Phosphate modified ZSM-5 for the shape-selective synthesis of para-diethylbenzene: Role of crystal size and acidity

Article abstract of DOI:10.1016/j.apcata.2014.07.006

Pore engineered ZSM-5 zeolite in extrudate form was prepared and used as shape-selective catalyst for vapor phase ethylation of ethylbenzene to selectively form para-diethylbenzene. The physico-chemical properties of the catalyst were established by XRD, N₂ sorption, FTIR, FESEM, NH ₃-TPD and ³¹P MAS NMR. Alkylation of ethylbenzene with ethanol was carried out in a continuous, down-flow, tubular reactor, at atmospheric pressure and H₂ as a carrier gas in vapor phase. Effect of silica to alumina ratio (SAR), crystal size, acidity of phosphate modified ZSM-5, stepwise phosphate modification and reaction conditions were studied in detail. ZSM-5 with SAR 187 was found to contain optimum acidity for phosphate modification to achieve good conversion and high selectivity for p-diethylbenzene. Under optimized reaction conditions, viz. temperature = 380 °C, ethylbenzene:ethanol mole ratio = 4:1, WHSV = 3 h^-1, H ₂/reactants = 2, 5PZSM-5 W catalyst gave 22.8% of ethylbenzene conversion with ～98% selectivity for para-diethylbenzene.

Full text of DOI:10.1016/j.apcata.2014.07.006

Applied Catalysis A: General 484 (2014) 8–16
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Phosphate modified ZSM-5 for the shape-selective synthesis of
para-diethylbenzene: Role of crystal size and acidity
Janardhan L. Hodala, Anand B. Halgeri, Ganapati V. Shanbhag∗
Materials Science Division, Poornaprajna Institute of Scientific Research (PPISR), Bidalur Post, Devanahalli, Bengaluru 562110, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pore engineered ZSM-5 zeolite in extrudate form was prepared and used as shape-selective catalyst for
vapor phase ethylation of ethylbenzene to selectively form para-diethylbenzene. The physico-chemical
properties of the catalyst were established by XRD, N2 sorption, FTIR, FESEM, NH3-TPD and 31P MAS NMR.
Alkylation of ethylbenzene with ethanol was carried out in a continuous, down-flow, tubular reactor, at
atmospheric pressure and H2 as a carrier gas in vapor phase. Effect of silica to alumina ratio (SAR), crystal
size, acidity of phosphate modified ZSM-5, stepwise phosphate modification and reaction conditions were
studied in detail. ZSM-5 with SAR 187 was found to contain optimum acidity for phosphate modification to
achieve good conversion and high selectivity for p-diethylbenzene. Under optimized reaction conditions,
viz. temperature = 380 ◦C, ethylbenzene:ethanol mole ratio = 4:1, WHSV = 3 h−1, H2/reactants = 2, 5PZSM-
5 W catalyst gave 22.8% of ethylbenzene conversion with ∼98% selectivity for para-diethylbenzene.
© 2014 Elsevier B.V. All rights reserved.
Received 28 April 2014
Received in revised form 23 June 2014
Accepted 4 July 2014
Available online 12 July 2014
Keywords:
Phosphate modification
Shape selectivity
Zeolite
ZSM-5
Alkylation
Vapor phase
p-diethylbenzene
1. Introduction
a desorbent in PAREX process designed by UOP. In PAREX pro-
cess, p-xylene is separated from its regioisomers and used as a
Zeolites (microporous crystalline aluminosilicates) have been
widely used as eco-friendly catalysts, and have successfully
replaced corrosive and environmentally unsafe catalysts. They
exhibit shape-selectivity for the chemical reactions due to their
microporous channels [1] and are applied for selective organic
transformations. To enhance the selectivity for the desired product,
pores of zeolites are regulated and usually done by post synthesis
modification. Silica deposition on the external surface, metal/non
metal oxide impregnation and controlled coking are widely applied
as post synthesis methods to regulate pore size [2–8]. Phosphate
impregnation on ZSM-5 is one such method studied by several
groups [9–21] but the studies were mainly restricted to the shape
selective synthesis of p-xylene from alkylation of toluene with
methanol and physico-chemical changes of zeolite ZSM-5 after
phosphate modification. Recently, Janardhan et al. reported the
effect of generation of new acid sites and effect of water treat-
ment on phosphate modified ZSM-5, which showed remarkable
improvement in shape selectivity while retaining good activity
[22].p-Diethylbenzene (PDEB) is an important chemical used as
raw material for polyester synthesis such as PET and PTA. Ethy-
lation of ethylbenzene over acidic catalysts in vapor phase gives
thermodynamic equilibrium mixtures of o,m,p-diethylbenzenes. A
high purity PDEB is required for PAREX process, which can be syn-
thesized by the reduction of p-ethylacetophenone, known to be
an expensive multistep process. Hence, it is necessary to design a
selective catalyst for ethylation of ethylbenzene which forms only
para isomer. Few research groups have reported catalysts like sil-
ica coated ZSM-5, ZSM-5 impregnated with lanthanum and cerium
oxides, aluminophosphates, phosphotungstic acid supported on
aluminophosphates AlPO and modified zeolites to selectively syn-
thesize PDEB from ethylation of ethylbenzene with ethanol or
ethylene [23–31]. Halgeri and co-workers had extensively worked
on silanation of ZSM-5 for the synthesis of PDEB by ethylation of
ethylbenzene [28–31]. Unlike other methods, phosphate modifica-
tion is an easy and inexpensive route for the catalyst modification,
which makes it a good prospect for a commercial process.
