Spectroscopic and kinetic study of the alkylation of phenol with dimethyl carbonate over NaX zeolite

Article abstract of DOI:10.1039/a706356c

The interaction of phenol and dimethyl carbonate (DMC) with acid-base sites in zeolite NaX has been studied using FTIR, UV and mass spectroscopy.At room temperature, phenol is predominantly adsorbed by hydrogen bonding to basic oxygen atoms of NaX.At higher temperature, phenol is partially deprotonated over basic sites to form phenolate anions; the latter undergo H-bonding to zeolitic hydroxy groups and Na⁺ ions.DMC forms a chelating complex with Na⁺ ions at room temperature which decomposes at 150 deg C into dimethyl ether and CO2 FTIR experiments of coadsorbed reactants show that, in the presence of phenol, DMC is mainly bonded by its carbonyl oxygen to Lewis acid sites.The formation of anisole sets in at 150 deg C and proceeds presumably via nucleophilic attack at the methyl carbon of DMC by the oxygen atom of H-bonded phenol or phenolate.Kinetic measurements reveal that the reaction order is negative in phenol but positive in DMC pointing to site blocking by strongly adsorbed phenol.

Full text of DOI:10.1039/a706356c

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Spectroscopic and kinetic study of the alkylation of phenol with
dimethyl carbonate over NaX zeolite
Tilman Beutel
L aboratoire de Reactivite de Surface Universite P. et M. Curie, 1106 URA du CNRS 4, Place
  
Jussieu, 75252 Paris, France
The interaction of phenol and dimethyl carbonate (DMC) with acidÈbase sites in zeolite NaX has been studied using FTIR, UV
and mass spectroscopy. At room temperature, phenol is predominantly adsorbed by hydrogen bonding to basic oxygen atoms of
NaX. At higher temperature, phenol is partially deprotonated over basic sites to form phenolate anions; the latter undergo
H-bonding to zeolitic hydroxy groups and Na` ions. DMC forms a chelating complex with Na` ions at room temperature which
decomposes at 150 ¡C into dimethyl ether and CO . FTIR experiments of coadsorbed reactants show that, in the presence of
2
phenol, DMC is mainly bonded by its carbonyl oxygen to Lewis acid sites. The formation of anisole sets in at 150 ¡C and
proceeds presumably via nucleophilic attack at the methyl carbon of DMC by the oxygen atom of H-bonded phenol or phenol-
ate. Kinetic measurements reveal that the reaction order is negative in phenol but positive in DMC pointing to site blocking by
strongly adsorbed phenol.
Aryl alkyl ethers represent an important group of chemicals
used as reactants for the production of dyes, agricultural
chemicals, antioxidants or stabilizers for polymers.1 They may
also be used as octane boosters and replace currently used
non-metallic additives such as methyl tert-butyl ether.2 The
simplest representative of this class of compounds is phenyl
methyl ether, anisole, which is produced commercially by
alkylation of phenol with methanol. Alkylation can take place
on the aromatic ring or the heteroatom, leading to cresol or
anisole, respectively. The selectivity depends, besides experi-
mental factors, on the acidÈbase properties3,4 and pore
structure5,6 of the catalyst. Namba et al.7 studied the selec-
tivity of phenol alkylation with methanol over H/Na/Y cata-
lysts. They found that the yield of anisole increased with the
Na : H ratio over zeolite Y. The nature of the alkylating agent
seems, likewise, to play a role in the selectivity, although no
systematic study exists as yet. The use of DMC yields high
selectivities in the N- and O-alkylation of aniline and phenol,
respectively, when basic zeolites are used as catalysts.8 Re-
cently, Fu and Ono9 reported a selectivity of 93% for the O-
alkylation of phenol at a conversion of 82% over NaX zeolite
using DMC as the alkylating reactant. The environmentally
friendly properties of DMC make it preferable to toxic and
corrosive alkylating reactants such as dimethyl sulfate or
methyl halides.10 NaX proved to be the most active and stable
catalyst for this particular alkylation reaction.9
during decomposition of DMC over NaX. Finally, kinetic
measurements on the alkylation of phenol with DMC were
carried out with the aim to determine the reaction orders in
both reactants. Based on spectroscopic and kinetic data, the
structure and reactivity of surface complexes with the individ-
ual adsorbates phenol and DMC and a reaction scheme for
the formation of anisole are proposed.
Experimental
Sample preparation
The reactants, DMC and phenol, were adsorbed at room tem-
perature (rt) on a commercial NaX catalyst (Union Carbide,
Lot No: 12 LBS 1360028) with an Si : Al ratio of 1.2. Prior to
adsorption of the organic reactants, the catalyst was calcined
in a Ñow of 50 ml min~1 oxygen (Linde) for 2 h at 500 ¡C at a
heating rate of 0.8 K min~1 and subsequently evacuated at
500 ¡C for 20 min.
