The mechanism of phenol methylation on acid and basic zeolite catalysts

Article abstract of DOI:10.1134/S0036024406060094

The alkylation of phenol with methanol on HY and CsY/CsOH catalysts was studied in situ under static conditions by ¹³C NMR spectroscopy. Attention was largely given to the identification of intermediate compounds and mechanisms of anisole, cresol, and xylenol formation. The mechanisms of phenol methylation were found to be different on acid and basic catalysts. The primary process on acid catalysts was the dehydration of methanol to dimethyl ether and methoxy groups. This resulted in the formation of anisole and dimethyl ether, the ratio between which depended on the reagent ratio, which was evidence of similar mechanisms of their formation. Subsequent reactions with phenol gave cresols and anisoles. Cresols formed at higher temperatures both in the direct alkylation of phenol and in the rearrangement of anisole. The main alkylation product on basic catalysts was anisole formed in the interaction of phenolate anions with methanol; no cresol formation was observed. The deactivation of acid catalysts was caused by the formation of condensed aromatic hydrocarbons that blocked zeolite pores. The deactivation of basic catalysts resulted from the condensation of phenol and formaldehyde with the formation of phenol-formaldehyde resins. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S0036024406060094

ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 6, pp. 892–898. © Pleiades Publishing, Inc., 2006.
Original Russian Text © I.B. Borodina, E.B. Pomakhina, Ts.M. Ramishvili, O.A. Ponomareva, A.I. Rebrov, I.I. Ivanova, 2006, published in Zhurnal Fizicheskoi Khimii, 2006,
Vol. 80, No. 6, pp. 1022–1028.
CHEMICAL KINETICS
AND CATALYSIS
The Mechanism of Phenol Methylation
on Acid and Basic Zeolite Catalysts
I. B. Borodinaa, E. B. Pomakhinaa, Ts. M. Ramishvilib, O. A. Ponomarevaa, A. I. Rebrovc,
and I. I. Ivanovaa
a Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119899 Russia
b Melikishvili Institute of Physical and Organic Chemistry, Academy of Sciences of Georgia, Tbilisi, Georgia
c Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 117912 Russia
e-mail: OAPonomareva@phys.chem.msu.ru
Received October 20, 2005
Abstract—The alkylation of phenol with methanol on HY and CsY/CsOH catalysts was studied in situ under
static conditions by 13C NMR spectroscopy. Attention was largely given to the identification of intermediate
compounds and mechanisms of anisole, cresol, and xylenol formation. The mechanisms of phenol methylation
were found to be different on acid and basic catalysts. The primary process on acid catalysts was the dehydra-
tion of methanol to dimethyl ether and methoxy groups. This resulted in the formation of anisole and dimethyl
ether, the ratio between which depended on the reagent ratio, which was evidence of similar mechanisms of
their formation. Subsequent reactions with phenol gave cresols and anisoles. Cresols formed at higher temper-
atures both in the direct alkylation of phenol and in the rearrangement of anisole. The main alkylation product
on basic catalysts was anisole formed in the interaction of phenolate anions with methanol; no cresol formation
was observed. The deactivation of acid catalysts was caused by the formation of condensed aromatic hydrocar-
bons that blocked zeolite pores. The deactivation of basic catalysts resulted from the condensation of phenol
and formaldehyde with the formation of phenol-formaldehyde resins.
DOI: 10.1134/S0036024406060094
INTRODUCTION
reactions to be identified and the pathways of their
transformations determined.
The demand for cresols and xylenols is many times
greater than their resources in coal-tar raw materials
[1]. Synthetic methods for their production therefore
remain an object of interest in current research. One
such method is the alkylation of phenol with metha-
nol. This method can be used to obtain various prod-
ucts of phenol methylation in the aromatic nucleus
and at the functional group depending on the acid–
base properties of the catalyst used. These products
are used as starting materials for the preparation of
phenol resins, antioxidants, dyes, plasticizers, and
perfumes.
The purpose of this work was to study the mecha-
nism of phenol alkylation with methanol on acid and
basic catalysts by 13ë NMR spectroscopy in situ.
EXPERIMENTAL
The catalysts used were zeolites HY and CsNaY
modified with cesium hydroxide (CsY/CsOH). The
degree of sodium ion exchange with protons and
cesium ions was 77 and 65%, respectively. The content
of the modifier recalculated to Cs2é was 34.3 wt % in
CsY/CsOH. A detailed description of the preparation of
catalyst samples and their characteristics was given in
[12–14].