Only few studies have been reported on alkylation of ethylben-
zene and benzene with ethanol/ethylene over phosphate treated
ZSM-5 for selective synthesis of PDEB [11,18]. In these reports,
activities of the catalysts reported were very low and hence high
concentrations of alkylating agent were used. These catalysts con-
tained ionic phosphates in the channels, which created diffusion
problems [22].
∗
Corresponding author. Tel.: +91 80 27408552; fax: +91 80 23611836.
E-mail addresses: shanbhag@poornaprajna.org, gvshanbhag@yahoo.co.in
(G.V. Shanbhag).
http://dx.doi.org/10.1016/j.apcata.2014.07.006
0926-860X/© 2014 Elsevier B.V. All rights reserved.

J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
9
In previous study, the authors showed that water treatment
after phosphate impregnation and calcination of ZSM-5 remark-
ably enhances the activity while retaining product selectivity with
probe reactions viz. toluene methylation, ethylbenzene ethyla-
tion and disproportionation, and competitive reaction of m-xylene
and ethylbenzene [22]. In the present article, detailed studies on
role of crystal size of ZSM-5 and acidity of water treated phos-
phate modified ZSM-5 was carried out for ethylbenzene alkylation
with ethanol reaction. Effect of different operating parameters
on the conversion and product selectivity was also studied in
detail.
was then purged by dry He for another 1 h. The desorption of NH3
was carried out in He flow (30 ml min−1) by increasing the tem-
perature to 550 ◦C at 10 ◦C min−1 using TCD detector. Point of zero
charge (PZC) was determined by mass titration method. In a typi-
cal procedure, 3 g of catalyst was added in multiples of 200 mg of
catalyst to 100 ml of 0.1 M KCl solution with stirring. The pH was
measured (with calibrated digital pH meter) each time after adding
200 mg catalyst and 30 min of equilibrium. PZC was determined
from the plot of pH v/s catalyst weight. That pH value is considered
as PZC, after which, addition of the catalyst to the solution has neg-
ligible effect on pH [32]. Solid state nuclear magnetic resonance
with magic angle spinning (MAS NMR) was performed in Bruker
DSX 300 using standard procedure. Sample was placed in a 4 mm
probe and was spun at 5 KHz. Chemical shifts were plotted with
respect to reference standards, NH4H2PO4 and Al(NO3)3 for 31P
and 27Al nuclei respectively. Phosphorus content in the modified
catalyst was estimated by colorimetry [33]. In a typical procedure,
catalyst was digested with a mixture of 1 g of HCl, 0.5 g HF and
1 g HNO3. The mixture was then neutralized with H3BO3 and this
mixture was diluted to 200 ml. 2 ml of this solution was taken in
50 ml volumetric flask and 5 ml of 2 M H2SO4 was added followed
by 5 ml of 0.01 M ammonium paramolybdate. To this solution, 4 ml
of mixed reagent (1:1 mixture of freshly prepared 10% ascorbic
acid: 0.004 M antimony potassium tartrate solution) was added to
get intense blue color. After 10 min, optical density (OD) was mea-
sured at 890 nm in PerkinElmer UV-Vis spectrophotometer. Similar
method was followed to make NH4H2PO4 standard solutions and
phosphorus was estimated in the catalyst samples using 5 point
calibration curve obtained from standard solutions. Morphology
of the catalysts were studied with FESEM. In a typical procedure,
a small amount of catalyst was smeared on carbon tape glued to
the stage, degassed in vacuum for 24 h, sputtered with gold for
1 min and image was collected in Zeiss instrument with different
magnifications.
2. Experimental
2.1. Catalyst preparation
Zeolite ZSM-5 with different silica to alumina ratios (SAR) were
kindly donated by Süd-Chemie India Pvt Ltd. Sodium form of zeo-
lites were exchanged with ammonium ions by refluxing in 1 M aq.
NH4NO3 solution for 4 h, filtered and dried, and this procedure was
repeated twice to ensure maximum ion-exchange. The final prod-
uct was calcined at 550 ◦C for 3 h to get HZSM-5. Further, HZSM-5
was modified post synthetically by wet impregnation method by
treatment with phosphorus reagent. In step 1, required amount
of NH4H2PO4 was dissolved in 22 ml water at 60 ◦C to which 5 g
of zeolite was added with stirring and evaporated at 90 ◦C. It was
then dried at 120 ◦C for 4 h and calcined initially at 330 ◦C for 3 h
and then the temperature was increased to 560 ◦C for 10 h. This cal-
cined catalyst powder was cooled to room temperature and stirred
in water at 80 ◦C for 12 h, then filtered and dried at 120 ◦C for 4 h
(Step 2). In step 3, it was then extruded with silica binder (Ludox
AS 40), dried in an oven for 4 h at 120 ◦C and calcined at 500 ◦C
for 8 h. Thus prepared catalysts were designated as XPZSM-5W
where X is percentage of P (as phosphate) added during impreg-
nation and W is the water treatment after P impregnation and
calcination.