The desorption and decomposition of adsorbed molecules
was studied by heating the sample stepwise to increasing tem-
peratures in vacuum. After each heat treatment, an IR or UV
spectrum was taken at rt. Both the IR and UV cell allowed in
situ pretreatments up to 500 ¡C under gas atmosphere or in
vacuum (p \ 10~5 mbar) and allowed to dose controlled
amounts of the organic reactants via vapour-phase adsorp-
tion. Calibration measurements were made in which DMC
was evaporated into a dose volume of 5 ml at 0 ¡C , while
phenol was dosed into a container of 150 ml at rt before being
dosed onto an NaX wafer. The quantities of DMC and phenol
per dose volume correspond to ca. 0.1 and 0.2 molecules,
respectively, per supercage. Phenol and DMC were puriÐed
before use by sublimation and distillation, respectively, onto
activated molecular sieve 4A.
The formation of anisole by alkylation of phenol with
DMC proceeds according to:
2
PhOH ] DMC F 2 PhOMe ] CO ] H O
(I)
2
2
Elementary steps have been proposed for this reaction, based
on catalytic test results over alkali-metal exchanged X and Y
zeolites, which include the formation of phenolate ions as a
reaction intermediate.8 IR spectroscopic data on the inter-
action of phenol with solid catalysts are scarce and pertain to
acidic supports11 or oxidation catalysts.12,13 No IR spectro-
scopic data have been reported so far on the adsorption of
phenol or DMC on basic zeolites. The incentive of this study
is, therefore, to characterize the adsorption states of DMC
and phenol on NaX and to discriminate potential reaction
intermediates in the alkylation reaction by IR and UV spec-
troscopy. The study of the adsorbed phase is supplemented by
a mass spectroscopic analysis of the gaseous products evolved
Spectroscopic techniques
For FTIR experiments, the sample was pressed at 100 bar into
a self-supporting wafer (8 mg cm~2) and mounted in a port-
able Pyrex cuvette equipped with CaF windows. Spectra
2
were recorded on a Bruker IFS66 vacuum spectrometer in the
transmission mode. A DTGS detector was used, accumulating
128 scans at a spectral resolution of 2 cm~1.
J. Chem. Soc., Faraday T rans., 1998, 94(7), 985È993
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For UV measurements, 300 mg of sample were pressed at
5 bar and mounted into a quartz cell behind an optical
2
window of Suprasil 300. Spectra were taken in the range
between 200 and 500 nm in di†use reÑectance on a Varian
Cary 5 spectrometer.
For mass spectroscopic studies, 50 mg of sample were given
a standard pretreatment in a reactor attached to a Schlenck
line as described above. After exposure to phenol or DMC at
rt, the reactor was brieÑy evacuated before being transferred
to a Balzer mass spectrometer. The desorption of gaseous pro-
ducts was continuously monitored while heating the sample to
300 ¡C at a rate of 5 K min~1.
Catalytic test
Gas-phase alkylation of phenol by DMC has been carried out
in a continuous Ñow reactor at a reaction temperature of
2
00 ¡C. The phenol and DMC partial pressures were in the
Scheme 1
range 1È4 mbar at a GHSV of ca. 200 000 h~1. Conversions
were kept below 10%.
of phenol. These bands are superimposed on a broad and
strong absorption around 3100 cm~1. The broadness, inten-
sity and low frequency shift of this band indicate the presence
of perturbed OH groups which belong most likely to phenol
H-bonded to zeolite oxygen atoms. The position of this band
is considerably red shifted as compared with monomers of
phenol18 which are characterized by a narrow OwH-
stretching band at 3657 cm~1. In addition, there is a series of
bands at 2955, 2840, 2719 and 2619 and 2506 cm~1 which are
equally spaced by ca. 120 cm~1.