Catalytic experiments on phenol methylation with
methanol were conducted in a flow reactor at atmo-
spheric pressure, 673 K, and weight rate of supplying
reagents 1 g/(g h). The molar ratio between phenol,
methanol, and N2 was 1 : 1 : 3. The conditions of chro-
matographic analyses of liquid and gaseous reaction
products are described in [15].
Zeolites are promising alkylation catalysts. They
have certain advantages over Friedel–Crafts catalysts
currently used in industry and mineral acids [2–5]. The
creation of high-effectiveness heterogeneous catalysts,
perfection of the technology of their preparation, and
prediction of their catalytic properties is, however,
often impeded by the absence of detailed data on the
mechanisms of chemical transformations on these
materials.
In recent years, the method of NMR spectroscopy in
The mechanism of phenol methylation was studied
situ [6–11] has been successfully used to study the in situ by NMR spectroscopy. Air-dry catalyst samples
mechanisms of reactions on the surface of zeolites. This (0.08–0.10 g) were placed into NMR ampules, evacu-
method allows intermediate compounds formed in the ated at 10–5 torr and 673 K, and cooled to 298 K. The
892

THE MECHANISM OF PHENOL METHYLATION
893
Selectivity (mol %) of phenol methylation on HY and CsY/CsOH; 673 K, 1 g/(g h), phenol : methanol : N2 = 1 : 1 : 3 (mol);
c is the conversion of phenol
Cresols
Methylani-
soles
Other prod-
ucts
Catalyst
Anisole
Xylenols
c
p-
m-
o-
HY
CsY/CsOH
21.7
0.8
0.0
20.0
0.0
14.1
0.0
37.1
0.0
4.8
0.0
1.3
0.0
10.0
6.5
100.0
adsorption of reagents was then performed. The sole with a 100% selectivity. Changes in phenol con-
reagents were 13C-methanol (ICON Services Inc.), version during the reaction are shown in Fig. 1. The
13C-dimethyl ether, 13C-methyl iodide (Cambridge activity of both catalysts decreases as time passes,
Isotope Laboratories), and 13C-para-formaldehyde but the CsY/CsOH catalyst is deactivated at a higher
(Sigma-Aldrich) enriched in 13C by 99%. We also rate.
used phenol labeled at the first carbon atom (Cam-
Intermediate compounds and primary and second-
bridge Isotope Laboratories) and unlabeled phenol
ary alkylation products were studied in situ by NMR
(Reakhim). The amount of adsorbed reagents varied
spectroscopy. This allowed us to establish the most
from 1 to 40 molecules per zeolite unit cell (mole-
important mechanisms of phenol alkylation with meth-
cules/u.c.) in various experiments. After adsorption,
anol and catalyst deactivation.
NMR ampules were cooled to 77 K, sealed off, and
placed into a standard 7 mm rotor for recording
NMR spectra with magic-angle rotation. The proce-
dure for conducting experiments included heating
the ampule to the required temperature during a fixed
time in a furnace capable of rapidly increasing tem-
perature. The NMR spectra were recorded at 298 K
after the ampule with a sample was cooled at a high
rate. After recording the spectrum, the ampule was
again placed into the furnace, and the experiment
was continued. This procedure is described in more
detail in [14, 16].
The Methylation of Phenol on Zeolite HY according
to the in Situ 13C NMR Data
The 13C NMR magic-angle rotation spectra
obtained under static conditions during the reaction
between 13C-methanol and phenol on zeolite HY at
various surface coverages with the reagents are
shown in Fig. 2.
The initial spectra recorded at room temperature
contain signals with chemical shifts of 49, 51.5, and
57 ppm. The signal with δ = 49 ppm corresponds to the
carbon atom of the methanol methyl group. If a metha-
nol molecule is adsorbed on a catalyst active center
together with a phenol molecule, the methyl group sig-
nal shifts downfield to 51.5 ppm. The signal with a
chemical shift of 57 ppm corresponds to surface meth-
oxy groups formed in the reaction between methanol
and zeolite acid centers. All signals are accompanied by
rotation-induced side lines, which is evidence of fairly
strong adsorption of methanol on the surface. The most
substantial changes in the spectra are observed at
423 K.
The NMR spectra were recorded on an MSL-300
Bruker spectrometer. The instrument was calibrated
13
against C-glycine (its carbonyl group chemical shift
was 176.03 ppm with respect to the tetramethylsilane
13
signal). The NMR spectra were obtained with the
suppression of 13ë–1ç spin-spin interaction. In several
experiments, the 13ë–1ç cross-polarization method
was used. The proton-decoupled 13C NMR spectra are
given below unless otherwise stated.
