Influence of phosphorus content on physicochemical and catalytic properties of H-pentasil in m-xylene isomerization and toluene methylation

Article abstract of DOI:10.1134/S1070427206100363

Physicochemical and catalytic properties of H-pentasils in m-xylene isomerization and toluene methylation were studied. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S1070427206100363

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 10, pp. 1723 1725.
Pleiades Publishing, Inc., 2006.
Original Russian Text
S.E. Mamedov, E.I. Akhmedov, F.Sh. Kerimli, M.I. Makhmudova, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79,
No. 10, pp. 1741 1743.
BRIEF
COMMUNICATIONS
Influence of Phosphorus Content on Physicochemical
and Catalytic Properties of H-Pentasil in m-Xylene
Isomerization and Toluene Methylation
S. E. Mamedov, E. I. Akhmedov, F. Sh. Kerimli, and M. I. Makhmudova
Baku State University, Baku, Azerbaijan
Received February 21, 2006
Abstract Physicochemical and catalytic properties of H-pentasils in m-xylene isomerization and toluene
methylation were studied.
DOI: 10.1134/S1070427206100363
Control of the zeolite pore size and acid properties
by elimination of the strongest acidic centers and
by variation of their nature is one of probable
ways to modify the catalytic and molecular-sieve
properties of zeolites. Chemical modification of
the zeolite surface with compounds of B, Mg, P,
Si, etc. [1 3], which have a para-orienting effect
upon the activity of zeolite catalysts in transforma-
tions of aromatic hydrocarbons, is promising in this
respect.
The IR spectra were recorded on a Specord spec-
trophotometer. Zeolites were compacted into thin pel-
lets containing 6 8 mg of the substance per square
centimeter. Before adsorption, they were heated in
a vacuum for 2 h at 500 C. Adsorption of pyridine
was performed at room temperature and saturated
vapor pressure. The IR spectra in the region of alu-
minosilicate framework vibrations were recorded with
samples pelletized with KBr. Adsorption measure-
ments were performed by the method described in [4].
Since pentasil-type high-silica zeolites offer wide
opportunities in oil refining, it is interesting to study
how modification of such zeolites with a phosphorus
compound will affect their catalytic and physico-
chemical characteristics in m-xylene isomerization
and toluene methylation.
The experiments were performed on a flow-type
device with a fixed catalyst bed (4 cm3), in a plug-
flow reactor, at atmospheric pressure within the 250
450 C range, volumetric flow rate of the raw material
1
of supply 1 h , and m-C6H4(CH3) : H2 and C7H8 :
CH3OH : H2 molar ratios of 3 : 1 and 2 : 1 : 2, re-
spectively. The reaction products were analyzed chro-
matographically [5].
EXPERIMENTAL
Pentasil-type zeolite with a SiO2/Al2O3 molar ratio
of 58 was used. It was converted into the NH4 form
by ion exchange as in [4] and then into the H form
by thermal decomposition of the NH4 form for 4 h
at 500 C. Catalysts modified with 0.5 4 wt % phos-
phorus were prepared by impregnation of the zeo-
lite H form with ammonium hydrophosphate for 6 h
at 80 C. The samples were dried in air for 16 h, then
for 4 h in a desiccator at 110 C, and finally calcined
in a muffle for 4 h at 550 C.
The X-ray phase analysis showed that the crystal-
line structure of the zeolite is preserved during mod-
ification with ammonium hydrophosphate. The IR
study confirmed this conclusion. In the region of alu-
1
minosilicate framework vibrations (400 1300 cm ),
which are the most sensitive to the degree of crystal-
linity of the zeolite, the bands of the external Si O
1
bonds at 550 560 and 790 800 cm remain virtual-
ly unchanged upon modification with phosphorus.
An increase in the intensity of the high-frequency
branch of the asymmetrical stretching band (1000
The catalysts obtained were subjected to chemical
analysis, X-ray phase analysis, and adsorption anal-
ysis and studied by IR spectroscopy.
1
1100 cm ) may be due to interaction of phosphorus
atoms with structural aluminum atoms [6].
1723