Coke was estimated by calcination of spent catalyst in air to get
constant weight (till the coke was completely removed). Difference
in weight before and after calcinations is considered as amount of
coke formed during reaction. It was normalized to hundred and
expressed in terms of percentage coke formed.
Another set of catalysts were prepared by two-step addition
of phosphates. For this, the procedure followed was similar to
the previously mentioned one except that phosphate impreg-
nation was repeated once after Step 2. Catalyst prepared were
1 + 4PZSM-5 W, 2 + 3PZSM-5 W, 3 + 2PZSM-5 W (a + bPZSM-5 W,
where a = % P added during first step, b = % P added during second
step).
2.3. Catalyst evaluation with alkylation of ethylbenzene with
ethanol
Ethylbenzene ethylation was carried out in a vapor phase con-
tinuous down-flow quartz reactor. Quartz reactor was connected
to a preheater at ∼250 ◦C and a condenser maintained at 2 ◦C with
the help of cryostat. About 2 g of extruded, calcined catalyst of
3–6 mm length and 1.5 mm Ø was taken in the quartz reactor. Inter-
nal diameter of the reactor was 14 mm and ratio of the reactor
diameter to catalyst diameter was always in the range of 9–10.
Catalyst bed was spread evenly with ceramic beads on the top and
a thermocouple to sense the bed temperature. It was then placed
inside the furnace and heated to a desired temperature. In a typ-
ical procedure, catalyst was activated in-situ at 500 ◦C for 1 h in
air before reaction and reaction temperature was set to 380 ◦C. A
mixture of ethylbenzene and ethanol was taken with 4:1 mol ratio
and fed into the preheater at a required rate through a syringe
pump (New Era pump system Inc). Carrier gas was fed through
the calibrated rotameter. Gaseous product stream was condensed
and collected in gas liquid separator maintained at 2–5 ◦C. Liquid
and gas products were analyzed (off-line) through GC using HP-
Innowax capillary column; 60 m × 0.25 ␮m × 0.25 mm and porapak
Q packed column; 4 m × 200 mm respectively. The components
were identified and quantified with standards, and confirmed by
GCMS.
2.2. Catalyst characterization
X-ray diffraction studies were performed with Bruker D2 Phaser
diffractometer equipped with Cu K␣ source. X-ray diffraction data
collected was from 5◦ to 50◦ Bragg angle with steps of 0.02 with
an interval of 0.5 s. FTIR spectra were obtained on Bruker Alpha
instrument in ATR mode. Spectral data were collected from 500
to 1500 cm−1 with a resolution of 4 cm−1 and average of 32 scans.
Nitrogen sorption measurements were carried out over Autosorb-
1 C (Quantachrome, USA) unit. The isotherms were measured at
77 K after degassing samples below 10−3 Torr at 300 ◦C for 4 h. The
BET specific surface area was estimated using adsorption data as
per the ASTM 4365 method applicable for microporous solids. The
total pore volume was estimated from the amount adsorbed at a
relative pressure of about 0.95. The acidity of the catalysts was mea-
sured by temperature programmed desorption (TPD) of NH3. In a
typical procedure, 0.1 g of sample was taken in a quartz tube of
1/4 in. packed with silica wool from both sides to remove dead vol-
ume and sample was dehydrated at 550 ◦C for 1 h. The temperature
was decreased to 100 ◦C and NH3 was adsorbed by passing a stream
of 10% NH3 in He through catalyst bead for 1 h. Physisorbed NH3

10
J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
3P SAR 38
3P SAR 84
3P SAR 187
3P SAR 272
3P SAR 408
ZSM-5 SAR 187 0.22 mmol/g
1PZSM-5W
3PZSM-5W
5PZSM-5W
6PZSM-5W
9PZSM-5W
0.22 mmol/g
0.33 mmol/g
0.24 mmol/g
0.37 mmol/g
0.31 mmol/g
100
200
300
Temperature oC
400
500
100
200
300
400
500
Temperature oC
Fig. 2. TPD of ammonia trace and acidity of phosphate modified ZSM-5.
Fig. 1. TPD of ammonia trace and acidity of 3PZSM-5W with different SAR.
TPD of ammonia showed that the stronger acid sites were
replaced with weaker acid sites after phosphate modification
(Figs. 1 and 2). It is evident that the phosphate salt reacts with the
acid sites creating new kinds of acid (active) sites. Interestingly,
it is evident from the TPD plot that stronger acid sites decreased
after phosphate modification but total acidity marginally changed
(Tables 1–3).
Nitrogen sorption shows the decrease in surface area and pore
volume after P-modification at phosphate loadings >1%. TPD of
ammonia analysis of P-modified and unmodified zeolite samples
shows a marginal change in the acidity of zeolite. The shift of peaks
at lower temperatures showed that stronger acid sites were killed
after P-modification.