Results
Adsorption of phenol at room temperature
Fig. 1(a) shows the FTIR spectrum of phenol adsorbed on
NaX at rt. Strong absorption bands are detected at 1591,
1
492, 1475 and 1238 cm~1. The band at 1591 cm~1 and its
shoulder at 1599 cm~1 can be readily assigned to the 8a and
8
bands are slightly displaced to lower wavenumbers as com-
b deformation vibrations of the aromatic ring.14,15 These
Fig. 3(a) displays the UV spectrum of phenol after rt
adsorption on NaX. Two main absorption bands can be dis-
tinguished at 221 and 269 nm, which are attributed to the
primary (1B ^ 1A ) and secondary (1B ^ 1A ) electronic
pared with phenol in CCl solution where they occur at 1596
and 1604 cm~1.15 Bands at 1492 and 1475 cm~1 are ascribed
4
to normal modes 19a and 19b of the ring. While the position
of vibration 19b is the same as that of neat phenol at 1474
cm~1, there is a red shift of 8 cm~1 for vibration 19a of
adsorbed phenol. The intensive absorption band at 1238 cm~1
is ascribed to the CwO stretching vibration of self-associated
phenol. A broad triplet of bands consisting of a maximum at
1u
1g
2u
1g
transitions of the p-electron system of phenol.19 Note that the
primary band at 221 nm is red shifted by 10 nm as compared
with the pure substance. It has been shown that both bands
undergo shifts to lower energies with increasing electron-
donating ability of the substituent.20 The observed shift poss-
ibly indicates an H-bonded state of phenol in which the
oxygen atom is more negatively polarized and hence a better
electron donor than free phenol. A small shoulder can be dis-
cerned at ca. 296 nm in Fig. 3(a) indicating the presence of
small amounts of phenolate ions, in accordance with literature
data reporting electronic transitions of phenolate at 235 and
1386 cm~1 and two shoulders around 1404 and 1360 cm~1 is
mainly attributed to the in-plane CwOwH-bending vibration
of phenol. However, coupling of this mode to a CxC-
stretching vibration has also been discussed,16 as well as the
occurrence of phenol polymers.17 The spectrum in the CwH-
stretching region is depicted in Fig. 2. A poorly resolved
feature with absorption maxima at 3065 and 3047 cm~1 is
typical of the CwH-stretching vibrations of the aromatic ring
287 nm.20
Fig. 1 FTIR spectra in the region of aromatic ring vibrations after
adsorption of phenol on NaX (ca. 0.3 molecules per supercage) at
Fig. 2 FTIR spectra in the region of CwH- and OwH-stretching
vibrations after adsorption of phenol on NaX (ca. 0.3 molecules per
supercage) at 25 ¡C followed by (a) 1 h evacuation at 25 ¡C, (b) 200 ¡C,
(c) 250 ¡C, (d) 300 ¡C and (e) 350 ¡C
2
3
5 ¡C followed by (a) 1 h evacuation at 25 ¡C, (b) 200 ¡C, (c) 250 ¡C, (d)
00 ¡C and (e) 350 ¡C
986
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Thermal treatment of adsorbed phenol
As the intensity of the high-frequency component of per-
turbed OwH-stretching bands is depleted, the cluster of
bands between 2955 and 2506 cm~1 seems to be proportion-
ally diminished in intensity. Complete deprotonation of
phenol over NaX is achieved at 300 ¡C, as evident from the
disappearance of all spectral features characterizing H-bonded
phenol in Fig. 2(d). The deprotonation manifests itself also in
a shift of the CwH-stretching vibrations at 3065 and 3047
cm~1 of phenol to 3058 and 3032 cm~1 of phenolate.
Remarkably, there are few free zeolitic hydroxyl groups
detected after heating to 300 ¡C, indicating that those groups
are either strongly perturbed or further condensation has
occurred.
Fig. 3(b)È(f) illustrate the e†ect of a stepwise heat treatment
to 250 ¡C in vacuum on the UV spectrum of phenol. SigniÐ-
cant changes are observed after heating to 100 ¡C; these are
characterized by the appearance of a shoulder at ca. 240 nm.
Concomitantly, the weak band at 296 nm, which was present
as a shoulder in Fig. 3(a) and (b), becomes prominent in Fig.
Fig. 1(b)È(e) show the e†ect of thermal treatment in vacuum
on the IR spectrum of phenol adsorbed on NaX. Heating to
2
1
1
00 ¡C leads to depletion of band intensity at 1475, 1386 and
238 cm~1, while the band at 1492 cm~1 grows. The band at
238 cm~1 is simultaneously shifted to 1254 cm~1, and a
shoulder at 1270 cm~1 appears, together with a band at 1297
cm~1. As the sample is further heated to 300 ¡C, the band at
1475 cm~1 in Fig. 1(c) disappears and the broad band at 1386
cm~1, characteristic of H-bonded phenol, is eroded. An isos-
bestic point exists between the bands at 1492 and 1475 cm~1,
indicating a transformation of phenol from state A into state
B, C or D in Scheme 1. State A can be assigned to H-bonded
phenol. States B, C and D are characterized by the loss of the
d
vibration and a blue shift of the l
vibration and cor-
COH
C~O
respond most likely to the deprotonated form of phenol. The
high-frequency shift of the l vibration to 1254 cm~1 after
C~O
heating to 200 ¡C is in line with an enhancement of the CO
bond strength when phenol undergoes H-bonding to basic
oxygen atoms of the zeolite lattice. This is consistent with the
assumption that the increased negative charge on the oxygen
atom of phenolate should enhance the double bond character
of the CwO bond by resonance e†ects.21 The appearance of
CwO-stretching bands at 1270 and 1297 cm~1 after heating
to 200 ¡C is indicative of deprotonated phenol, in which the
CO bond is further strengthened. A high-frequency shift of the
3(c). Simultaneously, the intensity of the original bands at 221
and 269 nm decrease. Bands at 240 and 296 nm can be
ascribed to phenolate ions, in accordance with literature data
reporting electronic transitions at 235 and 287 nm.19 Further
heating, to 250 ¡C, depletes the overall band intensity and
leads to an inversion of the intensity ratio of bands at 269 and
296 nm.
