This article was downloaded by: [Princeton University]On: 28 September 2013, At: 02:58Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Petroleum Science and TechnologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lpet20

Nanosized Platinum-Loaded H-ZSM-5 Zeolite Catalysts for n-HexaneHydroconversionA. K. Aboul-Gheit a , S. M. Abdel-Hamid a & D. S. El-Desouki aa Process Development Department, Egyptian Petroleum ResearchInstitute, Nasr City, Cairo, EgyptPublished online: 21 Sep 2011.

To cite this article: A. K. Aboul-Gheit , S. M. Abdel-Hamid & D. S. El-Desouki (2011) NanosizedPlatinum-Loaded H-ZSM-5 Zeolite Catalysts for n-Hexane Hydroconversion, Petroleum Science andTechnology, 29:22, 2346-2360, DOI: 10.1080/10916461003716640

To link to this article: http://dx.doi.org/10.1080/10916461003716640

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Petroleum Science and Technology, 29:2346–2360, 2011

Copyright © Taylor & Francis Group, LLC

ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916461003716640

Nanosized Platinum-Loaded H-ZSM-5 Zeolite

Catalysts for n-Hexane Hydroconversion

A. K. ABOUL-GHEIT,1 S. M. ABDEL-HAMID,1 AND

D. S. EL-DESOUKI1

1Process Development Department, Egyptian Petroleum Research Institute,

Nasr City, Cairo, Egypt

Abstract A series of nanosized platinum-containing catalysts was successfully

loaded on/in zeolite H-ZSM-5 via exchanging the zeolitic proton with platinum from Pttetramine dichloride complex. Another series of Pt/H-ZSM-5 catalysts was prepared

via wet impregnation of H2PtCl6 solution for comparison. The latter series was found

to produce lower Pt dispersion. Pt dispersion was determined by H2 chemisorption.Catalyst characterization via ammonia temperature-programmed desorption (TPD),

temperature-programmed reduction (TPR), and transmission electron microscopy(TEM) was examined for all catalysts and showed large differences in particle sizes.

The data on n-hexane reactions of the catalysts of both series confirmed the formationof Pt nanoparticles in the exchanged catalysts. The relatively lower density and

strength of acid sites acquired by Pt-exchanged catalysts contributed to this difference;stronger acid sites in the impregnated catalysts are in favor of hydrocracking reactions,

which inhibit isomerization selectivity.

Keywords H2PtCl6, hydroconversion, nanosize, n-hexane, Pt, Pt(NH3)4Cl2

1. Introduction

In general, in petroleum naphtha fractions, branched paraffins have a higher octane rating

than their respective n-paraffins; for example, linear hexane has an octane number of 25,

whereas 2,2-dimethylbutane (hexane isomer) has an octane number of 92. Therefore,

isomerization of linear paraffins is a process used to improve gasoline quality. This

process consumes very low hydrogen and occurs at relatively low temperatures.

The most widely applied alkane isomerization catalysts are composed of platinum

supported on chlorinated alumina. Zeolite-supported catalysts are characterized by their

outstanding tolerance to feed poisons such as sulfur and water, whereas chlorinated

alumina-supported catalysts suffer from extreme sensitivity to the majority of feed con-

taminants in addition to being corrosive (Weyda and Köhler, 2003; Ramos et al., 2008).

In naphtha stream hydroisomerization, catalysts containing Pt (Ramos et al., 2007) or Pd

(Ramos et al., 2005) on zeolites are frequently used. Pt/H-ZSM-5 catalyst was also used

in the hydroisomerization of alkanes with simultaneous saturation of benzene (Gopal and

Smirniotis, 2003). Nevertheless, the metal content in the hydroisomerization catalysts

has a crucial effect on the catalytic activity and selectivity as investigated by several

Address correspondence to Ahmed K. Aboul-Gheit, Process Development Department,Egyptian Petroleum Research Institute, P.O. Box 9540, Nasr City, Cairo 11787, Egypt. E-mail:aboulgheit2000@hotmail.com

2346

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2347

authors: in n-octane isomerization over BEA-agglomerated zeolite-based catalysts (de

Lucas et al., 2005); in n-hexane hydroconversion using hydrothermally modified Pt-MOR

catalysts (Aboul-Gheit et al., 1998, 2003, 2004, 2008); in n-hexane hydroconversion us-

ing ethylenediaminetetraacetic acid (EDTA)-dealuminated Pt/H-ZSM-5 catalysts (Aboul-

Gheit et al., 2003); in n-hexane hydroconversion using Pt, Pd, and Pt-Pd with various

contents of the metals on H-MOR or H-BEA zeolites (Aboul-Gheit et al., 2004); and in

n-hexane hydroconversion using Pt, Ir, and Pt-Ir on H-ZSM-5 zeolite and studying the

effect of hydrochlorination and hydrofluorination of these catalysts (Aboul-Gheit et al.,

2008).