3. Results and discussion
3.1. Catalyst characterization
XRD patterns of unmodified HZSM-5 matched well with all
phosphate modified samples. This showed the retention of struc-
tural integrity of ZSM-5 after phosphate modification followed by
water treatment (Figs. S1, S2A and S3). It is further confirmed by
the IR spectrum that the bands corresponding to pentasil chain (at
1220–1230 cm−1) and pentasil unit (at 540–550 cm−1) did not shift
positions as compared with zeolite without modification (Figs. S2B,
S4, S5 and S6).
Surface area of PZSM-5 after phosphate modification decreased
(Table S1) from 525 to 368 m2/g which could be due to the impreg-
nated phosphate groups occupied both in external and micropore
areas. Surface area was completely restored after 1% P loading and
subsequent water treatment whereas at higher loadings, surface
area decreased by ∼30%. Pore volume after phosphate modification
also decreased marginally.
Point of Zero Charge (PZC) measurement gives the information
regarding the surface coverage [29]. The plot of PZC vs wt% of phos-
phate on ZSM-5 shows a break at monolayer coverage. This break
was observed at 4% P loading, indicating the completion of mono-
layer islands formation at this loading. Phosphates added were
chemisorbed to form monolayer to multilayer. Better performance
of the catalyst can be achieved at monolayer formation because
Table 1
Acidity, P-content and activity studies of PZSM-5 with different crystal sizes.
Morphology
and average
crystal size ␮m
SAR Acidity
(mmol/g)
P added
(wt%)
P remaining after Acidity After water P/Al
water treatment P modification
aEB Conversion aBenzene
DEB Selectivity aPDEB selectivity
(wt%)
selectivity
(wt%)
(wt%)
(wt%)
(wt%)
(mmol/g)
Spherical
0.63
240 0.22
272 0.19
280 0.20
250 0.24
0
–
n/a
–
35.6
12.9
85.1
38.6
2
5
0.45
0.94
0.29
0.30
0.47
0.98
27.7
23.7
8.1
7.8
86.1
88.4
53.5
88.3
Irregular
1.2
0
–
n/a
–
16.7
3.6
98.9
66.13
2
5
0.51
1.05
0.29
0.16
0.60
1.24
23
20.3
3.5
3.4
97.3
95.5
80.4
98.4
Cuboid
1.1#
0
–
n/a
–
16.3
6.6
91.5
62.7
2
5
0.40
1.01
0.17
0.18
0.39
0.99
15.5
13.5
6.3
5.2
91.4
92.5
72.1
91.5
Inter grown
cube
0
–
n/a
–
25.7
10.9
86
61.4
3.9
2
5
0.45
1.02
0.28
0.19
0.49
1.11
19.9
16.1
10.4
10.0
84
88
87.1
97.6
a
Conditions: temperature = 380 ◦C WHSV = 3 h−1, catalyst weight = 2 g, H2: reactants = 2, ethylbenzene:ethanol = 4:1, time = 4 h; # based on volume equivalent sphere
diameter.

J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
11
Table 2
Acidity and P-content of PZSM-5 with different SAR.
HZSM-5 (SAR)
HZSM-5 acidity (mmol/g)
P added (wt%)
P remaining water treatment (wt%)
Acidity after water treatment (mmol/g)
P/Al
38
84
187
272
408
1.74
1.5
0.22
0.19
0.17
3
3
3
3
3
1.78
0.88
0.89
0.92
0.87
0.76
0.48
0.33
0.21
0.21
0.29
0.32
0.72
1.09
1.55
Table 3
Stepwise phosphate modification of HZSM-5 (SAR-187).
Catalyst
Step 1 P added Step 2 P added P remaining after
Acidity after water
P/Al
aEB conversion aEthanol
aPDEB selectivity
(wt%)
(wt%)
(wt%)
water treatment (wt%) treatment (mmol/g)
(wt%)
conversion (wt%)
HZSM-5
0
1
2
3
5
0
4
3
2
0
0
0.22
0.20
0.28
0.20
0.24
–
17.3
21
21
21
22.8
99.9
99.9
99.9
99.9
99.9
70
97
97
97
1 + 4PZSM 5W
2 + 3 PZSM-5W
3 + 2PZSM-5W
5 PZSM-5W
0.66
0.90
0.66
0.81
0.54
0.94
0.54
0.66
98.5
a
Conditions: temperature = 380 ◦C WHSV = 3 h−1, catalyst weight = 2 g, H2: reactants = 2, ethylbenzene:ethanol = 4:1, time = 4 h.
27Al MAS NMR (Fig. 3(A and B)) spectra confirm that all the Al
are tetrahedral in nature in both ZSM-5 (SAR 187) and 5PZSM-5 W
(SAR 187). This indicates that Al sites are retained in the zeo-
lite framework after phosphate modification and dealumination
did not occur. Interestingly, the peak at 53.3 ppm correspond-
ing to framework Al shifted downfield marginally to 56.5 ppm
after phosphate treatment. This could be due to phosphate and
framework Al interactions as shown in Scheme 1. 31P MAS NMR
spectra of PZSM-5W showed pyrophosphate, polyphosphate, and
aluminophosphate species (−5.72, −22.05, −32.87 ppm respec-
tively) (Fig. 3(C)). This shows that orthophosphate interaction with
zeolite matrix. However, pyrophosphate, polyphosphate, and alu-
minophosphate formed a stable interaction with the zeolite hence
it was retained even after water treatment. Similar observation for
water treated PZSM-5 was reported by Derewinski et al. [34]. There
higher loadings result in multilayer of phosphates which may block
the pores to create diffusion problems [22].