l
vibration to 1270 cm~1 has been reported for phenolate
C~O
DMC adsorption at room temperature
in basic aqueous solution.22
When the sample is further heated to 350 ¡C, the band at
254 cm~1, corresponding to state A, vanishes, while bands at
270 and 1297 cm~1 remain. After heat treatment to 350 ¡C
Fig. 4(a) shows the carbonyl and CwH-deformation vibra-
tions of DMC adsorbed on NaX at rt. Three main absorp-
tions can be discerned at 1753, 1461, and 1312 cm~1. The
feature with a maximum at 1753 cm~1 represents the CxO-
stretching vibration of DMC.23 There are two shoulders at
1769 and 1742 cm~1, which possibly indicate di†erent adsorp-
tion states of DMC. The intense absorption band with
maximum at 1311 cm~1 corresponds to the antisymmetric
1
1
the amount of adsorbed phenol decreased by ca. 50% of the
initial amount of adsorbed phenol. This is due to desorption
of phenol, but formation and subsequent desorption of reac-
tion products such as diphenyl ether may also contribute to
the loss of aromatic compounds after heat treatment to 200 ¡C
and higher temperatures. Fig. 2 demonstrates the e†ect of
thermal treatment on the OwH- and CwH-stretching bands.
As evident from Fig. 2(b), the broad feature of perturbed OH
groups is signiÐcantly depleted on its high-frequency side at
ca. 3300 cm~1, while intensity remains on the low-frequency
wing when the sample is heated to 200 ¡C. This seems to indi-
cate the presence of two di†erent H-bonded phenol species
with di†erent degree of perturbation of the phenolic hydroxyl
group.
stretching vibration of the CO entity in DMC. It is
2
broadened by a shoulder on the low-frequency side. An
exhaustive interpretation of the vibrational spectrum of DMC
has been given by Collingwood et al.24 Bands at 1461 and
1433 cm~1 are assigned to deformation vibrations of the
methyl groups of DMC. A third band at 1475 cm~1 which is
absent in dilute solutions of DMC in CCl appears in Fig.
4(a). Fig. 5 depicts the corresponding CwH-stretching region
4
Fig. 4 FTIR spectra in the carbonyl-stretching region of DMC
adsorbed on NaX (ca. 0.1 molecules per supercage) at 25 ¡C followed
by (a) 15 min evacuation at 25 ¡C and after subsequent evacuation for
1 h at (b) 50, (c) 80, (d) 100, (e) 150 and (f) 200 ¡C
Fig. 3 UV di†use reÑectance spectra of phenol adsorbed on NaX at
2
5 ¡C followed by (a) 1 h evacuation at 25 ¡C, (b) 100, (c) 150, (d) 200
and (e) 250 ¡C
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
987

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and a band at 2850 cm~1 which grow in concert with the
CwH-deformation band at 1454 cm~1. At the same time
another absorption band at 2822 cm~1 appears together with
a shoulder at 2945 cm~1 and a weak and broad band around
2
900 cm~1. The latter three bands represent, most likely, a
third type of methyl group.
The thermal decomposition of DMC adsorbed on NaX at
rt (ca. 1.5 molecule per supercage) has been followed by mass
spectroscopy. Fig. 6 illustrates the evolution of peaks corre-
sponding to m/z \ 45, 44, 29 and 15. The peak at m/z \ 44
can readily be ascribed to CO . The peaks at 15, 29, and 45
2
are representative of both DMC and dimethyl ether (DME).
Injection of pure DME and DMC into the mass spectrometer
yielded ratios of m/z \ 15 to m/z \ 45 of 1.3 and 5, respec-
tively. It should be mentioned that, under the experimental
conditions encountered in this study, the relative abundance
of fragments with m/z \ 15 and 45 is di†erent from literature
data.29 A plot of the ratio m/z \ 15 to m/z \ 45 [Fig. 6(b)]
reveals indeed that, during decomposition of DMC over NaX,
the ratio is 5 initially, in accordance with the desorption of
DMC. This ratio starts dropping at temperatures above
100 ¡C and reaches a constant value of 1.4 above 200 ¡C which
corresponds to desorption of DME. The declining part of the
curve indicates the onset of DME desorption. Evolution of
Fig. 5 FTIR spectra in the CwH-stretching region of DMC
adsorbed on NaX (ca. 0.1 molecules per supercage) at 25 ¡C followed
by (a) 15 min evacuation at 25 ¡C and after subsequent evacuation for
1
h at (b) 50, (c) 80, (d) 100, (e) 150 and (f) 200 ¡C
CO
sets in at around 60 ¡C. The appearance of CO2
in the
2
gas phase is evidence of the decomposition of DMC. Inter-
estingly, the onset of detection of peak 15 occurs at ca. 100 ¡C.
This means that DMC decomposes between 60 and 100 ¡C
while its methyl groups are retained on the surface. The peak
ratio m/z \ 15 to m/z \ 45 is high at temperatures below
of adsorbed DMC. Bands at 3027 and 2965 cm~1 in Fig. 5(a)
have been ascribed to the antisymmetric and symmetric
stretching vibrations of methyl groups, whereas the small
absorption situated at 2861 cm~1 has been interpreted as a
combination band of two CwH-deformation vibrations.24
100 ¡C, corresponding to desorption of DMC. The peak ratio
decreases above 100 ¡C, indicating the release of DME.