The metal and acid sites in a catalyst require optimum balance to give maximum

isomerization efficiency. Deviation from the optimum balance of these functions may

seriously alter the isomerisation/hydrocracking balance (Guisnet et al., 1995; Kinger and

Vinek, 2001; Kinger et al., 2000, 2002).

2. Experimental

2.1. Preparation of the Metal/H-ZSM-5 Catalysts

The sodium ions in Na-ZSM-5 zeolite were five times exchanged with ammonium

nitrate (NH4NO3) molar solution; each time with a fresh solution for 8 hr at 70ıC.

The zeolite was then separated, washed with distilled water until free of the NOC

3,

dried at 110ıC overnight, and then calcined in air at 530ıC for 3 hr. The requisite

quantity of each platinum precursor was used in preparing the current catalysts (platinum

tetramine dichloride in case of exchanged catalysts or hexachloroplatinic acid in case of

impregnated catalysts). Excess distilled water was used as solvent for these precursors.

A small quantity of citric acid was added to enhance penetration of the solution in the

pores of the catalytic support (Aboul-Gheit, 1979). This preparation was left overnight

at room temperature then dried again overnight at 110ıC. The catalyst was calcined in

air for 4 hr at 530ıC and finally reduced at 500ıC in an H2 flow of 20 cm3 min�1 in a

flow reactor for 8 hr.

2.2. Reactor System and Product Analysis

A stainless steel tubular reactor system containing 0.2 g of a catalyst was used in all

hydroconversion runs. The reactor was heated in a metal block furnace thermostated to

˙1ıC. Hydrogen gas was used as a carrier gas and simultaneously as a reactant in the

reactions under study at a flow rate of 20 cm3 min�1. The n-hexane feed was introduced

into the reactor as pulses of 1 �L then the gaseous goes to a Perkin-Elmer Autosystem XL

gas chromatograph using a 15-m capillary column of Carbowax 20M bonded in fused

silica to be analyzed, a flame ionization detector (FID), and a Turbochrom Navigator

Programme.

2.3. Temperature-Programmed Reduction of Platinum Oxide in

Pt/H-ZSM-5 Catalysts

Temperature-programmed reduction (TPR) experiments were carried out using the cur-

rent catalysts containing 0.15, 0.30, and 0.60% Pt/H-ZSM-5 either prepared via cation

exchange (Figure 5) or via impregnation (Figure 6). These catalysts were first calcined at

530ıC for 3 hr in a flow of purified air of 30 cm3 min�1 in the TPR furnace. Reduction

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2348 A. K. Aboul-Gheit et al.

Figure 1. Hydroisomerization of n-hexane using (a) 0.15% Pt/H-ZSM-5, (b) 0.30% Pt/H-ZSM-5,

and (c) 0.60% Pt/H-ZSM-5 catalysts prepared by Pt exchange.

Figure 2. Hydroisomerization of n-hexane using (a) 0.15% Pt/H-ZSM-5, (b) 0.30% Pt/H-ZSM-5,

and (c) 0.60% Pt/H-ZSM-5 catalysts prepared by H2PtCl6 impregnation.

Figure 3. Isomerization activities in the conversion product using exchanged Pt/H-ZSM-5.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2349

Figure 4. Isomerization activities in the conversion product using impregnated Pt/H-ZSM-5.

Figure 5. TPR of exchange-prepared Pt/H-ZSM-5 catalysts.

Figure 6. TPR of impregnated Pt/H-ZSM-5 catalysts.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2350 A. K. Aboul-Gheit et al.

of the catalyst sample was then carried out at atmospheric pressure in a flowing gaseous

mixture containing 5.0% H2 in an Ar stream while increasing the reduction temperature

from ambient up to 600ıC using a programmed heating rate of 5ıC min�1. The H2 uptake

was indicated by use of a thermal conductivity detector and Ar as the reference gas.

2.4. Temperature-Programmed Desorption of NH3

The procedure adopted by Aboul-Gheit (1991, 1997) using differential scanning calorime-

try (DSC) for detecting desorption of presorbed ammonia from the catalyst was used.

Ammonia was primarily adsorbed in a silica tube furnace. After evacuation at 1:33�10�3

Pa while heating at 500ıC and subsequent cooling under vacuum to 50ıC, ammonia was

introduced through the catalyst at a flow rate of 50 cm3 min�1. The samples were

then measured in a DSC unit (Mettler TA-3000) using standard Al crucibles. Primarily

adsorbed ammonia on the current catalysts was desorbed in the DSC cell in a nitrogen

purge at a flow rate of 30 cm3 min�1. The heating rate was 10 K min�1 and the full-scale

range was 25 mW. The DSC thermograms obtained for the desorption of ammonia from

the 0.15, 0.30, and 0.60% Pt/H-ZSM-5 are shown in Figure 7. The DSC thermograms

obtained using the corresponding impregnated Pt/H-ZSM-5 catalysts (not given) were

similar, and their difference from those in Figure 7 is shown in Table 1.