The % of phosphorus added during preparation was plotted
with phosphorus remaining after water treatment. In all the prepa-
rations of PZSM-5 catalysts, phosphate to aluminium mole ratio
added was >1. The plot shows that, as the phosphate impregna-
tion increased from 0 to 3%, phosphate content in zeolite after
water treatment increased linearly and becomes almost constant
at 0.8–0.9% up to 6% impregnation [22]. This shows the satura-
tion of phosphate monolayer islands inside the pores. At higher
phosphate impregnations (>6%), there is an increase in phosphate
content which may be due to the formation of multilayer of phos-
phates. These monolayer phosphate sites catalyze the reaction and
increase the selectivity by imparting additional diffusion barrier to
formed isomers.
Fig. 3. 27Al MAS NMR of (A) ZSM-5, (B) 5PZSM-5W, (C) 31P MAS NMR of 5PZSM-5-W.

12
J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
to generation of new kind of low strength acid sites [22]. In pre-
vious reports, ZSM-5 with lower SAR < 100 were used invariably
for preparing PZSM-5 and reported decrease in acidity after mod-
ification. In present study, decrease in acidity after P-modification
is true only for ZSM-5 with low SAR and interestingly, marginal
increase acidity was observed for higher SAR. This change in acidity
can be explained with respect to the possible modes of interac-
tion of phosphates with zeolite framework as shown in Scheme 1
(adopted from Refs. [12,15,37,38]). At lower SAR, possibility of for-
mation of structures C and D is relatively high due to high acid site
density. For higher SAR, formation of A and B is possible due to
well dispersed acid sites which results in the formation of two acid
sites by replacing one acid site and hence acidity increased. Forma-
tion of species C, D, and E result in decrease of acid sites. 31P MAS
NMR spectra of 5PZSM-5W (SAR 187) suggests that all 5 species
can form but relative change in acidity can possibly be attributed
to relative abundance of these species over PZSM-5 with different
SAR.
Scheme 1. Phosphate interaction with zeolite framework as adopted from A [12],
B [37], C [15], D [38], E [15].
During 2-step phosphate addition, phosphates retained was
almost similar for all catalysts except 2 + 3 PZSM-5 which showed
maximum phosphorus retention of 0.9% (0.94 P/Al) (Table 3, Fig S7).
Some Al sites which left unreacted during first step addition may
react with phosphates during second step. However, the reaction
of phosphate groups formed during the first step with phosphates
added in the second step cannot be ruled out. Acidity of different
PZSM-5W prepared with 2-step phosphate additions were in the
range 0.20–0.28 mmol H+/g whereas unmodified HZSM-5 showed
acidity of 0.22 mmol H+/g.
was no aggregation or change in morphology for 5PZSM-5 W con-
trary to the observations made by the van der Bij et al. where water
treatment was not carried out [35]. Morphologies of the catalysts
before and after modification were spherical with particle size in
the range of 1.5–3.1 ␮m (Fig. 4(A and B)).
To understand the effect of acidity on phosphate modification,
3PZSM-5 W catalysts with different SAR were prepared and charac-
terized for phosphate content and acidity by TPD of ammonia. For
unmodified HZSM-5 catalysts, acidity decreased with increase in
SAR as expected (Fig. 1, Table 2). After P-modification, phosphates
per Al site increased with increase in SAR even though phosphate
added to the zeolite was constant (3 wt%). It could be attributed to
the interaction of phosphates predominantly with Al sites at lower
SAR and increase in SAR increases phosphate-phosphate interac-
tions forming poly-phosphates. Phosphorus retained on zeolite was
almost constant for all SAR except SAR 38. Interestingly, the acidity
of ZSM-5 increased for higher SAR of 187, 272, 408 after phosphate
treatment. The increase in acidity after modification could be due
3.2. Alkylation of ethylbenzene with ethanol
Alkylation of ethylbenzene with ethanol in
a continuous
down-flow reactor in vapor phase gives mainly a mixture of o,m,p-
diethylbenzenes (hereafter DEB). Minor products such as benzene,
toluene, xylene, ethyltoluene, triethylbenzene were also formed
during the reaction.
Fig. 4. Scanning electron micrographs of (A) HZSM-5(SAR 187), (B) 5PZSM-5W (SAR 187), (C) HZSM-5(SAR-240), (D) HZSM-5(SAR-250), (E) HZSM-5(SAR-272), (F) HZSM-
5(SAR-280).

J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
13
3.2.1. Effect of silica to alumina ratio of ZSM-5
380 to 400 ◦C, EB conversion improved by 2% however, DEB selec-
tivity decreased due to higher dealkylation to form benzene.