1724
MAMEDOV et al.
higher than 2 wt %, the p-xylene content noticeably
decreases, but the para-selectivity substantially in-
creases (from 70 to 85%). A different trend is observed
for o-xylene. An increase in the phosphorus content
decreases the o-xylene content within the entire range
of phosphorus contents and, especially at those higher
than 2 wt %. For example, the o-xylene yield de-
creases from 12.0 to 1.2% when the phosphorus con-
tent is raised from 2 to 4%. In this case, the conden-
sate is enriched in p-xylene, i.e., the selectivity with
respect to p-xylene formation increases.
The temperature dependence of the m-xylene and
toluene conversion and selectivity with respect to
p-xylene on H-pentasil modified with 4 wt % phos-
phorus is shown in Fig. 2. As seen, a decrease in the
conversion of m-xylene and toluene is accompanied
by an increase in the para-selectivity of the catalyst.
At 250 C and 12% conversion of the aromatic hydro-
carbon, the selectivity with respect to p-xylene reaches
95%. This shows that p-xylene is the primary product
of m-xylene isomerization and toluene methylation
on the phosphorus-containing zeolite.
Fig. 1. Yield Y of (1) p-xylene and (2) m-xylene, and
(3) selectivity S with respect to p-xylene vs. the phosphorus
content of H-pentasil.
The differences in the distribution of the products
of m-xylene isomerization and toluene methylation
and in the selectivity of the reaction on modified
pentasils become more understandable when the re-
sults obtained are considered from the viewpoint of
control of the reaction rate by diffusion into the vol-
ume. Reactions in zeolites occur with the diffusion
rate determined by the shape and size of both the re-
acting molecules and zeolite channels (configuration
diffusion) [5]. In this case, the increased selectivity
with respect to p-xylene can be accounted for by
the higher diffusion rate of p-xylene molecules, which
have a smaller kinetic diameter than molecules of
m- and o-xylenes, in the channels of the modified
pentasil.
Fig. 2. Conversion k of (1) m-xylene and (3) toluene, and
selectivity S with respect to p-xylene vs. temperature T
in (4) isomerization of m-xylene and (2) methylation of
toluene.
Analysis of the activity and selectivity of the mod-
ified catalysts shows that the phosphorus content
strongly affects the m-xylene isomerization and tolu-
ene methylation. As can be seen from Fig. 1, the yield
of p-xylene somewhat grows as the phosphorus con-
tent increases to 1.0 wt %. At a phosphorus content
Indeed, the modification with phosphorus is ac-
companied by a chemical reaction with the zeolite, as
judged from a substantial decrease in the intensity of
absorption of zeolite OH groups in the IR spectra of
the modified samples as compared to the initial zeo-
lite. In addition, polymeric structures of P O P
type probably appear during modification [6], along
with PO4 fragments formed in reactions of phosphorus
compounds with zeolite OH groups, which undoubt-
edly affects the accessibility of the zeolite channels.
This, in turn, decreases the zeolite sorption capacity
(see table) and is reflected in molecular-sieve proper-
ties of pentasil in reactions sensitive to the pore size:
m-xylene isomerization and toluene methylation.
Adsorption of water and benzene vapors on H-pentasils
(HP) modified with phosphorus (P/Ps = 0.1, T = 20 C)
1
Adsorption, cm3 g
Zeolite
H2O
G6H6
NP
0.078
0.071
0.059
0.043
0.018
0.0083
0.046
0.040
0.031
0.023
0.0084
0.0037
0.5% P/NP
1.0% P/NP
2.0% P/NP
3.0% P/NP
4.0% P/NP
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 10 2006

INFLUENCE OF PHOSPHORUS CONTENT ON PHYSICOCHEMICAL PROPERTIES OF H-PENTASIL 1725
It is known that a change in the number and strength
of acid centers is the main reason for variation of
the catalytic activity of zeolites [7]. Since the isomer-
ization is catalyzed by Brønsted acid centers faster
than by Lewis centers, a decrease in the Brønsted
acidity and a change in the ratio of Brønsted to Lewis
acidic centers will affect the catalyst activity [8].
The IR spectra of the initial zeolite and that mod-
ified with ammonium hydrophosphate (Fig. 3) show
that the band at 3615 cm 1 in the region of the stretch-
ing vibrations of the strongly acidic bridging OH
groups is observed in the spectra of all the samples
studied. Its intensity substantially decreases with
an increase in the phosphorus content of the sample.
This fact suggests that phosphorus partially dealu-
minates the zeolite and thus neutralizes acid centers
situated at entrances into channels or at their inter-
sections [6]. Thus, modification of zeolites with phos-
phorus decreases the total concentration of hydroxy
groups.
Fig. 3. IR spectra of H-pentasils modified with phosphorus
and heated in a vacuum at 500 C for 2 h. (1) H-pentasil;
P (wt %): (2) 1.0, (3) 2.0, and (4) 4.0. (1 4 ) After pyridine
adsorption at 20 C for 1 h and desorption at 200 C.
( ) Wavenumber.
In the region of pyridine bending vibrations (1440
1
1700 cm ), the spectra of all the samples contain
the following pyridinium ion (PyH+) bands: a band
decreases the zeolite sorption capacity. The ratio of
Lewis to Brønsted acid centers increases and the zeo-
lite channels become narrower, which makes higher
the catalyst selectivity with respect to p-xylene.
1
at 1550 cm corresponding to Brønsted acid centers
1
(B centers), band at 1450 cm belonging to Lewis
1
acid centers (L centers), and band at 1490 cm , be-
longing to B and L centers; their intensity decreases
with the phosphorus content increasing from 1.0 to
4.0 wt %. Thus, not only the amount of strong acid
centers decreases, but also the ratio of L to B centers
increases more substantially, which is probably a rea-
son for the change in the activity and selectivity of
the catalysts modified with phosphorus.
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phosphorus affects not only the concentration, but also
the nature and properties of the active centers in zeo-
lite. In addition, the zeolite channels become nar-
rower. All these facts affect the adsorption and dif-
fusion characteristics of the catalysts, which largely
determine the high selectivity with respect to forma-
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CONCLUSION
Chemical modification of H-pentasil with am-
monium hydrophosphate redistributes acid centers and
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 10 2006