2.5. Dispersion of Platinum in Catalysts

The dispersion of Pt in the catalysts, also called the fraction of metal exposed, was

determined via hydrogen chemisorption using a pulse technique similar to that described

by Freel (1972). The calcined catalysts were heated in the chemisorption furnace at

500ıC for 3 hr in a flow of oxygen-free hydrogen of 50 cm3 min�1. The flow was then

replaced with oxygen-free nitrogen gas for 2 hr at a flow rate of 30 cm3 min�1 at 500ıC

(degassing). The furnace was shut off and the catalyst was cooled to room temperature.

Hydrogen was then pulsed into the nitrogen carrier gas to saturation; that is, until the

appearance of hydrogen peaks equivalent to complete volumes of unchemisorbed pulses.

Figure 7. TPD-DSC thermograms for Pt-impregnated catalysts supported on H-ZSM-5.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2351

Table 1

TPD values for acid strength distribution in Pt/H-ZSM-5 catalysts

Pt impregnated Pt exchanged

Catalyst

NH3 �Hd ,

Jg�1

Peak

temperature,ıC

NH3 �Hd ,

Jg�1

Peak

temperature,ıC

H-ZSM-5 110 512

0.15% Pt/H-ZSM-5 96.8 499 90.5 480

0.30% Pt/H-ZSM-5 90.7 492 86.7 473

0.60% Pt/H-ZSM-5 86.9 484 81.2 469

The hydrogen uptake was calculated as hydrogen atoms adsorbed per total Pt atoms. The

surface area of the metal was also calculated, based on an area of one hydrogen molecule

of 12 � 10�16 m�2.

We used the above-mentioned technique to compare the chemisorption of platinum

metal in three Pt/H-ZSM-5 catalysts prepared via cation exchange and three other Pt/H-

ZSM-5 catalysts prepared via H2PtCl6 solution impregnation.

3. Results and Discussion

During n-hexane hydroconversion using Pt/H-ZSM-5 catalysts, hydroisomerization and

hydrocracking take place via catalytic bifunctionality, where Pt acts as the hydrogenation–

dehydrogenation sites, whereas the zeolite support provides the strong acid sites required

to produce the carbenium ion, whereupon molecular rearrangement and/or C-C splitting

take place. Indeed, the metal and acid sites in a catalyst require an optimum balance to

provide maximum isomerization efficiency, and deviation from the optimum balance of

these functions may seriously alter the isomerization/hydrocracking balance (Aboul-Gheit

et al., 1998, 2003, 2004, 2008).

3.1. Hydroisomerization of n-Hexane

Hydroisomerization generally occurs at relatively low temperatures on Pt-containing

catalysts. It is of prime importance to investigate the effect of Pt loading techniques

on the zeolitic supports to shed light on how Pt is incorporated and activated. Hence, in

the present work, 0.15, 0.30, and 0.60 wt% Pt loadings on H-ZSM-5 zeolite were carried

out as follows:

1. By exchanging the zeolite with platinum tetramine dichloride complex (Pt(NH3)4Cl2)

or by impregnating the zeolite with chloroplatinic acid (H2PtCl6) solution.

2. In platinum tetramine dichloride complex, Pt is the cation, whereas in chloroplatinic

acid, Pt is a part of the anion (PtCl6)2�. The Pt loadings chosen in the present work

range between 0.15 and 0.60 wt%. This range is indeed within those used in industrial

naphtha hydroisomerization and catalytic reforming catalysts, as recently reviewed by

Aboul-Gheit and Ghoneim (2008).

Typically, during the lower reaction temperature range, the hydroconversion of n-hexane

takes place principally producing isohexanes, where isoparaffins increase with tempera-

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2352 A. K. Aboul-Gheit et al.

ture to reach a maximum, beyond which they decline in the product with a further increase

of temperature (Figures 1 and 2). According to this isohexanes maximum, the operating

conditions are set up to be sustained for several years of time-on-stream before the catalyst

requires regeneration. The decline of isohexanes is principally due to hydrocracking

producing lower-molecular-weight hydrocarbons (C1–C5) and dehydrocyclization of n-

hexane to benzene. Hydrocracking is considered the principal isomerization competi-

tor, such that selective isomerization catalysts have to enjoy minimum hydrocracking

activities.

The overall n-hexane hydroisomerization activities of the current Pt/H-ZSM-5 cat-

alysts containing 0.15–0.60 wt% Pt, prepared via cation exchange from a Pt complex

(Figure 1) and impregnation of a Pt precursor (Figure 2), are compared. Significantly

varying behaviors are exhibited. Principally, the three exchanged catalysts, with their

widely different Pt contents, do not exhibit significant variation in hexane isomers pro-

duction, whereas the impregnated catalysts exhibit wide differences. Slides 1 and 2 show

the transmission electron microscopy (TEM) photographs of the 0.60% Pt/H-ZSM-5

catalysts prepared by Pt exchange and Pt impregnation, respectively.

