ORIGINAL PAPER

Enhancing the Production of Light Olefins by Catalytic Crackingof FCC Naphtha over Mesoporous ZSM-5 Catalyst

M. A. Bari Siddiqui • A. M. Aitani •

M. R. Saeed • S. Al-Khattaf

Published online: 5 October 2010

� Springer Science+Business Media, LLC 2010

Abstract The enhanced production of light olefins from

the catalytic cracking of FCC naphtha was investigated

over a mesoporous ZSM-5 (Meso-Z) catalyst. The effects

of acidity and pore structure on conversion, yields and

selectivity to light olefins were studied in microactivity test

(MAT) unit at 600 �C and different catalyst-to-naphtha

(C/N) ratios. The catalytic performance of Meso-Z catalyst

was compared with three conventional ZSM-5 catalysts

having different SiO2/Al2O3 (Si/Al) ratios of 22 (Z-22), 27

(Z-27) and 150 (Z-150). The yields of propylene (16 wt%)

and ethylene (10 wt%) were significantly higher for Meso-

Z compared with the conventional ZSM-5 catalysts.

Almost 90% of the olefins in the FCC naphtha feed were

converted to lighter olefins, mostly propylene. The aro-

matics fraction in cracked naphtha almost doubled in all

catalysts indicating some level of aromatization activity.

The enhanced production of light olefins for Meso-Z is

attributed to its small crystals that suppressed secondary

and hydrogen transfer reactions and to its mesopores that

offered easier transport and access to active sites.

Keywords Cracking � Naphtha � Olefins � ZSM-5 �Mesoporous � Ethylene � Propylene

1 Introduction

Light olefins (mainly ethylene and propylene) are impor-

tant feedstocks for the production of useful materials, such

as polyethylene and polypropylene, vinyl chloride, ethyl-

ene oxide, ethylbenzene and others. Polyolefins remain the

largest sector of light olefins demand showing the highest

overall growth rate compared with other derivatives.

Annual projected growth rate for ethylene and propylene

demand is estimated at 4.5 and 5.4%, respectively [1]. The

major sources of propylene are steam crackers and fluid

catalytic cracking (FCC) units. While ethylene is the main

product of steam crackers and gasoline is the also the main

product in FCC units, propylene remains a byproduct in

both units. In conventional FCC, typically about 2 wt%

ethylene and 3–6 wt% propylene yields are obtained [1].

About, 30% of world’s propylene is supplied by refinery

FCC operations, 64% is co-produced from steam cracking

of naphtha or other feedstocks, and the remaining is pro-

duced on-purpose using metathesis or propane dehydro-

genation processes [1]. Compared with thermal steam

cracking, the catalytic cracking of naphtha is carried out at

lower temperatures and consumes 10–20% less energy.

The reduction of carbon dioxide emissions is also another

driving force for the low temperature naphtha catalytic

cracking [2].

Many types of zeolites have been investigated for the

catalytic cracking of naphtha to produce ethylene and

propylene including ZSM-5 zeolites, zeolite A, zeolite X,

zeolite Y, zeolite ZK-5, zeolite ZK-4, synthetic mordenite,

dealuminated mordenite, as well as naturally occurring

zeolites including chabazite, faujasite, mordenite [2–5].

High selectivity to light olefins depends on the extent of

reaction, reaction path and residence time, which are

determined by the zeolite acidity, pore structure and crystal

M. A. Bari Siddiqui � A. M. Aitani � M. R. Saeed �S. Al-Khattaf (&)

Center of Research Excellence in Petroleum Refining

& Petrochemicals, King Fahd University of Petroleum

& Minerals (KFUPM), Dhahran 31261, Saudi Arabia

e-mail: skhattaf@kfupm.edu.sa

123

Top Catal (2010) 53:1387–1393

DOI 10.1007/s11244-010-9598-1

size [2]. Selectivity to light olefins is also enhanced by

lower conversion (weak acidity), shorter residence time

(small crystal size) and increased accessibility (mesopores)

which allow full utilization of zeolitic volume by the feed

molecules.