RESULTS AND DISCUSSION
The results of catalytic experiments are given in
the table. We see that the reaction direction strongly
depends on the acid–base properties of the catalyst.
Aromatic ring alkylation preferably occurs to pro-
duce o-, m-, and p-cresols, methylanisoles, and xyle-
nols on zeolite HY characterized by high acidity.
Trimethyl- and ethylphenols are also formed in
insignificant amounts. Simultaneously, methanol is
transformed into dimethyl ether (DME), water, ë1–ë3
hydrocarbons (largely methane and ethane), and
traces of ë6–ë10 aromatic hydrocarbons (benzene,
toluene, ethylbenzene, xylenes, and tri- and tetrame-
thylbenzenes).
c, %
12
8
HY
4
CsY/CsOH
0
50
100
τ, min
Fig. 1. Time dependences of phenol conversion during phe-
nol methylation on HY and CsY/CsOH at 673 K, 1 g/(g h),
Phenol alkylation at the functional group occurs
on strongly basic zeolite CsY/CsOH to produce ani-
phenol : methanol : N = 1 : 1 : 3 (mol); c is the conversion
2
of phenol.
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894
BORODINA et al.
(‡)
(b)
15.5
20
23.5
15.5
20
62
62
59.5
59.5
473 K, 1 h
423 K, 1 h
473 K, 1 h
423 K, 1 h
57
51.5
49
62.5
20 15.5
* *
*
51.5
57
62
49
57
298 K
**
298 K
*
**
0
*
0
200 160 120
80
δ, ppm
40
200 160 120
80
δ, ppm
40
13
Fig. 2. C NMR magic-angle rotation spectra recorded under static conditions as the temperature increased in steps during the reac-
13
tion after C-methanol and phenol adsorption on zeolite HY in amounts of (a) 7 and 21 molecules/u.c. and (b) 40 and 10 mole-
cules/u.c., respectively.
At a high ratio between methanol and phenol (meth- we assigned this signal to the methyl group in the
adsorption complex of anisole. To check this hypothe-
sis, we performed an additional experiment with ani-
sole with the natural content of 13C adsorbed on zeolite
HY. The spectrum obtained contained a weak signal at
δ = 58 ppm that likely corresponded to the adsorption
complex of anisole with the terminal methyl group ori-
entation. By analogy with dimethyl ether, we might
expect the existence of one more adsorption complex of
anisole similar to the DME complex with the side
methyl group orientation, which is characterized by a
δ = 62 ppm signal in 13C NMR spectra. It appears that
this signal was not observed for adsorbed anisole
because we used a high anisole concentration to obtain
anol/phenol = 4 : 1), the intensity of adsorbed methanol
signals decreases substantially compared with the ini-
tial spectrum as the system is heated to 423 K. The
major reaction product is then dimethyl ether with two
forms of adsorption on acid zeolite HY centers charac-
terized by NMR signals at δ = 59.5 and 62.5 ppm,
respectively. These signals correspond to the terminal
and side configurations [17]. Heating to 473 K causes
the appearance of signals at δ = 15.5 and 20 ppm and a
decrease in the intensity of signals at δ = 49–63 ppm.
The new signals can be assigned to ortho- and para-
cresols (and/or xylenols), respectively. The appearance
of a narrow signal at δ = 23.5 ppm corresponds to meth-
anol transformation into isobutane.
13
signals from C atoms present in their natural abun-
dance. Conversely, the concentration of anisole was
fairly low in phenol methylation experiments, and the
side configuration might therefore be energetically
favorable.
At low methanol/phenol ratios, the intensity of a sig-
nal with a 62 ppm chemical shift increases as the inten-
sity of methanol and methoxy group signals decreases.
This signal appears in the spectrum already at room
temperature. Its chemical shift is similar to that charac-
terizing the side form of the adsorption of dimethyl
ether formed on zeolite HY as a result of methanol
etherification. The formation of this complex is, how-
ever, hardly probable under these conditions, because it
is usually observed in systems with high surface cover-
ages by methanol and is always accompanied by the
formation of the adsorption complex of DME with the
terminal methyl group orientation (δ = 59.5 ppm).
Since phenol, like methanol, can experience etherifica-
The spectrum of a system with a comparatively high
content of phenol heated to 423 K contains two new
lines with the chemical shifts δ = 15.5 and 20 ppm.
They correspond to ortho- and para-cresols, respec-
tively [18]. The signals of methylanisoles and xylenols
are also observed in this spectral region.
The intensity of all signals at δ = 49–63 ppm
decreases substantially as the system is heated to 473 K.