Figure 1 shows that the maximum isomers production amounts to 64.4, 68.6, and

66.7% at 300ıC using the exchanged 0.15, 0.30, and 0.60% Pt/H-ZSM-5 catalysts,

respectively. This reveals that the balance of the metal/acid functions is quite attainable

Figure 8. TEM photograph for exchanged Pt/H-ZSM-5 (magnification 40,000�).

Figure 9. TEM photograph for impregnated Pt/H-ZSM-5 (magnification 40,000�).

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2353

throughout a platinum loading range of 0.15–0.60% used in the current catalysts. Nev-

ertheless, at temperatures below the maximum isomers production (250ıC–300ıC), the

hydroizomerization activities of the exchanged catalysts are in the order 0.30 wt% Pt >

0.60 wt% Pt > 0.15 wt% Pt. The optimum Pt content in the majority of modern industrial

light naphtha hydroizomerization is �0.40%. The 0.30 wt% Pt catalyst in the present work

also economizes half of the Pt content compared to the 0.60 wt% Pt catalyst in this series.

On the other hand, the impregnated Pt/H-ZSM-5 catalysts (Figure 2) exhibit a significant

increase of the hydroisomerization activity as a function of Pt loading. Using the 0.15,

0.30, and 0.60 wt% Pt/H-ZSM-5 catalysts, maximum isohexanes production amounted

to 41.3, 50.6, and 69.0%, respectively, indicating that the optimum Pt/acid sites balance

lies within a narrow range of Pt content (�0.60 wt% Pt). The main difference between

the exchanged and impregnated catalysts is that the Pt dispersion in the zeolite (Table 2)

in the exchanged catalysts presents principally in the atomic state because it is located

in the ZSM-5 channels of the exchanged catalysts, whereas in the impregnated catalysts

it is present mostly as clusters on the external surface of the zeolite.

Another economic advantage of the exchanged 0.15 wt% Pt catalyst compared to

its impregnated version is that in addition to providing much higher maximum isomers

(64.4 vs. 41.3%), the operating temperature is 50ıC lower (250ıC vs. 300ıC). This

also indicates that the concentration of the metal/acid sites balance in the impregnated

catalyst is significantly far from the optimum balance. Evidently, the higher acid density

and strength in the impregnated catalysts than in the exchanged catalysts may contribute

to this off-balance (Table 1). The order of hydroisomerization activity as a function of Pt

loading using the Pt-impregnated catalysts is 0.15 wt% Pt < 0.30 wt% Pt < 0.60 wt%

Pt/H-ZSM-5.

3.1.1. Hydroisomerization Selectivity Relative to n-Hexane Conversion. The

hydroisomerization–conversion relationship can reflect a practical comparison of

the hydroisomerization selectivities of different catalysts via a so-called Waterman’s plot

(Figures 3 and 4), which may be easily considered in refinery correlations. In Figures

3 and 4, all points on the diagonal line represent 100% isomerization selectivity; that

is, they reveal that all hydroconverted n-hexane molecules are isohexanes. Traditionally,

the hydroisomerization selectivity D 100 � (isomers %)/total conversion.

Figure 3 shows that the exchanged 0.30 wt% Pt and 0.60 wt% Pt/H-ZSM-5 catalysts

overlap in a single isomerization–conversion curve after deviating from the diagonal

100% selectivity line, indicating closely similar change of isomers production with

hexane conversion, whereas the 0.15 wt% Pt-containing catalyst exhibits a somewhat

lower isohexanes selectivity change as a function of conversion. Nevertheless, the three

Table 2

Percentage of platinum dispersion in the

Pt/H-ZSM-5 catalysts

Catalyst

Pt

exchanged

Pt

impregnated

0.15% Pt/H-ZSM-5 88.7 75.5

0.30% Pt/H-ZSM-5 86.6 67.6

0.60% Pt/H-ZSM-5 83.8 61.8

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2354 A. K. Aboul-Gheit et al.

impregnated Pt/H-ZSM-5 catalysts (Figure 4) show broad differences in hexane isomers

selectivities as conversion proceeds via different Pt loadings. The 0.30 and 0.60 wt% Pt

catalysts exhibit parallel decrease of selectivity with conversion, whereas the 0.15 wt%

Pt catalyst shows a more gradual decrease of selectivity with conversion. The catalytic

selectivity for isohexanes at maximum production is an important technical parameter for

evaluating industrial hydroisomerization catalysts, which briefly define the magnitude of

side reactions practically taking place; that is, hydrocracking and benzene formation. In

general, it can be assumed that at a given platinum loading, the hexane isomerization se-

lectivity is higher on the exchanged catalysts than on the corresponding impregnated ones.