ZSM-5 is considered the catalyst of choice when shape

selectivity influences the preferential formation of light

olefins. Several strategies have been proposed to enhance

the accessibility to active sites in ZSM-5, such as the use of

nanocrystals [4], composite materials [5–7] low level of

acid concentration provided by high Si/Al molar ratio, and

incorporation of intra-crystalline mesoporosity [8]. The

latter can be accomplished either by carbon templating

[9–11] or by post synthesis modification by alkali treatment

[12–15]. The generation of mesopores by secondary tem-

plating with carbon particles during synthesis increases the

accessibility of acid sites in ZSM-5 and diminishes the

role of transport [16]. In such a case, zeolites are able to

nucleate around mesoporous particles of carbon matrix and

form typical ZSM-5 crystals. After the removal of carbon

particles by combustion, ZSM-5 crystals with a controlled

pore size distribution and a highly crystalline micro-mes-

oporous hierarchical structure are formed. Mesoporous

ZSM-5 has shown improved catalytic performance in the

cracking of model compounds due to the hierarchical

porosity of the pore walls and to the easier transport and

access to the active sites. Hierarchical ZSM-5 combines the

high activity, shape selectivity, and hydrothermal stability

of conventional ZSM-5 [16].

In this study, the enhancement of light olefins yield from

FCC naphtha cracking over mesoporous ZSM-5 catalyst

was investigated. The catalytic performance of Meso-Z

was compared with three conventional ZSM-5 catalysts

having low and high Si/Al ratios. The catalytic activity and

selectivity were discussed to elucidate the effects of acidity

and pore structure on the selectivity to light olefins and

aromatics.

2 Experimental

2.1 Catalysts and Naphtha Feed

The H-form of Z-27 and Z-150 (ZSM-5 zeolites) were

procured from Catal International, while the H-form of

Z-22 zeolite was obtained from Industrial Chemicals, USA.

The Meso-ZSM-5 zeolite having a Si/Al of 30 was

obtained from the J. Heyrovsky Institute of Physical

Chemistry, Czech Republic. It was synthesized using car-

bon black (CBP 2000, Cabot Corporation) as a secondary

template with an average particle diameter of 12 nm as per

the procedure outlined by Pavlackova et al. [17] and

Al-Khattaf et al. [18].

Cataloid AP-3 (which contains 5.4 wt% alumina,

3.4 wt% acetic acid, and water as balance) was used as

alumina binder for the four catalysts. The required quantity

of alumina was mixed with demineralised water which has

been acidified to a pH of 5 with dilute nitric acid. ZSM-5

sample was added to the alumina-water slurry, while stir-

ring. The slurry was dried at 120 �C for 2 h followed by

calcination at 600 �C for 3 h. The calcined catalyst was

pelletized, crushed and sieved to obtain 80–90 microns size

catalyst particles.

FCC naphtha feed was obtained from Saudi Aramco’s

Jeddah refinery and was used in all MAT experiments. The

composition of the naphtha feedstock comprising paraf-

fins (iso and normal), olefins, naphthenes and aromatics

(PIONA), is presented in Table 1.

2.2 Catalyst Characterization

The surface area of the catalysts were measured in a

Quantochrome NOVA 1200 gas sorption analyzer by the

adsorption of nitrogen at 77 K according to standard

ASTM D3663 method. Prior to nitrogen sorption, catalyst

samples were evacuated for 2 h while being heated up to

350 �C at a steady rate of increase of temperature. Micro

pore volume was determined by t-plot method. Total pore

volume was determined by BJH method. Difference of

total pore volume and micro pore volume gives the mes-

opore volume, Vme.

Bronsted and Lewis acid sites density of zeolites was

determined by adsorption of pyridine as a probe molecule

followed by FTIR spectroscopy. Zeolites were activated in

a form of self-supporting wafers at 450 �C overnight. The

adsorption of pyridine was carried out 170 �C, according to

Gil et al. [19].