Thermal treatment of adsorbed DMC
In order to follow the reactivity of DMC, the sample was
heated stepwise to 200 ¡C in vacuum. A spectrum was taken
after each thermal treatment. Fig. 4(b) shows the sample after
heating at 50 ¡C for 1 h. It is evident that major losses of
absorbance occurred at 1753 and 1296 cm~1. These bands are
close to 1756 and 1281 cm~1 of neat DMC25 and can most
likely be attributed to physisorbed DMC. At the same time,
the band intensity at 1769 cm~1 increased. As the sample is
further heated, the antisymmetric and symmetric OwCwO-
stretching bands diminish continuously and vanish completely
at 150 ¡C. Fig. 4(b) reveals the appearance of additional bands
at 1664 and 1345 cm~1 which have been attributed to biden-
tate carbonate groups ligated to Na` ions.26,27 At the same
time, a shoulder at 1454 cm~1 emerges on the low frequency
side of the d vibration of DMC methyl groups. The carbon-
CH
ate bands and the new CwH-deformation band at 1454 cm~1
grow in concert when the sample is further heated to 80 and
100 ¡C under vacuum. The bidentate carbonate bands and the
CwH-deformation band at 1454 cm~1 decrease when the
sample is heated above 150 ¡C, and an additional band pair at
1
485 and 1428 cm~1 develops. The latter bands have been
assigned by Jacobs et al.28 to Na`-bonded symmetric carbon-
ate ions.
Fig. 5 illustrates the changes occurring in the CwH-
stretching region upon heat treatment of DMC. The change in
the adsorption state of DMC, which is induced by heating to
5
0 ¡C, entails a slight blue shift of the symmetric CwH-
stretching vibration to 2969 cm~1. After heating to 80 ¡C, the
carbonyl vibrations of DMC are strongly depleted. There is,
however, no loss in intensity of the band at 2969 cm~1. Fig.
5
(c) shows an additional absorption band at 2997 cm~1 and
an enhancement of absorbance at 2863 cm~1. It is plausible to
assume the formation of a second type of CH group at 80 ¡C
Fig. 6 Evolution of CO (m/z \ 44), DME and DMC (m/z \ 15, 29,
3
2
which is characterized by bands at 2997, 2969 and 2863 cm~1.
Further heating to 100 ¡C generates a shoulder at 2956 cm~1
45) as a function of time under vacuum as followed by mass spectrom-
etry from DMC-loaded NaX (ca. 0.8 molecules per supercage)
988
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Coadsorption of DMC and phenol at room temperature
tion of DMC is retarded in the presence of phenol. Further
heating to 150 ¡C causes a decrease in band intensity of
adsorbed DMC. The aromatic ring vibration at 1591 cm~1 is
shifted to 1597 cm~1 and exhibits a shoulder at 1585 cm~1.
Concomitantly, the CwH-deformation band of DMC methyl
groups at 1461 cm~1 becomes broader and a band at 1238
cm~1 arises. In addition, a broad band around 1685 cm~1
and a shoulder around 1330 cm~1 grow in. These bands can
Fig. 7 shows the e†ect of phenol adsorption on the state of
preadsorbed DMC at rt. Fig. 7(a) was taken after adsorption
of DMC and Fig. 7(b) after subsequent adsorption of phenol.
The band at 1238 cm~1 indicates the presence of H-bonded
phenol. The di†erence spectrum (b) [ (a) reveals the disap-
pearance of band intensity at 1769 cm~1 and an increase in
band intensity at 1742 cm~1 of DMC. The same result (not
shown) was also obtained when the order of adsorption was
reversed.
be assigned to HCO~ ions. Further heating to 200 ¡C erodes
3
all carbonyl bands ascribed to DMC but leaves bands at 1597,
1238 cm~1 and a doublet at 1454 and 1446 cm~1. Those
bands are indicative of anisole which is formed by O-
alkylation of phenol. The band assignment for anisole has
been checked by adsorption of anisole on NaX in a separate
experiment. Note that for Fig. 8(b) and (c) the IR cell was
closed during heat treatment in order to trap the reaction pro-
Reactivity of phenol and DMC
Fig. 8 demonstrates the e†ect of heating coadsorbed phenol
and DMC. Fig. 8(a) was obtained after adsorption of phenol
and DMC at rt and subsequent heating to 100 ¡C. Remark-
ably, the intensity of the DMC carbonyl band at 1742 cm~1 is
signiÐcantly higher than when DMC was heated in the
absence of phenol [cf. Fig. 4(c)]. Apparently, the decomposi-
ducts. The l
and l
stretching vibrational regions pre-
sented in Fig. 9 reveal the typical features of H-bonded phenol
together with DMC. Upon heating to 150 ¡C, new bands at
O~H
C~H
3
320, 2958, 2841 and 2818 cm~1 appear. The intense and
broad band around 3320 cm~1 is characteristic of perturbed
hydroxyl groups. Since the typical feature of H O hydrogen
2
bonded to NaÉ É ÉO acidÈbase pair sites is missing in Fig. 9,
z
the band at 3320 cm~1 is likely to be attributed to H-bonded
methanol. Bands at 2958 and 2841 cm~1 can accordingly be
ascribed to the antisymmetric and symmetric stretching vibra-
tions of methyl groups in anisole and methanol.