Ethanol utilization towards DEB increased as the temperature was
increased upto 380 ◦C and then remained constant. Hence, 380 ◦C
was optimum temperature with good conversion (23%), high PDEB
selectivity (98.5%) and ethanol utilization towards DEB (69.64%).
Silica to alumina ratio (SAR) of zeolite is important for any
acid catalyzed reaction as the conversions depend upon the acid
site concentration and strength. It is generally observed that, with
increase in SAR, amount of acid sites decreases so is the conversion.
As the acidity of catalyst decreases, alcohol to olefin side reaction
also decreases. This leads to a decrease in coke formation and hence
catalyst life increases. Phosphate modification of ZSM-5 not only
modifies the acid sites but also narrows the pores. To find the best
SAR, 3% P loaded zeolites of different SAR were prepared and stud-
ied (Fig. 5A, Table 2). With increase in SAR, conversion decreased
as expected, but selectivity for para isomer increased progressively
from SAR 38 to 407. As added phosphates react with aluminum
sites, P/Al increases with increase in SAR with identical 3% P loading.
Hence for higher SAR, lower amount of phosphates are sufficient for
narrowing of pores and as a result, para selectivity increases with
increase in SAR. 3PZSM-5W with SAR 187 showed better perfor-
mance in terms of EB conversion (22.3%) with lesser side products
(diethylbenzene selectivity of 85%) and hence this was chosen for
further studies.
3.2.5. Effect of reactants mole ratio
Effect of mole ratio was studied at 380 ◦C, WHSV of 3 h−1 and
H2/reactants mole ratio of 2 (Fig. 5D). Limiting reactant, ethanol
conversion was invariably >99% in all the experiments. With
decrease in EB to ethanol mole ratio from 8:1 to 1:1, the conversion
of EB increased from 17.2 to 33.5%. With increase in concentra-
tion of alkylating agent, total DEB increased from 72.6 to 90.8%
instead of heavy aromatics. This could be due to the restricted pore
size of PZSM-5, which avoids the formation of bulkier products
inside the channels. Even though the conversion of EB increased
with increase in alkylating agent, it did not increase proportion-
ately with respect to increase in ethanol concentration in the feed
because more amount of ethanol converted into gaseous products
at higher ethanol concentrations (Table S2). Due to this, ethanol
utilization towards DEB decreased substantially from 77.3 to 29.6%
with decrease in EB:Ethanol from 8:1 to 1:1. Optimum mole ratio
was found to be 4:1, considering good conversion (22.8%) and high
ethanol utilization (69.6%).
3.2.2. Optimization of phosphate content
Impregnation of phosphate to ZSM-5 increased the EB conver-
sion as well as selectivity to PDEB (Fig. 5B). Moreover, weaker acid
sites generated in 3PZSM-5W (NH3-TPD, Fig. 2), decrease the gas
formation resulting in increased ethanol utilization towards total
DEB. Increase in P from 3 to 5% increased PDEB selectivity from
93 to 98.5% but conversion was almost the same (∼22%) for both
the catalysts. Further increase in P > 5% increased the selectivity for
PDEB but decreased the conversion. This could be due to the diffu-
sion limitation imparted by narrowed zeolite pores even to reactant
molecules at higher P concentrations. Due to smaller molecular
size, ethanol can easily diffuse into pores and reach active sites
resulting in gas forming reactions leading to decreased ethanol uti-
lization towards alkylation. Maximum of 99.5% PDEB selectivity
was achieved with 9% P on ZSM-5 (SAR 187) with 9% EB conversion.
Considering both conversion and selectivity, optimum concentra-
tion of P was 5 wt% which gave 23% conversion with 98.5% PDEB
selectivity in 4 h time on stream.
3.2.6. Effect of space velocity
Weight hourly space velocity (WHSV) gives a correlation
between throughput and contact time. With increase in WHSV
from 1 to 10 h−1, conversion progressively decreased from 23.9 to
12.5% (Fig. 5E) due to decrease in effective contact time of reac-
tants with the catalyst. At lower WHSV, the side product formation
was higher whereas at higher WHSV side products decreased with
improvement in total DEB formation. At WHSV = 1 h−1, conversion
was 23.9% and DEB selectivity was 77%, whereas at WHSV = 3 h−1
conversion decreased marginally to 22.8% but total DEB increased
considerably (85.3%). With further increase in WHSV from 3 to
10 h−1, conversion of EB decreased progressively whereas DEB
selectivity improved only marginally. Considering both conversion
and selectivities for DEB and PDEB, WHSV of 3 h−1 was chosen as
the optimum space velocity.
3.2.3. Effect of 2-step phosphate modification
It is shown that phosphates interact with aluminium sites of
ZSM-5 and form phosphate islands with increase in phosphate con-
centration [22]. Modification of ZSM-5 with phosphate was carried
out in 2 steps to understand the phosphate and zeolite interaction
(Table 3). For this, total phosphates added in 2 steps were kept con-
stant at 5 wt% (as described in experimental section). Phosphate
modified catalysts prepared in two steps and in the single step
contained similar amount of phosphates (0.7–0.9%) as well as sim-
ilar acidity (in the range of 0.17–0.22 mmol/g). This is despite the
fact that water treatment was carried out in every step to remove
water-soluble phosphates. Interestingly, activity and selectivity of
different catalysts prepared in two steps (with % P addition; 1 + 4,
2 + 3, 3 + 2) were same with 21% EB conversion with 97% selectiv-
ity whereas for single step addition, conversion was 23% with 98%
selectivity for PDEB. This shows that, single step addition of phos-
phates gave marginally better performance than two-step addition.