2.3 Catalytic Evaluation

The cracking of FCC naphtha was carried out in a fixed-

bed microactivity test (MAT) unit, manufactured by

Table 1 Composition of FCC naphtha feed (wt%)

Carbon

no.

n-Paraffins i-Paraffins Olefins Naphthenes Aromatics

C4 0.7 1.2 4.0 – –

C5 0.9 5.7 9.8 1.0 –

C6 0.8 5.5 8.4 1.5 0.5

C7 0.8 5.0 5.3 2.4 2.4

C8 0.7 3.7 3.2 2.3 6.2

C9 0.4 2.8 0.7 1.8 7.7

C10–C12 1.2 4.1 0.6 0.5 8.2

Total 5.5 28.0 32.0 9.5 25.0

1388 Top Catal (2010) 53:1387–1393

123

Sakuragi Rikagaku, Japan. The procedure is carried out

according to ASTM D-3907 which is a standard method

used to determine the activity and selectivities of FCC

catalysts. MAT tests were performed at 600 �C and 30 s

time-on-stream (TOS). Conversion was varied by chang-

ing the catalyst/naphtha ratio (C/N) in the range of

1.0–5.0. This variable was changed by keeping constant

the amount of feed (1.0 g) and changing the amount of

catalyst. Prior to MAT test, the system was purged with

N2 flow at 30 cc/min of for 30 min. About 1.0 g of

naphtha was then fed to the reactor during 30 s. After the

reaction mode, the stripping of the catalyst was carried

out for 15 min using 30 cc/min of N2. During the reaction

and stripping modes, liquid products were collected in a

glass receiver kept in an ice-bath. Gaseous products (for

GC analysis) were collected in a gas burette by water

displacement.

A thorough gas chromatographic analysis of all MAT

products was conducted to provide detailed yield patterns

and information on the selectivity of the catalyst being

tested. The gases were analyzed using two Varian gas

chromatographs (GC) equipped with FID and TCD detec-

tors. This allowed the quantitative determination of all light

hydrocarbons up to C4, hydrogen and fixed gases. Hydro-

carbon distribution of feed and cracked naphtha was

determined by a GC PIONA analyzer manufactured by

Shimadzu, Japan. The GC was equipped with FID detector,

a 100 m capillary column (CP-Sil5) and software that

identifies and quantifies different hydrocarbon compounds

in naphtha. Coke on spent catalyst was determined by

Horiba Carbon–Sulfur Analyzer Model EMIA-220V.

Naphtha conversion is defined as the sum of gaseous

product yields and coke [20]. Mass balance was considered

acceptable when comprised in the limits 95–105% of the

injected naphtha.

3 Results and Discussion

3.1 Catalyst Characterization

The textural properties of the catalysts, presented in

Table 2, were determined from nitrogen adsorption iso-

therms. Meso-Z has significantly higher mesopore volume

(Vme) of 0.41 cc/g compared with Z-27 (0.23 cc/g), Z-22

(0.26 cc/g) and Z-150 (0.38 cc/g) which includes inter-

crystal void volume. The high Vme of Meso-Z catalyst is

due to actual mesopores and does not represent intracrys-

talline void volume [19]. This is evident from the steep rise

in adsorption isotherm and hysteresis effect in desorption

curve for Meso-Z in the P/Po range of 0.6–0.9, as illus-

trated in Fig. 1. The isotherms of conventional Z-27

catalyst showed a gradual rise in P/Po of 0.6–0.9.

Intercrystalline void volume is reflected by a steep rise in

the nitrogen adsorption isotherm at P/Po of 0.9–1.0 [21].

Surface area, calculated from nitrogen adsorption isotherm,

was highest for Meso-Z (370 m2/g) compared with the

lowest Z-27 at 284 m2/g. As shown in Table 2, the Bron-

sted acidity of Meso-Z was 0.08 mmol/g compared with

0.10 mmol/g for Z-150, 0.31 mmol/g for Z-22 and

0.28 mmol/g for Z-27. Total acidity of the catalysts

decreased in the following order: Z-150 \ Meso-Z \Z-22 \ Z-27.

3.2 Catalytic Evaluation

The catalytic cracking of FCC naphtha was carried out at

600 �C over the four catalysts in MAT unit. Product yields

were mapped over a reasonable range of conversions by

varying the C/N in the range of 1.5–4.0. Conversion and

product yields were plotted as a function of C/N ratio.

Typical plots for conversion, propylene, ethylene and

C2–C3 olefins yields are shown in Fig. 2.