Using the exchanged 0.15 wt% Pt/H-ZSM-5 catalyst, maximum isohexanes production

amounts to 64.4% with a selectivity of 95.6%, whereas using the 0.30 wt% Pt/H-ZSM-5

version, maximum isohexanes amount to 68.6% with selectivity of 96.2%. Again, using

the 0.60 wt% Pt/H-ZSM-5–exchanged catalyst, maximum isohexanes amount to 66.7%

with selectivity of 98.5%. All of the isomers’ maxima are attained at 300ıC, assuring

well-balanced functions.

On the other hand, using the impregnated 0.15% Pt/H-ZSM-5 catalyst, the selectivity

at maximum isohexanes amounts to 85.4%. Using the 0.30 and 0.60% Pt/H-ZSM-5

catalyst, at maximum isohexanes, the isomers selectivity amounts to 96.6 and 96.7%,

respectively. However, the 0.60 wt% catalyst can be selected as the most active and

selective impregnated catalyst despite its high platinum content. Pt exchange in the zeolite

produces nanoparticles of platinum. These advantages may be principally attributed to

the higher dispersion of Pt in the exchanged catalysts than in the impregnated catalysts

(Table 2).

3.1.2. TPR for the Exchanged and Impregnated Catalysts. Figures 5 and 6 show that

reduction of PtO (formed via calcination of the Pt precursor in the catalysts) to the

Pt metal in the exchanged and impregnated series of catalysts is of different activities.

For the exchanged series of catalysts, the curves in Figure 5 all attain a maxima at

300ıC, whereas for the impregnated series, the maxima of the curves in Figure 6 are

attained at significantly lower temperatures. The curves representing reduction of PtO

in the impregnated 0.15 and 0.30% Pt/H-ZSM-5 catalysts attain a maxima at 225ıC,

whereas the curve representing the 0.60% Pt/H-ZSM-5 version attains a maximum at

205ıC. This indicates that the higher content of impregnated Pt is easier to reduce; that

is, it is reduced at lower temperature, most probably due to its main presence on the

external surface of the zeolitic support compared to the lower Pt contents (0.15 and

0.30% Pt). Nevertheless, the higher reduction temperatures of the exchanged catalysts

can be safely attributed to the higher dispersion of the PtO sites, which are deeply located

in the zeolitic channels where the majority of the exchange positions exist. Hence, the

TPR curves in Figures 5 and 6 provide strong evidence that the catalysts prepared by the

Pt cation exchange method are more successful for acquiring ordered and well-dispersed

nanosized Pt compared to the catalysts prepared by impregnation.

3.1.3. Ammonia Temperature-Programmed Desorption for the Exchanged and Impreg-

nated Catalysts. Figure 7 depicts the curves of temperature-programmed desorption

(TPD) of NH3 from catalysts prepared by impregnation of Pt in H-ZSM-5. For each

catalyst, the TPD curve is composed of two peaks, one located in the low-temperature

region (between 30ıC and 340ıC) and another peak in the high-temperature region

(between 340ıC and 700ıC). The first peak corresponds to NH3 desorption from the

weak acid sites on the catalyst, whereas the second peak corresponds to NH3 desorption

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2355

from the strong acid sites. The latter peak is of interest because it is concerned with the

strong acid sites that are effective in the current hydroconversion reaction. Figure 7 shows

that the unloaded H-ZSM-5 (cited as a reference sample) acquires the highest acid site

density (desorption enthalpy, �Hd ) among all samples, and its peak maximum is located

at 510ıC. Incorporation of 0.15% Pt in H-ZSM-5 results in decreasing the ammonia

desorption enthalpy, that is, decreasing strong acid sites density, as well as decreasing

the temperature maximum of NH3 desorption peak (decreasing acid sites strength). With

a further increase of Pt in the catalyst to 0.30% and then to 0.60%, both the density and

strength of the acid sites decrease. This indicates that each increment of Pt incorporation

in H-ZSM-5 masks a part of the acid sites and the values obtained indicate that the

masking is selective for the strongest acid sites.

3.2. Hydrocracking of n-Hexane Using Pt/H-ZSM-5 Catalysts

As an endothermic reaction, hydrocracking increases with temperature. During n-hexane

hydroconversion using the current Pt/H-ZSM-5 catalysts, hydrocracking took place via

bifunctionality, where Pt sites perform the hydrogenation–dehydrogenation steps, whereas

the strong acid sites provided by the zeolite create carbenium ions. These metallic and

acid sites require an optimum balance in the catalyst to provide maximum isomer-

ization efficiency. Deviation from the optimum balance will not only seriously affect

isomerization but also hydrocracking reactions. When the acid sites density and strength

overcompensate for the balancing effectiveness of metal sites, hydrocracking is enhanced

and may become excessive and cause feed loss.