It is important to note that bands at 1685 and 1330 cm~1
are also formed when monomethyl carbonate (MMC), as gen-
erated from DMC under dry conditions, is exposed to water
and stepwise heated to 80, 100 and 150 ¡C in the closed IR
cell. The heat treatment of MMC with water leads to the
depletion of bands at 1664 and 1345 cm~1 while bands at
1
685 and 1330 cm~1 build up, together with a band at 3320
cm~1. This result can be rationalized assuming hydrolysis of
MMC into hydrogen carbonate and methanol and conÐrms
the band assignment given above.
The reaction of phenol with DMC was also followed by UV
spectroscopy. However, reference experiments showed that
DMC does not absorb in the wavelength range between 200
and 500 nm while anisole, presenting bands at 224 and 266
nm, appears to be indistinguishable from phenol.
Coadsorption experiments revealed that the formation of phe-
nolate is suppressed at 150 ¡C in the presence of excess DMC,
Fig. 7 FTIR spectra in the carbonyl-stretching region of NaX after
(
a) adsorption of DMC at 25 ¡C, (b) followed by adsorption of phenol
at 25 ¡C and after subtraction (b) [ (a). Equal amounts of DMC and
phenol, corresponding to ca. 0.2 molecules, respectively, per unit cell
have been dosed.
Fig. 9 FTIR spectra in the CwH- and OwH-stretching region of
NaX after coadsorption of DMC and phenol at 25 ¡C followed by
brief evacuation and 1 h heating to (a) 100, (b) 150 and (c) 200 ¡C in
the closed cell. Quantities of adsorbed reactants correspond to those
of Fig. 7.
Fig. 8 FTIR spectra in the carbonyl-stretching region of NaX after
(
a) coadsorption of DMC and phenol at 25 ¡C and followed by brief
evacuation and 1 h heating to (b) 100 and (c) 150 ¡C in the closed cell.
Quantities of adsorbed reactants correspond to those of Fig. 7.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
989

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seems likely, but cannot be inferred from the present spectro-
scopic data. Quantum chemical calculations by Matsamura et
al.30 show that phenol interacts with Na` ions preferentially
by the oxygen atom. Based on this result, structure A is pro-
posed in Scheme 1.
The presence of a second broad contribution at 3300 cm~1
in the OH-stretching region of phenol adsorbed at rt is
ascribed mainly to self-associated phenol or phenol adsorbed
on the external surface of the zeolite. Adsorption of phenol on
silica yielded a perturbed OH band at 3357 cm~1. Relatively
narrow bands at 3530 and 3654 cm~1 for phenol dimers and
at 3441 and 3449 cm~1 for trimers in the gas phase have been
measured in ionization-detected IR experiments by Ebata et
al.31 These bands may be present in Fig. 2(a) but masked by
the broad feature around 3300 cm~1. The depletion of absorb-
ance around 3300 cm~1 becomes evident in Fig. 2 and corre-
sponds to three processes: (i) Desorption of phenol, (ii)
migration of phenol to basic zeolite oxygen atoms and (iii)
reaction of phenol to phenolate and/or diphenyl ether. The
band at 3300 cm~1 may contain contributions of small
amounts of phenolate ions interacting with zeolitic hydroxyl
groups via the aromatic ring, as depicted in structure B of
Scheme 1. It is known that acidic hydroxyl groups in H-ZSM-
5
are shifted by ca. 300 cm~1 upon interaction with adsorbed
benzene.32 It is difficult to predict the displacement of an OH
band in HNaX when interacting with the p system of phenol-
ate ions. The acid strength of hydroxyl groups in HNaX is
weaker than in H-ZSM-5 but the phenolate ion is a stronger p
electron donor than benzene. In addition, the adsorption
geometry changes from C in the benzene proton adduct to a
Fig. 10 Reaction orders in DMC (A) and phenol (B) as determined
from the dependence of the rate of anisole formation on the partial
pressure of the reactants measured at 200 ¡C
6
v
lower symmetry in the phenolate proton adduct.