The results show that naphtha conversion increased

with increased C/N (Fig. 2), as is expected in MAT

Table 2 Textural and acidic properties of catalystsa

Item Meso-Z Z-22 Z-27 Z-150

SiO2/Al2O3 ratio 30 22 27 150

Total pore volume, cc/g 0.496 0.344 0.298 0.462

Micro pore (Vmi), cc/g 0.088 0.079 0.075 0.081

Meso pore (Vme), cc/g 0.408 0.265 0.233 0.381

Surface area, m2/g 370 293 284 320

Bronsted acidity, mmol/g 0.08 0.13 0.22 0.10

Lewis acidity, mmol/g 0.32 0.31 0.28 0.16

Total acidity, mmol/g 0.40 0.44 0.50 0.26

a Catalysts were mixed with 33.3 wt% alumina binder

30

80

130

180

230

280

330

380

0 0.2 0.4 0.6 0.8 1

Volu

me

adso

rbed

, cc/

g

Relative pressure, P/Po

Fig. 1 Nitrogen adsorption–desorption isotherms for Meso-Z (filledtriangle) and Z-27 (empty circle)

Top Catal (2010) 53:1387–1393 1389

123

experiments. The highest conversion was obtained for

Z-150, it varied from 49 to 54% as C/N was increased from

1.5 to 4. Within the same C/N range, Z-27 and Z-22 gave

naphtha conversion of 46–50%. Meso-Z exhibited naphtha

conversion of 44–50%. The difference in conversion

between the four catalysts was in the range of 4–5%, not

much significant change.

The trends of propylene and ethylene yield with

increased C/N ratio are shown in Fig. 2. Propylene yield

decreased with increased C/N ratio for the four catalysts.

Significant enhancement in propylene yield was achieved

by the use of meso-Z catalyst. Propylene yield was also

enhanced by Z-150 compared with Z-27 and Z-22. Pro-

pylene yield of 13–16 wt% and 9–12 wt% was observed

for Meso-Z and Z-150, respectively compared with

5–9 wt% for Z-27 and Z-22. The effect of mesopores on

propylene enhancement was clearly evident. The mesop-

ores allowed fast elution of propylene from the zeolitic

pores before secondary, olefin consuming oligomerization

reactions take place.

Ethylene yield was also higher for Meso-Z and Z-150

(8–10 wt%) compared with 6–7 wt% obtained for Z-27

and Z-22. The extent of increase (30%) in ethylene yield

for Meso-Z was lower compared with the increase (80%) in

propylene yield. Ethylene yield increased slightly as C/N

ratio increased up to about 3.0 and then remained constant

as the C/N was further increased. At lower C/N, ethylene

was produced by direct catalytic cracking and by secondary

cracking of propylene [22].

3.3 Catalytic Performance at Constant Catalyst/

Naphtha Ratio

The difference between the catalytic performances of the

four catalysts was elucidated by comparing conversion and

product yields at constant C/N of 3.0 (the data were cal-

culated by interpolation). Naphtha conversion, product

yields (gaseous and liquids) and other values determined at

a constant C/N ratio are presented in Table 3. Meso-Z

catalyst exhibited a naphtha conversion of 47% compared

with 48.4% for Z-22, 49.4% for Z-26 and 50.4% for Z-150.

A major portion of the FCC naphtha used in this study

consists of olefins, which are known to be highly reactive

hydrocarbons. Because of presence of highly reactive

olefins in the naphtha feed, no strong correlation between

acidity and conversion was observed.

Besides ethylene and propylene, significant amounts of

methane, butenes, and LPG were detected over all cata-

lysts. The highest propylene yield of 14.6 wt% was

obtained for Meso-Z, compared with 7.8–8.2 wt% obtained

for Z-22 and Z-27 containing conventional ZSM-5 having

low S/Al ratio similar to Meso-Z. Propylene selectivity was

also enhanced by similar margins for Meso-Z. Higher Si/Al

catalyst, Z-150, gave 9.4% propylene yield which was

40

45

50

55

60

Co

nver

sio

n, %

C/N Ratio

Meso-Z Z-22 Z-27 Z-150

4

8

12

16

20

Pro

pyl

ene

Yie

ld, w

t.%

C/N Ratio

Meso-Z Z-22 Z-27 Z-150

4

6

8

10

12

Eth

ylen

e Y

ield

, wt.