The overall hydrocracking activities of the catalysts prepared via Pt exchange in H-

ZSM-5 are depicted in Figure 10. Using 0.15 and 0.30 wt% Pt catalysts, hydrocracking is

almost indifferent throughout the whole reaction temperature range investigated. However,

the activity of this reaction is somewhat lower using the exchanged 0.60 wt% Pt catalyst,

particularly at temperatures beyond 450ıC, where the hydrocracked product declines

steadily with temperature until 500ıC. The lower hydrocracking activity using the highest

Pt-loaded catalyst can be attributed to the decreased acid sites number and strength in

this catalyst such that the balancing metal sites increase (Table 1). A larger portion

of Pt in the exchanged 0.60 wt% Pt catalyst should have exchanged a larger number

of Brönsted acid sites (HC) inside the zeolitic channels where the reaction principally

Figure 10. Hydrocracking of n-hexane using exchanged Pt/H-ZSM-5 catalysts.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2356 A. K. Aboul-Gheit et al.

Figure 11. Hydrocracking of n-hexane using impregnated Pt/H-ZSM-5 catalysts.

takes place, and the result will be a significant decrease of the acidity of this catalyst

compared to the 0.15 and 0.30 wt% Pt catalysts. In addition to this chemical effect,

there is another physical effect that decreases the hydrocracking activity of this catalyst.

The significant loading of Pt along the zeolitic channels of this catalyst should have

also resulted in significant diffusion restriction and caused an obvious decrease of the

rate of hydrocracking particularly at higher temperatures. It is well known that the effect

of diffusion appears at higher reaction temperatures because the rate of the chemical

reaction increases with temperature is many times greater than the rate of diffusion such

that diffusion becomes rate controlling.

This behavior significantly differs from that occurring in the corresponding 0.60 wt%

Pt/H-ZSM-5 catalyst prepared via Pt impregnation. In the latter case, Pt is distributed

between the internal and external surfaces such that it does not cause significant inhibition

of the acidity (Table 1) or diffusion limitation; accordingly, n-hexane hydrocracking over

the catalysts containing 0.15, 0.30, and 0.60% Pt prepared via impregnation increases

significantly with Pt loading. For instance, at 450ıC, the hydrocracked product amounts

to 40.2, 55.2, and 62.9% using these catalysts, respectively (Figure 11).

The hydrocracking activities on the Pt/H-ZSM-5 catalysts prepared via Pt exchange

are in the order 0.15% Pt � 0.30% Pt > 0.60% Pt, whereas the hydrocracking activities

on the catalysts prepared via Pt impregnation are in the order 0.60% Pt > 0.30% Pt >

0.15% Pt. The order of hydroisomerization activities of the catalysts prepared via Pt

exchange is 0.15% Pt � 0.30% Pt � 0.60% Pt, whereas the order of hydroisomer-

ization activities of catalysts prepared via Pt impregnation is 0.60% Pt > 0.30% Pt >

0.15% Pt.

The arrangement of the orders of hydrocracking and hydroisomerization activities of

the Pt/H-ZSM-5 catalysts prepared via Pt impregnation is sequential where the activities

of both reactions are enhanced with increasing Pt loading. This indicates that Pt may

have better control in impregnated catalysts supported on H-ZSM-5 zeolite. On the other

hand, to generalize the comparative picture of the activities of the catalysts prepared via Pt

exchange, it can be assumed that all Pt loadings provide almost similar hydroisomerization

and hydrocracking activities, except for a single case: hydrocracking on the 0.60% Pt/H-

ZSM-5 catalyst where the uniqueness of Pt distribution, characteristic for uniform Pt

dispersion, is interrupted principally by a significant diffusion (physical) effect.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2357

3.2.1. Individual Hydrocracked Components in the Product. The effect of metal content

in the current catalysts’ hydrocracking activities is significantly evident from propane

yield. Propane is the major component in hydrocracked products over all current catalysts.

For instance, using the exchanged 0.15 wt% Pt catalyst, propane in †C1 ! C5 comprises

78.6–66.9%.

3.3. Dehydrocyclization of n-Hexane Using Pt/H-ZSM-5 Catalysts

Benzene is principally formed on the metal component and normally increases with

increased metal content in the catalyst (Bernard, 1980; Tamm et al., 1988; Lane et al.,

1991; Ostgard et al., 1992). Using the three exchanged catalysts, benzene formation is not

markedly different with increased Pt content (Figure 12). The activities of these catalysts

can be arranged in the order 0.60% Pt > 0.30% Pt > 0.15% Pt.

At a reaction temperature of 450ıC, for instance, benzene production on these cat-

alysts amounts to 29.8, 26.1, and 23.1%, respectively. These relatively small differences

in benzene yields as a function of Pt content can be attributed to the uniform locations of

the Pt sites in the exchanged catalysts where the differences in the rates of the diffusion

processes in the zeolitic channels are not significant. Figure 10 shows that at temperatures

beyond 425ıC, the effect of increased reaction temperature on enhancing catalytic activity

is modest.