3.2.7. Effect of Hydrogen
Effect of carrier gas H2 on the catalyst performance was stud-
ied by varying H2 to reactants mole ratio (Fig. 5F). Without carrier
gas (H2/reactant = 0), dehydration of ethanol predominated over
the alkylation forming high amount of gaseous side products. Due
to this, the conversion without carrier gas was low (16.7%) with
total DEB of 90%. With the introduction of hydrogen as carrier
gas (H2: Reactants = 1), conversion increased to 21% whereas total
DEB decreased to 84.0%. With further increase in hydrogen flow of
H2: Reactants = 2, there was marginal variation in conversion and
DEB (22.8% and 85.3% respectively). With further increase in the
hydrogen flow (H2: Reactant = 4), conversion and DEB selectivity
decreased considerably to 19.8% and 82.5% respectively. Selectivity
towards PDEB was constant (∼98%) at different carrier gas concen-
trations in the feed and for further studies were carried out with
H2: Reactant = 2.
3.2.4. Effect of temperature
With increase in temperature from 320 to 400 ◦C, activity of the
catalyst increased as expected (Fig. 5C). EB conversion increased
from 7.1 to 25% whereas PDEB selectivity remained almost the
same (∼98%) at all temperatures. However, total DEB selectivity
increased from 90.9 to 93.4% as the temperature increased from
320 to 360 ◦C and it decreased to 84.9% with further increase in
temperature to 380 ◦C. As the temperature was increased from
Interestingly, PDEB selectivity in the mixture of DEB was very
high and remained almost constant (98–99%) under various reac-
tion conditions such as temperature, reactants mole ratio, space
velocity and carrier gas concentration in the feed. This suggests
that high selectivity achieved for PDEB is independent of oper-
ating parameters and truly due to shape selectivity obtained by
phosphate modification of ZSM-5 catalyst.

14
J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
100
90
EB Conversion
Benzene
PDEB Selectivity
DEBs
Ethanol utilization
Benzene Selectivity
80
70
Conv
DEBs
PDEB
60
B
C
A
EB Conversion
PDEB Selectivity
DEBs
Ethanol utilization
50
40
30
20
10
320 340 360 380 400
Temperature oC
38
84 187 272 408
SAR
0
3
5
7
9
wt % P loading
100
80
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
EB Conversion
Toluene
Benzene
PDEB selectivity
DEBs
Ethanol utilization
Benzene Selectivity
E
D
F
EB conversion
PDEB Selectivity
DEBs
EB conversion
PDEB Selectivity
DEBs
60
Benzene Selectivity
Benzene Selectivity
40
20
8:1 6:1 4:1 2:1 1:1
Ethylbenzene : EtOH
Mole ratio
0
2
4
6
8
10
0
1
2
3
4
H2/Feed
WHSV
Fig. 5. (A) Effect of P modification on ZSM-5 with different SAR; (B) Optimization of P content on ZSM-5(SAR 187) (conditions for (A and B): temperature = 380 ◦C WHSV = 3 h−1
,
catalyst weight = 2 g, H2: reactants = 2, ethylbenzene:ethanol = 4:1, time = 4 h); (C) effect of temperature (conditions: WHSV = 3 h−1, catalyst weight = 2 g, H2: reactants = 2,
ethylbenzene:ethanol = 4:1, time = 4 h); (D) effect of reactant mole ratio (conditions: temperature = 380 ◦C WHSV = 3 h−1, catalyst weight = 2 g, H2: reactants = 2, time = 4 h);
(E) Effect of space velocity (conditions: temperature = 380 ◦C, catalyst weight = 2 g, H2: reactants = 2, ethylbenzene:ethanol = 4:1, time = 4 h); (F) effect of hydrogen as carrier
gas (conditions: temperature = 380 ◦C WHSV = 3 h−1, catalyst weight = 2 g, ethylbenzene:ethanol = 4:1, time = 4 h).
3.2.8. Effect of crystal size and morphology
25.5% whereas PDEB selectivity decreased marginally (61.4%). For
zeolite with smaller crystal size, product can diffuse easily without
much diffusion barrier and hence the activity of the catalyst was
high. It is found that benzene formation due to dealkylation of EB
was higher for unmodified ZSM-5 with small and large crystals.