It is interesting to note that, after rt adsorption of phenol, a
series of bands at 2955, 2840, 2719, 2619 and 2605 cm~1 is
observed on the low-frequency wing of the band correspond-
ing to perturbed hydroxyl groups in H-bonded phenol. A
similar phenomenon has been observed recently by Binet et
al.33 for pyrrole adsorbed on NaX and was attributed to the
formation of pyrrolate anions interacting with OH groups of
the support. In this study, UV experiments suggest that only
small amounts of phenolate should be present at rt. The inten-
sity of the progression of these bands seems to follow the
decrease in perturbed OH intensity at 3100 cm~1 as the
sample is heated to higher temperatures, suggesting that they
are related to the presence of H-bonded phenol. Separate
experiments using 13C H OH and C D OH showed that the
spacing of these bands is sensitive to isotopic substitution of
the ring carbon atoms by 13C as well as deuteration of the
ring. This suggests that vibrations which include the motion of
ring atoms and the hydroxy group are involved in this pheno-
menon. The origin of this phenomenon is not understood at
present and needs further investigation.
indicating that phenol and/or phenolate reacted with DMC at
that temperature.
Kinetic measurements have been carried out in a Ñow
reactor in order to determine the reaction orders of both reac-
tants. Fig. 10 shows the dependence of the reaction rate on the
partial pressure of each reactant. Evaluation of the slopes of
the bilogarithmic plot yields a positive reaction order in the
range ]1 for DMC and a negative reaction order of ca. [1
for phenol. The negative reaction order in phenol signiÐes that
phenol is strongly adsorbed on the surface and blocks sites for
the adsorption of DMC. This in turn means that DMC does
not react from the gas phase in an EleyÈRideal type mecha-
nism but from an adsorbed state.
6
5
6 5
Discussion
Reactivity of phenol
IR data reveal that phenol adsorbed on NaX is predominantly
H-bonded at rt. This is evident from the broad OwH-
stretching vibration at ca. 3100 cm~1 of phenol undergoing a
hydrogen bond to basic oxygen atoms of the zeolite lattice.
The red shift of 10 nm of the primary band in the UV spec-
trum of phenol adsorbed on NaX as compared to that of neat
phenol, is in agreement with the assumption of H-bonded
phenol. The primary and secondary electronic transitions of
the p-electron system of phenol are sensitive to the electron
density of the aromatic ring. Substituents with electron lone
pairs can transfer electron density to the ring by resonance
e†ects. Electron donors raise the energy of the HOMO, but
a†ect the energy level of the LUMO to only a small extent,
thus resulting in a low-frequency shift of the electronic tran-
sitions of the p system.20 A bathochromic shift with respect to
free phenol is, therefore, anticipated as a result of H-bonding
as it enhances the electron density of the hydroxyl oxygen and
hence the donating power of the substituent. An additional
interaction of phenol with adjacent Na` ions by either its
hydroxyl oxygen atom or the p electrons of the aromatic ring
Fig. 3(b) shows that phenol is partially deprotonated after
heating to 200 ¡C, as evident from the enhancement of the sec-
ondary electronic transition of phenolate at 296 nm relative to
that of phenol at 269 nm. The broad and strong absorption at
3100 cm~1 and the d
vibration in Fig. 1(b) and Fig.
C~O~H
2(b), respectively, have substantially decreased, indicating that
H-bonded phenol has been deprotonated. This is further con-
Ðrmed by the hypsochromic shift of the CwO-stretching
vibration from 1238 cm~1 in self-associated phenol to 1254
cm~1 in zeolite-bonded phenol. After 300 ¡C, all spectral fea-
tures in the OwH-stretching region characteristic of H-
bonded phenol have vanished [see Fig. 2(d)], but there
remains about half of the original intensity in the range of the
aromatic ring vibrations [see Fig. 1(d)]. Obviously, part of the
phenol has desorbed. Another part of the phenol was depro-
tonated and formed phenolate ions which are most likely
bonded to Na` ions. The further blue shift of the CwO-
stretching vibration to 1297 cm~1 may be taken as evidence
for Na`-bonded phenolate ions. As neither free nor perturbed
990
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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OH groups are observed after heating to 300 ¡C, it is reason-
able to suggest their further reaction with phenol to form
surface phenyl ethers. The band at 1270 cm~1 is tentatively
Na` ions. Unland observed bands at 1662 and 1344 cm~1
after thermal decomposition of methanol over NaX and
ascribed them to bidentate carbonate groups.27 While MMC
assigned to the CO-stretching vibration of O Ph groups.
forms a complex with Na` ions, the CH groups are bonded
z
3
Bands at 1241 and 1260 cm~1 have been measured for
to surface oxygen atoms of the zeolite. Partial hydrolysis of
(
EtO) Si(OPh).34 The reaction of NaOPh with O Ph groups
DMC by traces of water, resulting in the formation of meth-
anol, can, however, not be excluded. In fact, bands at 2954,
2850 and 1450 cm~1 are typical of methanol as well as
3
z
may yield diphenyl ether. Neat diphenyl ether presents a band
at 1237 cm~1 which is not observed after vacuum treatments
at 300 and 350 ¡C. It is, therefore, concluded that diphenyl
ether, if formed, desorbs at those temperatures. The reactivity
of phenol is depicted in Scheme 1.