On the other hand, Figure 13 shows the effect of increased Pt content in the catalysts

prepared via impregnation on benzene production. Using the 0.60% Pt-containing cata-

lyst, the yield of benzene as a function of temperature does not encounter significant

restriction due to acquiring a large proportion of Pt on the external surface of the

zeolite. However, Figure 11 shows also that benzene production on the impregnated 0.15

and 0.30% Pt catalysts is significantly lower than on the respective exchanged catalysts

and interchange their activities beyond 400ıC. Below 400ıC, the order of activities is

normal; that is, 0.60% Pt > 0.30% Pt > 0.15% Pt, whereas beyond 400ıC the order

of activities is as follows: 0.60% Pt > 0.15% Pt > 0.30% Pt. Evidently, the normal

dehydrocyclization mechanism on the 0.30% Pt catalyst is no longer operative in the

425ıC–500ıC temperature range. The catalyst may have physically lost a large proportion

Figure 12. Dehydrocyclization of n-hexane using exchanged Pt/H-ZSM-5 catalysts.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2358 A. K. Aboul-Gheit et al.

Figure 13. Dehydrocyclization of n-hexane using impregnated Pt/H-ZSM-5 catalysts.

of its activity via blockage of the zeolitic channels with Pt that causes significant diffusion

limitation. Nevertheless, it cannot be said that the 0.15% Pt catalyst is free of diffusion

limitation at 425ıC–500ıC, still it suffers some limitation but less than in the 0.30% Pt

catalyst. The effect of diffusion is particularly evident in benzene formation because its

molecular size largely exceeds that of the paraffinic isomers.

4. Conclusion

Maximum isomers production is taken by the refiner as a processing parameter of prime

importance according to which the operating conditions sustain that maximum during

several years of time-on-stream before the catalyst requires regeneration. Maximum

n-hexane isomers production with the exchanged catalysts of 0.15, 0.30, and 0.60%

Pt on H-ZSM-5 attain close levels: 64.4, 68.6, and 66.7%, respectively. However, the

maximum hydroisomerization using the impregnated catalysts is 41.3, 50.6, and 69.0%,

respectively. This is attributed to the high dispersion of Pt in the exchanged catalysts,

compared to that in impregnated catalysts, in addition to acquiring deeper penetration

of Pt atomic particles in the protonic zeolitic positions in the channels of exchanged

catalysts, whereas in the impregnated catalysts, Pt clusters are distributed partly internally

and partly on the external surface of the zeolite support. Exchanged catalysts have

mostly nanosized Pt, with lower acid sites density and strength, compared to impregnated

catalysts. At the isomers maximum production, exchanged 0.15, 0.30, and 0.60% Pt/H-

ZSM-5 catalysts attain selectivities of 95.6, 96.2, and 98.5%, respectively, whereas the

respective impregnated catalysts attain selectivities of 85.4, 96.6, and 96.7%.

At the isomers maximum production, in presence of exchanged 0.15, 0.30, and

0.60% Pt/H-ZSM-5 catalysts, hydrocracking is very low, amounting to 2.9, 2.7, and

1.0%, respectively, an benzene is absent on all catalysts. On the respective impregnated

catalysts, hydrocracking amounts to 4.7, 1.8, and 2.3%, respectively, and benzene is also

absent on these catalysts.

Beyond the temperature range at which maximum isomers production occurs, both

hydrocracking and dehydrocyclization behave differently for exchanged and impregnated

catalysts. On impregnated catalysts, hydrocracking goes normally and increases with Pt

content, whereas on exchanged catalysts the activities of 0.15 and 0.30% Pt catalysts are

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

n-Hexane Hydroconversion 2359

almost the same and the activity of the 0.60% Pt catalyst is lower, most likely due to

excess loading with Pt that goes deeper in the channels where proton positions carry the

exchanged Pt atoms. Dehydrocyclization on the exchanged catalysts normally behaves

(increases) with respect to Pt loading. On impregnated catalysts, diffusion effects cause

inhibition of the reaction, particularly at temperatures beyond 400ıC. Diffusion effects

are more pronounced in the 0.30% Pt catalyst than in the 0.15% Pt catalyst. The 0.60%

Pt catalyst acquires its highest activity for benzene production due to the high percentage

of this metal deposited on the catalyst external surface.

References

Aboul-Gheit, A. K. (1979). The role of additives in the impregnation of platinum and ruthenium

on alumina catalysts. J. Chem. Tech. Biotechnol. 29:480–486.

Aboul-Gheit, A. K. (1991). Effect of decationation and dealumination of zeolite-Y on its acidity

as assessed by ammonia desorption using differential scanning calorimetry. Thermochim. Acta

191:233–240.

Aboul-Gheit, A. K. (1997). Reactivities of H-mordenite for the adsorption and oxidation of am-

monia and pyridine. Solid State Ionics 101–103:893–897.

Aboul-Gheit, A. K., Abdel-Hamid, S. M., and Awadallah, A. E. (2004). Pt/H-MOR and Pt/H-BEA

catalysts with various Pt contents and bimetallic PtPd/H-MOR, PtPd/H-BEA, PtIr/H-MOR,

PtIr/H-BEA catalysts with various secondary metal contents for the hydroconversion of n-

hexane. Oil Gas Eur. Mag. 30:40–44.