%

C/N Ratio

Meso-Z Z-22 Z-27 Z-150

10

15

20

25

30

1.0 2.0 3.0 4.0 5.0

1.0 2.0 3.0 4.0 5.0

1.0 2.0 3.0 4.0 5.0

1.0 2.0 3.0 4.0 5.0

C2-

C3

Ole

fin

s Y

ield

, wt.

%

C/N Ratio

Meso-Z Z-22 Z-27 Z-150

Fig. 2 Effect of catalyst/

naphtha ratio on naphtha

conversion, ethylene and

propylene yields for Meso-Z

(filled square), Z-22 (emptytriangle), and Z-27 (filledtriangle) and Z-150 (filledcircle) at 600 �C

1390 Top Catal (2010) 53:1387–1393

123

slightly higher compared with lower Si/Al ratio catalysts,

Z-22 and Z-27. Highest ethylene yield was also achieved

for Meso-Z (9.4 wt%) compared with ethylene yield

obtained for Z-22 (6.6 wt%) and Z-27 (7.8 wt%). Ethylene

yield was slightly increased for high Si/Al Z-150 (8.2 wt%)

compared with low Si/Al Z-27. Methane yield, which is an

indication of the extent of cracking reaction, was lowest for

Meso-Z (1.4 wt%) compared with 5.5 wt% for Z-22,

4.9 wt% for Z-27 and 3.1 wt% for Z-150.

Enhanced light olefins yield (ethylene and propylene)

was achieved for Meso-Z (24.0 wt%) compared with the

other three catalysts. For the other catalysts, light olefins

yield decreased in the order of Z-150 [ Z-27 [ Z-22. The

propylene/ethylene ratio of 1.6 was highest for Meso-Z

compared with 1.2 for Z-150 and 1.1 each for Z-22 and

Z-27. Coke yield was lowest for Meso-Z (0.47 wt%) and

highest for Z-27 (1.1 wt%). Figure 3 illustrates these

differences in product yields among the four catalysts.

3.4 Hydrogen Transfer Activity

One of the factors for light olefins enhancement by Meso-Z

was its low hydrogen transfer activity. Hydrogen transfer

index (HTI) is a measure of hydrogen transfer activity of

the catalyst during cracking reactions [23]. It is defined as

the ratio of sum of propane and butanes selectivity to

propylene selectivity. Z-22 and Z-27 showed the highest

HTI of 2.5–2.9 compared with 0.7 for Meso-Z and 2.2 for

Z-150. The values of HTI for Z-27 were similar to those

found by Zhu et al. [23]. Hydrogen transfer activity for low

Si/Al zeolites, Z-22 and Z-27 was high because of their

high acidity, as hydrogen transfer reactions require high

strength of acid sites.

Hydrogen transfer reactions occurred at the outer sur-

face of low Si/Al zeolites as higher C1 yield was observed

(Table 3) and steric constraints [24] did not play a role.

Reactions on external surface follow a radical mechanism,

which gives more C1 and C2 hydrocarbons. Corma and

Orchilles [24] showed that methane and ethane can be

formed by a protolytic cracking of branched products. Part

of these two gases and most of the ethylene are formed by a

radical type of cracking, in which extraframework alu-

minium plays an important role. Propane yield was sig-

nificantly low for Meso-Z because of its significantly low

hydrogen transfer activity, also evidenced by lower BTX

yields. Propane yield was higher for the other ZSM-5

catalysts because of their higher hydrogen transfer activity.