Simultaneously, the cresol and xylenol signal intensi-
ties increase. The intensity ratio between methyl
tion with the release of water and formation of anisole, group signals corresponding to the ortho and para
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 6 2006

THE MECHANISM OF PHENOL METHYLATION
895
positions is approximately 2 : 1. Anisole also isomer-
The following scheme of phenol alkylation with
izes into cresols; this was shown in a separate experi- methanol on zeolite HY was suggested on the basis of
ment.
the NMR data:
OCH3
OCH3
+C6H5OH
CH3OZ
I
–H2O
CH3
OH
CH3OH
OH
–H2O
II
+C6H5OH
,
CH3
CH3OCH3
CH3
CH3
where I and II are low and high catalyst surface cover- appearance of a signal at δ = 54.5 ppm, which corre-
ages with methanol. At 298–473 K, we largely observe sponds to the methyl group of anisole.
two processes, the dehydration of methanol and alkyla-
tion of phenol. The contributions of these reactions
depend on the ratio between the reagents and catalyst
surface coverage by them (Fig. 2). At a high content of
methanol at the initial reaction stage, phenol alkyla-
tion products are virtually not observed, and the major
reaction product is dimethyl ether. No more than
traces of methoxy groups are formed. Alkylation
products (cresols and xylenols) only appear after
methanol transformation into DME. Conversely, in
the presence of excess phenol, only traces of dimethyl
ether are observed, and methanol dehydration results
in the formation of methoxy groups. Alkylation then
yields both anisole and cresols, that is, the aromatic
ring and functional group are alkylated simulta-
neously. Anisole isomerizes into cresols at elevated
temperatures.
As distinct from experiments with acid catalysts, the
chemical shift of the signal of anisole adsorbed on
CsY/CsOH was close to the value tabulated for anisole
in the liquid phase [18]. Heating the system to a still
higher temperature (573 K) did not cause the appear-
ance of any other reaction products and only increased
the intensity of the signal with δ = 54.5 ppm corre-
sponding to anisole and decreased the intensity of the
signal with δ = 50 ppm corresponding to methanol. At
623 K, weak signals at δ = 20–45 ppm appeared in the
spectrum. They were assigned to the 13C atoms of CH2
phenol-formaldehyde resin groups on the basis of cal-
culations performed using the ACD-labs package. The
formation of phenol-formaldehyde resins is considered
below.
In the presence of excess methanol (Fig. 3b), a nar-
row signal with δ = 57 ppm and a low-intensity signal
at δ = 60 ppm appear at 543 K. Increasing temperature
to 623 K increases the intensity of the δ = 57 ppm signal
and causes the appearance of signals at δ = 58 and
167 ppm and at δ = 15–45 ppm. The δ = 60 and
167 ppm signals correspond to dimethyl ether and the
formate group, respectively, and the signals at 15–
45 ppm, to CH2 groups of phenol-formaldehyde resins.
Two intense narrow lines at δ = 57 and 58 ppm can be
assigned to intermediate products of the formation of
phenol-formaldehyde resins (hydroxybenzyl alcohols).
According to the data obtained in calculations with the
use of the ACD-labs package, the chemical shifts of the
signals of the CH2 groups of ortho- and para-hydroxy-
benzyl alcohols in solution are 62 and 64 ppm, respec-
tively. Adsorption on the surface of basic catalysts can
cause upfield shifts of these signals to 57 and 58 ppm
because of the formation of phenolates and quinone
methides.
In order to determine the nature of alkylating parti-
cles, we performed separate experiments in which phe-
nol interacted with dimethyl ether and surface methoxy
groups. It was found that these compounds are also
methylation agents and contribute to both O- and
C-alkylation of phenol.
The Methylation of Phenol on Zeolite CsY/CsOH
according to the in Situ 13C NMR Data
The 13C NMR magic-angle rotation spectra obtained
under static conditions during the reaction between
13C-methanol and phenol on zeolite CsY/CsOH at var-
ious catalyst surface coverages with the reagents are
shown in Fig. 3.
The spectra recorded immediately after the adsorp-
tion of methanol and phenol contain a single signal at
δ = 50 ppm. This signal corresponds to the methyl
group of adsorbed methanol. As distinct from the acid
Signals with δ = 183, 167, and 7 ppm were observed
form of zeolite Y, we do not observe coadsorption of at 523 K in addition to the DME signal (δ = 59.5 ppm)
methanol and phenol on CsY/CsOH. In the presence of in the experiment with methanol on the CsY/CsOH cat-
excess phenol (Fig. 3a), heating to 543 K results in the alyst. According to [19–21], these signals correspond to
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 6 2006

896
BORODINA et al.