Dealkylation decreased marginally after phosphate modification
but benzene still remained as major side product. At lower con-
centration of phosphates (2% P), PDEB selectivity increased with
increase in crystal size. At higher concentrations (5% P), small crys-
tal size (0.63 ␮m) attained 88.3% PDEB selectivity and it increased
to 91.5% for 1.1 ␮m with cuboid shape. Para isomer selectivity was
maximum for 1.2 ␮m with irregular shape and 3.9 ␮m size crystals
(∼98%). Difference in para selectivity for 1.1 ␮m size crystals could
be due to their different morphologies and marginally less conver-
sion due to marginal difference in SAR. Lower PDEB selectivity for
0.63 ␮m size catalyst suggests that more phosphates are needed
for smaller crystal size zeolite to achieve similar PDEB selectiv-
ity comparable with bigger crystal size zeolites. This could be due
to reduced path length for zeolite with small crystal size, which
increases the number of zeolite channels per unit volume com-
pared with zeolite with bigger crystals. At lower concentration of
phosphates, all the isomers are formed over the catalyst but due to
a diffusion barrier imparted by phosphates results in the increased
PDEB selectivity. Phosphates in the micropores of ZSM-5 not only
Study on the effect of zeolite crystal size is important as it gives
information on the mechanism of shape selectivity of the cata-
lyst [36]. Crystal size has a direct correlation with selectivity to
para isomer formed during aromatic substitution reactions. For a
catalyst with larger crystal size, the selectivity of para-isomer is
greater than that of selectivity in thermodynamic equilibrium mix-
ture of isomers. It could be observed from optimization of reaction
parameters that PDEB selectivity is not affected by the reaction con-
ditions and EB conversion levels because the selectivity for PDEB
is the property of shape selectivity of phosphate modified ZSM-5.
Hence, the difference in conversion levels obtained for different
crystal sizes could be ignored while correlating crystal size with
PDEB selectivity. Silica extrudates of three different crystal sizes of
ZSM-5 with similar SAR (similar acidity) were studied for ethyla-
tion of ethylbenzene under the same reaction conditions (Table 1,
Fig. 4). Unmodified HZSM-5 catalysts with different crystal sizes
were compared to understand the size effect. It is observed that
as the average crystal size increased from 0.3 to 1.2 ␮m, EB con-
version decreased from 35.6 to 16.7% whereas PDEB selectivity
enhanced from 38.6 to 66.1%. With crystal size of 1.1 ␮m and cuboid
morphology did not change the conversion and selectivity appre-
ciably (16.3% conversion and 62.7% selectivity). For ZSM-5 with
large crystals (Avg. crystal size = 3.9 ␮m), conversion increased to

J.L. Hodala et al. / Applied Catalysis A: General 484 (2014) 8–16
15
restricted transition state mechanism is also possible. Acid site
density of the catalyst is important as phosphates react with Al
and increase diffusion barrier. ZSM-5 with SAR 187 was found to
contain optimum acidity for phosphate modification to achieve
good conversion and high selectivity for p-diethylbenzene. With
increase in phosphate loading, para selectivity increased and 5% P
(as oxide) on ZSM-5 was found to be optimum loading with good
conversion (22.8%) and high PDEB selectivity (98.5%). Single step
phosphate modification was better than two-step modification to
achieve high activity and selectivity for p-diethylbenzene. High and
unchanged p-diethylbenzene selectivity in the mixture of diethyl-
benzenes (98–99%) under various reaction conditions suggested
that high selectivity achieved for p-diethylbenzene is indepen-
dent of operating parameters and truly due to shape selectivity
obtained by phosphate modification of ZSM-5 catalyst. Under opti-
mized reaction conditions, catalyst showed good stability towards
deactivation during 50 h of time on stream with minimum coke
deposition.
100
90
80
70
60
50
40
30
20
10
0
EB conversion
PDEB selectivity
DEBs
Benzene Selectivity
0
5
10 15 20 25 30 35 40 45 50
Time (h)
Fig. 6. Time on stream under optimized reaction parameters (conditions:
temperature = 380 ◦C WHSV = 3 h−1, catalyst weight = 2 g, H2: reactants = 2, ethyl-
benzene:ethanol = 4:1).
Acknowledgements
increase the diffusion barrier but also decrease the pore volume
(Table S1). Decrease in pore volume reduces the formation of tran-
sition state of bulkier m-DEB and o-DEB and hence low selectivities
for m and o-DEB were observed at higher phosphate concentra-
tions. These results show that, at lower phosphate concentrations,
phosphate modification imparts diffusion barrier to the formed iso-
mers and hence mechanism of product shape selectivity could be
proposed. However, at higher phosphate concentrations, restricted
transition state mechanism is also possible. It is reported elsewhere
that product shape selectivity depends on the length of inter crys-
talline diffusion path and hence the measurable selectivity effects
decrease with decrease in crystal size. If restricted transition state
shape selectivity is operative, the measurable selectivity will be
independent of the crystal size [36].
HLJ acknowledges Admar Mutt Education Foundation (AMEF),
Bangalore for a fellowship and Manipal University, India for per-
mitting this research as a part of the Ph.D. programme. Authors
gratefully thank Sophisticated Instrument Facility, Indian Institute
of Science, Bangalore for collecting MAS NMR experimental data
and Centre for Nanoscience and Engineering, Indian Institute of
Science, Bangalore for SEM images.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.07.006.
References
3.2.9. Time on stream (TOS)
TOS was carried out under optimized conditions of 380 ◦C,
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(Fig. 6). Catalyst initially showed high EB conversion of 22.5% after
4 h and then decreased gradually to 20% in 11 h TOS. The conversion
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during 50 h reaction.
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