There was an increasing trend of propane yield with

increasing hydrogen transfer index (Table 3). The highest

yield of light olefins was obtained for Meso-Z because of

Table 3 Naphtha conversion and product yields at constant catalyst/

naphtha ratio of 3.0 and 600 �C

Catalyst Meso-Z Z-22 Z-27 Z-150

Naphtha conversiona, % 47.4 48.4 49.5 49.6

Gaseous products, wt%

Ethylene 9.4 6.6 7.8 8.2

Propylene 14.6 7.1 8.2 9.4

Butenes 7.5 2.9 3.2 3.9

Methane 1.4 5.5 4.9 3.1

Ethane 1.6 5.1 4.2 3.6

Propane 6.7 15.7 14.9 14.6

i-Butane 3.5 2.5 3.0 3.6

n-Butane 2.2 2.2 2.3 2.6

Total gas yield 46.9 47.6 48.5 49.0

Coke 0.47 0.84 1.0 0.57

HTIb 0.8 2.9 2.5 2.2

Ethylene ? Propylene 24.0 13.7 16.0 17.6

Propylene/Ethylene 1.6 1.1 1.1 1.2

Selectivity, %

Ethylene 20.0 13.6 15.8 16.2

Propylene 31.0 14.8 16.5 18.7

Liquid products, wt%

n-Paraffins 0.74 0.1 0.3 0.3

i-Paraffins 6.9 2.1 2.5 2.1

Olefins 0.9 0.2 0.3 0.5

Naphthenes 2.3 1.0 1.1 0.8

Aromatics 42.0 48.0 46.0 46.0

Benzene 2.2 5.3 5.2 3.1

Toluene 11.4 17.3 17.2 15.4

m-Xylene 7.2 6.8 6.7 8.3

p-Xylene 2.7 2.5 2.6 2.7

o-xylene 3.0 2.9 2.7 3.6

C9? 15.5 13.3 12.1 12.8

a Naphtha conversion is the sum of total gas and cokeb HTI (hydrogen transfer index) ratio of propane and butanes selec-

tivity to propylene selectivity

Meso-Z Z-22 Z-27 Z-150

Fig. 3 Comparison of ethylene and propylene yields for Meso-Z,

Z-22, Z-27 and Z-150 at constant catalyst/naphtha ratio of 3.0 and

600 �C

Top Catal (2010) 53:1387–1393 1391

123

suppression of olefins consuming, secondary hydrogen

transfer reactions as indicated by its low HTI (Table 3).

These secondary reactions were suppressed predominantly

due to shorter residence time, and rapid elution of primary

reaction products caused by larger intracrystalline spaces

of Meso-Z and not because of its low Bronsted acidity. The

primary products can undergo secondary cracking, aro-

matization and hydride transfer reactions. Z-150 gave

higher light olefins yield compared with Z-27 because of

low density of acid sites and because of its higher meso-

pore volume.

3.5 Hydrocarbon Distribution in Liquid Product

The composition of liquid products in cracked naphtha at

constant C/N ratio for the four catalysts is given in Table 3.

The results show a significant decrease in iso-paraffins,

n-paraffins, olefins and naphthenes and an increase in

aromatics fraction. BTX aromatics were produced either by

cyclization of olefins or by hydrogen transfer or by dehy-

drogenation of corresponding naphthenes [25] Olefins,

comprising 32% of the naphtha feed, were reduced to

0.95 wt% for Meso-Z. For Z-22 and Z-27 catalysts, the

olefins were almost completely cracked. Olefins are known

to be very reactive compared with paraffins and naphth-

enes. Iso-paraffins fraction comprising 28 wt% of naphtha

feed was decreased to 7 wt%, for Meso-Z corresponding to

a decrease of 75%. For Z-22 and Z-27 and Z-150, iso-

paraffins were decreased to 2–5 wt%. Naphthenes com-

prising 10 wt% of naphtha feed was decreased to 2.3 wt%

by Meso-Z and decreased to about 1 wt% by Z-22, Z-27

and Z-150.

The aromatics content of increased to 42 wt% for Meso-

Z compared with 25 wt% in naphtha feed. An increase in

aromatics content was also observed for Z-22, Z-27 and

Z-150 catalysts. It increased to 46–48 wt% from 25 wt% in

the feed. The distribution of BTX in the liquid fraction of

cracked naphtha for the four catalysts is presented in

Table 3 and illustrated in Fig. 4. Benzene content for low

Si/Al catalysts Z-22 and Z-27 was higher (5 wt%) com-

pared with Z-150 (3 wt%). About 17 wt% toluene was

produced by Z-22 and Z-27 compared with 11 wt% for

Meso-Z. Xylenes yield was similar for the four catalysts.