57
54
(a)
(b)
58
50
20
28
20.5
34
623 K, 1 h
623 K, 1 h
573 K, 1 h
15
50
54.5
60.5
573 K, 1 h
543 K, 1 h
57
543 K, 1 h
298 K
49.5
40
50
40
298 K
200 160 120
80
δ, ppm
0
200 160 120
80
δ, ppm
0
13
Fig. 3. C NMR magic-angle rotation spectra recorded under static conditions as the temperature increased in steps during the reac-
13
tion after C-methanol and phenol adsorption on zeolite CsY/CsOH in amounts of (a) 7 and 21 molecules/u.c. and (b) 40 and
10 molecules/u.c., respectively.
carbon monoxide, formate group, and methane, respec- δ = 167, 52, and 48 ppm. Formaldehyde formed from
tively. The appearance of cesium formate and carbon
monoxide signals is evidence of intermediate formalde-
hyde formation during methanol transformations. To
check this suggestion and inquire into the special fea-
tures of formaldehyde transformations on CsY/CsOH,
we performed additional experiments with 13C-formal-
dehyde adsorbed on this zeolite. It was found that form-
aldehyde rapidly transformed into the formate group,
methoxy group, and methanol even at room tempera-
methanol enters into the condensation reaction with
phenol present on the surface of basic catalysts as phe-
nolate anions, as was shown in a separate experiment
on the adsorption of labeled phenol on zeolite
CsY/CsOH. Condensation yields various bakelite (phe-
nol-formaldehyde resin) fragments.
We used the spectral data obtained to suggest the
following scheme of processes that occurred on the sur-
ture. This followed from the appearance of signals at face of zeolite CsY/CsOH:
OH
OH
formate
groups
II
CH3OH
OH
[CH2O]
–H2
CH3OCH3
OCH3
CH2OH
OCs
I
+CsZ
CH3OH
–CsZ
Bakelite,
where I and II are low and high catalyst surface cover- late anions are formed on the surface of zeolite. These
ages with methanol.
anions interact with methanol to produce anisole.
In the presence of excess methanol, some of the
In the presence of excess phenol, phenol preferably
interacts with zeolite active centers; as a result, pheno- methanol transforms into formaldehyde, which inter-
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THE MECHANISM OF PHENOL METHYLATION
897
not observed at the exit of the flow reactor under typical
alkylation conditions because they rapidly transform
into phenol-formaldehyde resins and cause catalyst
deactivation.
(‡)
CxHy
(CH3)n
127–128
~135
The Mechanism of Deactivation of Zeolite Catalysts
160
120
80
40
0
To find out what the reasons are for the deactivation
of acid and basic catalysts during alkylation, we per-
formed a detailed analysis of reaction products formed
on the surface of catalysts at elevated temperatures. At
673 K, the interaction of phenol with methanol on HY
results in the formation of light aliphatic hydrocarbons
characterized by narrow signals at 10–30 ppm. Simul-
taneously, condensed aromatic hydrocarbons with δ =
126 and 137 ppm are produced (Fig. 4a). In all proba-
bility, it is these products that cause the deactivation of
catalysts.
δ, ppm
(b)
57
Bakelite
CH2 groups
58
50
CH3OH
54
33
60
20
27
15
65 60 55 50 45 40 35 30 25 20 15
δ, ppm
Phenol methylation at elevated temperatures on
CsY/CsOH causes the appearance of new NMR signals
at δ = 15, 20, 27, and 33 ppm (Fig. 4b). All these signals
can be assigned to methylene groups of phenol-formal-
dehyde resins (bakelites). Their formation is preceded
by the appearance of signals at δ = 57 and 58 ppm
assigned to ortho- and para-hydroxybenzyl alcohols.
The supposed mechanism of bakelite formation is as
follows:
13
Fig. 4. C NMR spectra recorded under static conditions
13
during the reaction; reaction time 1 h, C-methanol and
phenol concentrations 40 and 10 molecules/u.c., respec-
tively; (a) HY at 673 K and (b) CsY/CsOH at 623 K.
acts with phenolate anions to produce ortho- and para-
hydroxybenzyl alcohols. Note that these products are
OH
O
CH2
O–
CH2OH
O
CH2
Bakelite.
O–
O–
OH
CH2
According to this scheme, hydroxybenzyl alcohols are
transformed into quinone methides at the first stage.
The latter interact with phenolate anions following the
Michael condensation mechanism. As a result, the
anionic chain grows. The successive addition of
quinone methides and phenolate anions to this chain
results in the formation of bakelite three-dimensional
structure fragments in zeolite pores, which causes rapid
CsY/CsOH deactivation (Fig. 1).
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