Aboul-Gheit, A. K., and Aboul-Gheit, N. A. K. (2003). Influence of dealumination of H-ZSM-5 ze-

olite with EDTA and platinum incorporation for use as catalysts for n-hexane hydroconversion.

Erdol, Erdgas, Kohle 119:28–34.

Aboul-Gheit, A. K., Awadallah, A. E., Aboul-Gheit, N. A. K., Solyman, E. A., and Abdel-Aaty,

M. A. (2008). Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt-Ir/H-

ZSM-5 catalysts for n-hexane hydroconversion. Appl. Catal. Gen. 334:304–310.

Aboul-Gheit, A. K., and Ghoneim, S. A. (2008). Catalysis in the petroleum naphtha catalytic

reforming process. Recent Patents on Chemical Engineering 1:113–125.

Aboul-Gheit, A. K., Ghoneim, S. A., and Al-Owais, A. A. (1998). Effect of hydrothermal treatment

and ammonium-ion incorporation in platinum mordenite catalysts for n-hexane hydroconver-

sion. Appl. Catal. Gen. 170:277–283.

Bernard, J. R. (1980). Proceedings of the 5th International Zeolite Conference, Rees, L. V. V. (Ed.).

London: Heyden, p. 686–695.

de Lucas, A., Sánchez, P., Dorado F., Ramos, M. J., and Valverde. J. L. (2005). Effect of the metal

loading in the hydroisomerization of n-octane over beta agglomerated zeolite based catalysts.

Appl. Catal. Gen. 294:215–225.

Freel, J. (1972). Chemisorption on supported platinum: I. Evaluation of a pulse method. J. Catal.

25:139–148.

Gopal, S., and Smirniotis, P. G. (2003). Pt/H-ZSM-12 as a catalyst for the hydroisomerization of

C5–C7 n-alkanes and simultaneous saturation of benzene. Appl. Catal. Gen. 247:113–123.

Guisnet, M., Thomazeau, C., Lemberlon, J. L., and Mignared, S. (1995). Model reaction of the

in situ characterization of the hydrogenating and acid properties of industrial hydrocracking

catalysts. J. Catal. 151:102–110.

Kinger, G., Lugstein, A., Swagera, R., Ebel, M., Jentys, A., and Vinek, H. (2000). Comparison of

impregnation, liquid- and solid-state ion exchange procedures for the incorporation of nickel in

HMFI, HMOR and HBEA: Activity and selectivity in n-nonane hydroconversion. Microporous

and Mesoporous Materials 39:307–317.

Kinger, G., Mejda, D., and Vinek, H. (2002). n-Heptane hydroisomerization over Pt-containing

mixtures of zeolites with inert materials. Appl. Catal. A 225:301–312.

Kinger, G., and Vinek, H. (2001). n-Nonane hydroconversion on Ni and Pt containing HMFI,

HMOR and HBEA. Appl. Catal. A 218:139–150.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013

2360 A. K. Aboul-Gheit et al.

Lane, G. S., Modica, F. S., and Miller, J. T. (1991). Platinum/zeolite catalyst for reforming n-hexane:

Kinetic and mechanistic considerations. J. Catal. 129:145–158.

Ostgard, D. J., Kustov, L., Doeppelmeier, K. R., and Sachtler, W. M. A. (1992). Comparison

of Pt/KL catalysts prepared by ion exchange or incipient wetness impregnation. J. Catal.

133:342–357.

Ramos, M. J., Gómez, J. P., Dorado, F., Sánchez, P., and Valverde, J. L. (2005). Hydroisomerization

of a refinery naphtha stream over agglomerated Pd zeolites. Ind. Eng. Chem. Res. 44:9050–

9058.

Ramos, M. J., Gómez, J. P., Dorado, F., Sánchez, P., and Valverde, J. L. (2007). Hydroisomerization

of a refinery naphtha stream over platinum zeolite-based catalysts. Chem. Eng. J. 126:13–21.

Ramos, M. J., Lucas, A., Jiménez, V., Sánchez, P., and Valverde, J. L. (2008). Hydroisomerization

of different refinery naphtha streams by using a beta zeolite catalyst. Fuel Process. Tech.

89:721–727.

Tamm, P. W., Maher, D. H., and Wilson, C. R. (1988). Octane enhancement by selective reforming

of light paraffins. In: Studies in Surface Science and Catalysis, Ward, J. W. (Ed.). Amsterdam,

The Netherlands: Elsevier, p. 335–353.

Weyda, H., and Köhler, F. (2003). Modern refining concepts—An update on naphtha-isomerization

to modern gasoline manufacture. Catal. Today 81:51–55.

Dow

nloa

ded

by [

Prin

ceto

n U

nive

rsity

] at

02:

58 2

8 Se

ptem

ber

2013