature acid site was AlMCM-41 > SAPO-

11 > HDBEA. According to Sakthivel et al. [25], type (v)

acid sites are not responsible for the catalytic activity in acid

catalyzed reactions.

The skeletal isomerization of 1-pentene over acid catalyst

usually occurs accompanying direct cracking of pentene,

dimerization (and oligomerization) followed by cracking,

hydride transfer and coke deposition. It is generally established

that the linear pentenes act like one reactant while the branched

pentene isomers appear as primary products [17,24]. The main

products were iso-pentenes; small fractions of pentane,

isopentane and C1–C4 products were also obtained.

Table 1

Specific surface area (SSA) and chemical composition of the catalysts

Catalyst Surface area (m2/gcat) Chemical composition

SAPO-11 175 (Al0.50 P0.45 Si0.05)O2

HDBEA 593 H1.5(Al1.5 Si62.5 O128)

AlMCM-41 980 (SiO2)35(AlO2)1

Table 2

Acid distribution and acid strength of different molecular sieves

Catalyst Amount of acid sites (acid strength) (mmol/g) (8C)

Total acidity

(mmol NH3/gcat)

Weak acidity (250–280 8C)

(mmol NH3/gcat)

Moderate acidity (320–370 8C)

(mmol NH3/gcat)

Strong acidity

(470–510 8C) (mmol NH3/gcat)

SAPO-11 0.313 0.087 (252) 0.161 (319) 0.065 (467)

HDBEA 0.470 0.05 (255) 0.400 (365) 0.020 (507)

AlMCM-41 0.370 0.09 (272) 0.190 (351) 0.090 (492)

Fig. 5. Selectivity to skeletal isomers-deactivation curves for HDBEA at the

reaction temperatures studied.

C.M. Lopez et al. / Applied Catalysis A: General 340 (2008) 1–64

A blank experiment was carried out with only glass wood in

the reactor, and it was found that with 0.12 atm of 1-pentene, a

total flow of 15 ml/min, and between 250 and 400 8C, the

maximum level of pentene conversion was 5% at 400 8C. This

result evidences that the contribution of homogeneous reactions

is not appreciable under these reaction conditions.

Selectivity to skeletal isomers-deactivation curves are

shown in Figs. 4–6 for the catalysts, SAPO-11, HDBEA and

AlMCM-41, at 250, 300 and 400 8C. In Fig. 4, for SAPO-11

no deactivation was observed at 300 and 400 8C. The time on

stream (TOS) behaviour at 250 8C was different. Conversion

decreased from 19% to about 5% after 240 min TOS. The

selectivity to skeletal isomerization increased from 50 to

70%, selectivity to other products was very low. For

temperatures of 300 and 400 8C, conversion increased from

16% at 300 8C to 46% at 400 8C. The selectivity to skeletal

isomerization increased from 85% at 300 8C to 95% at

400 8C. As observed when the reaction temperature was

250 8C, selectivity to other products was very low. The higher

deactivation observed at 250 8C, can be attributed to a more

pronounced adsorption and coke formation at lower

temperature. This coke deposits can produce a shape-

selective effect which might prevent formation of C5 saturate

products. At higher temperatures, adsorption of olefins can be

reduced, which might lead to a decrease in the undesired

bimolecular reactions. Since isomerization of n-pentene can

take place via a monomolecular process, bimolecular

reactions that can lead to by-products and consequently

coke are minimized at 300 and 400 8C.

Fig. 4. Selectivity to skeletal isomers-deactivation curves for SAPO-11 at the

reaction temperatures studied.

A similar behaviour in function of reaction temperature was

observed for HDBEA and AlMCM-41, as showed in Figs. 5 and

6, respectively. In these cases, deactivation at 250 8C was lower

compared to SAPO-11; the differences in pore structure of

these solids could explain these results.

The effect of temperature on the catalytic activity and

product selectivity of the catalysts is best depicted in Fig. 7. For

all catalysts evaluated, the conversion increased with an

increase in temperature from 250 to 400 8C. For SAPO-11 and

AlMCM-41 the selectivity to skeletal isomerization increased

with the increase of reaction temperature; whereas for HDBEA,

Fig. 6. Selectivity to skeletal isomers-deactivation curves for AlMCM-41 at the

reaction temperatures studied.

Fig. 7. The conversion of 1-pentene and selectivity to iso-pentenes of the

catalysts evaluated as a function of reaction temperature.

Fig. 8. Distribution of normalized pentenes on (a) SAPO-11, (b) HDBEA and

(c) AlMCM-41 as a function of reaction temperature compared with thermo-

dynamics equilibrium.

C.M. Lopez et al. / Applied Catalysis A: General 340 (2008) 1–6 5

this selectivity decreases with the increase of temperature, due

to an increase in the selectivity to C1–C4 products was

observed. The observed order in conversion goes according

with the acidity of solids; HDBEA, with higher total acidity and

strong Bronsted acid sites had the highest conversion, and acid

sites can be activated towards production of C1–C4 products at

300 and 400 8C.

The similar behaviour observed for SAPO-11 and AlMCM-

41 strongly suggests that differences in pore diameter produce

few changes in the catalytic properties of the solids. The

observed differences seem to be produced mainly by the

acidity differences. This way, HDBEA with an open structure

showed a different behaviour compared to AlMCM-41, and

this latter showed a similar behaviour to that of SAPO-11. This

result could be explained invoking the monomolecular

pathway for the skeletal isomerization of 1-pentene over the

studied catalysts. In this case, the acidity density of the solids

could be adequate to avoid the occurrence of bimolecular

reactions that lead to the heavy products, independently of the

pore size.

Normalized distribution of pentenes at 250, 300 and

400 8C for SAPO-11, HDBEA andAlMCM-41, compared

with the equilibrium composition reported by Maurer [8] is

showed in Fig. 8. As seen in this figure, at 400 8C a mixture of

pentenes obtained with a composition similar to that of

equilibrium was obtained. At lower temperature (250 and

300 8C), a different composition was observed. This result

suggests that an increase in reaction temperature leads to a

thermodynamic control of the reaction for the studied

catalysts. In a previous work carried out in our laboratory

using SAPO-11 as catalysts [27], a good approach to the

equilibrium at 300 8C, space velocity of 1.32 h�1 and N2/1-

pentene molar ratio of 1.96 and no dependence of the

conversion of 1-pentene towards skeletal isomerization with

reaction temperature was observed. Changing the space

velocity and the N2/1-pentene ratio to the conditions used in

the present work seems to result in a change of the reaction

regime. Then at 250 and 300 8C the kinetic reaction regime is

more probable, whereas at 400 8C, the reaction seems to be in

a thermodynamic regime.

4. Conclusions

The aim of the present work was to give a qualitative

description of the effect of the pore size and acidity of acid

molecular sieves in the skeletal isomerization of 1-pentene. We

have found that under the reaction conditions studied, the

acidity of the catalysts seems to play a more important role than

the pore size. A monomolecular pathway for the skeletal

isomerization of 1-pentene could explain this result. The

skeletal isomerization seems to be kinetically controlled at 250

and 300 8C, whereas at 400 8C a thermodynamic regime is

C.M. Lopez et al. / Applied Catalysis A: General 340 (2008) 1–66

possible, however more work will be required in order to make

a complete kinetic analysis.

Acknowledgement

This work was supported by CDCH-UCV Projects 03-12-

5419-2004/2006 and 03-00-5738/2004.

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