Higher aromatics content (C9?) was lower for Z-22, Z27

and Z-150 catalyst compared with Meso-Z. This was due to

easier accessibility in large pores of Meso-Z catalyst.

3.6 Effect of Reaction Temperature

MAT experiments were conducted at 600–650 �C with

Meso-Z catalyst to study the effect of temperature on

conversion and light olefins yield. Figure 5 shows the

effect of temperature on conversion at three C/N ratios. A

linear increase in naphtha conversion with increased tem-

perature was observed. At C/N of 4.5, naphtha conversion

increased from 50% at 600 �C to 56% at 650 �C. This is

typical of acid catalyzed cracking reactions in which

cracking is higher at higher temperatures.

At lower C/N ratio light olefins yield was the highest.

Figure 6 shows the variation of propylene and ethylene

yields with temperature at the lowest C/N ratio of 1.5.

Propylene yield increased from 15.6 wt% at 600 �C to

17.1 wt% at 625 �C, an increase of 10%. On further

increase of temperature to 650 �C, propylene yield

increased only by 4% to 17.8 wt%. Ethylene yield

increased from 8 wt% at 600 �C to 9.2 wt% at 625 �C, an

increase of 14%. It increased further by the same extent to

10.5 wt% on further increase of temperature to 650 �C.

The extent of increase in ethylene yield at higher tem-

perature was higher than the increase in propylene yield.

This trend was due to their different selectivity trends.

Fig. 4 Comparison of BTX content in cracked naphtha for Meso-Z,

Z-22, Z-27 and Z-150 at constant catalyst/naphtha ratio of 3.0 and

600 �C

40

45

50

55

60

575 600 625 650 675

Co

nver

sio

n, %

Temperature,°C

Fig. 5 Effect of reaction temperature on naphtha conversion for

Meso-Z at catalyst/naphtha ratio of 1.5 (filled square), 3 (filledtriangle) and 4.5 (filled circle)

1392 Top Catal (2010) 53:1387–1393

123

Propylene selectivity was 36% at 600 �C and it did not

show any increase with increasing temperature (Fig. 6).

However, ethylene selectivity increased with increased

temperature. At higher temperatures, free radical mecha-

nism increased ethylene yield further. Ethylene is the main

product of thermal cracking. Because of hydrogen transfer

reactions at higher temperature, propylene was consumed

thereby reducing the increase in propylene yield.

4 Conclusions

High selectivity to light olefins was achieved over a novel

mesoporous ZSM-5 with enhanced diffusion rates of FCC

naphtha feed and products, mainly light olefins and aro-

matics. The mesoporosity of ZSM-5 has significantly

enhanced the production of light olefins and aromatics

showing a two-fold increase in propylene and BTX yields

compared with conventional ZSM-5 catalyst. Using high

Si/Al ZSM-5 catalysts enhanced light olefins yields com-

pared with low Si/Al ZSM-5 catalysts. However, signifi-

cant light olefins enhancement was achieved with

mesoporous ZSM-5. The yield of ethylene and propylene

over the mesopore ZSM-5 reached 24.0 wt% compared

with 13.7 wt% over conventional ZSM-5. Propylene-to-

ethylene ratio of about 1.6 was obtained for Meso-Z due to

the relatively large pore volume compared with Z-22 and

Z-27. Mesopores, low acidity, and small crystals were

important parameters to enhance light olefins yield from

FCC naphtha cracking. Olefins consuming hydrogen

transfer reactions were suppressed and secondary reactions

were reduced due to shorter residence time and rapid

elution of primary reaction products.

Acknowledgments The authors express their appreciation to the

support from the Ministry of Higher Education, Saudi Arabia, in

establishing the Center of Research Excellence in Petroleum Refining

& Petrochemicals at King Fahd University of Petroleum & Minerals

(KFUPM).

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0

5

10

15

20

25

30

Yie

ld, w

t.%

Temperature,°C

15

25

35

45

55

65

575 600 625 650 675 575 600 625 650 675

Sel

ecti

vity

, %

Temperature,°C

Fig. 6 Effect of reaction

temperature on the yield and

selectivity to propylene (filledsquare), ethylene (filledtriangle) and

propylene ? ethylene (filledcircle) for Meso-Z

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