SiwOCH groups. At 100 ¡C, the band intensity of MMC
3
methyl groups is diminished and a new band pair consisting
of a shoulder at 2950 cm~1 and a band at 2822 cm~1 arise.
MS analysis of the gas phase shows that CO is released. This
2
Reactivity of DMC
can be rationalized assuming a decomposition of MMC to
form Na`-bonded methoxy groups, as depicted in reaction (2)
of Scheme 3. The low-frequency position of the symmetric
CwH-stretching band at 2822 cm~1 points to an ionic
methoxy group. Bands in the 2820 cm~1 region can some-
times be due to overtones or combination bands of the funda-
mental CwH deformation vibrations of CH or CH groups.
A priori, two adsorption modes of DMC inside the supercages
of NaX can be envisaged and are illustrated in Scheme 2.
Complex E is characterized by an interaction of the carbonyl
oxygen with a Na` ion. In this structure the CxO double
bond is weakened as compared to liquid-phase DMC. IR
bands at 1742 and 1281 cm~1 seen in Fig. 4(a) after rt adsorp-
tion can be attributed to complex E. Another interaction con-
sists in the formation of a bidentate complex in which the
ester oxygen atoms of DMC chelate one Na` ion, as depicted
in structure F. For this complex, a strengthening accompanied
by a blue shift of the CxO bond is expected with respect to
neat DMC. Bands at 1769 and 1312 cm~1 may, accordingly,
be ascribed to complex F in Scheme 2. In both structures,
DMC acts as a Lewis base to form an acidÈbase complex with
Lewis acid sites of the zeolite. IR and MS data also revealed
the potential of DMC to react with Lewis acidÈbase pairs of
the zeolite. Heating to 80 ¡C leads to formation of two new
3
2
There is, however, no fundamental vibration in the appropri-
ate range of Fig. 4(c) and (d) as to account for an overtone at
2822 cm~1. The absence of this band in the spectra of DMC
adsorbed on HY is further support for Na` ions as the
responsible adsorption sites. The formation of Na`-bound
methoxy groups from MMC occurs without participation of
adjacent sites. This reaction, as it is not base assisted, may be
anticipated to require a higher activation energy. Bensitel et
al.35 studied the generation of MMC by reaction of CO with
2
methoxy groups adsorbed on ZrO . They ascribed bands at
1
2
583 and 1372 cm~1 to monomethyl carbonate ligated in a
bidentate fashion to Zr4` cations. The zirconia-bound MMC
complex decomposed completely into ZrwOCH and CO on
heating to 170 ¡C. At temperatures above 150 ¡C, band inten-
sity at 2850 and 2822 cm~1 decreases and formation of DME
is observed by mass spectrometry. Apparently, Na`-bonded
methoxy groups react with lattice oxygen bound methyl
groups to generate dimethyl ether as illustrated in reaction 3
of Scheme 3. This reaction is similar to the formation of ethers
from alcohols over alumina, which has been discussed by
types of CH groups which are characterized by bands at
3
2
997, 2969 and 2863 cm~1 for one type and by bands at 2954
3
2
and 2850 cm~1 for the other type. The latter band pair can be
attributed to either adsorbed methanol or silicon-bonded
methoxy groups. The band at 1454 cm~1 has to be assigned
to the corresponding CwH deformation band. The close
proximity of the position of the former three bands to those of
DMC suggests a great structural similarity. The triplet of
bands at 2997, 2969 and 2863 cm~1 may tentatively be
ascribed to monomethyl carbonate (MMC). The formation of
KnoŽ zinger36 in terms of a nucleophilic attack by the oxygen
atom of an O- and H-bonded alcohol to an adjacent alkoxy
group. A separate experiment of DME adsorbed at rt yielded
also a band at 2820 cm~1 which, however, disappeared after
vacuum treatment at 100 ¡C, because of desorption of DME.
This result is in agreement with the detection of DME by
mass spectroscopy in the course of DMC decomposition at
two new types of CH groups can be interpreted in terms of
3
an alkyl cleavage of DMC yielding methyl groups and MMC
according to reaction (1) of Scheme 3. The generation of
methoxy groups is also accompanied by the evolution of a
characteristic band pair at 1664 and 1345 cm~1 which is
attributed to MMC groups ligated in a bidentate fashion to
1
50 ¡C. On the contrary, the absorption band at 2822 cm~1
resulting from DMC decomposition is still observed after
heating in vacuum to 200 ¡C. When the sample is heated to
1
1
50 ¡C, two new absorption bands are observed at 1488 and
430 cm~1 which have been assigned to symmetric carbonate
groups. These bands have been shown to be generated from
CO and NaX in the dry state.26
2
Scheme 2
Reactivity of coadsorbed phenol and DMC
The coadsorption experiments show that phenol is able to
force DMC into one preferential adsorption state correspond-
ing to complex F in Scheme 2 while physisorbed and chelating
DMC are suppressed. This situation is realized regardless of
the order of adsorption of the reactants DMC and phenol.
After heating to 100 ¡C, much more DMC is retained on the
surface when phenol is present. This provides evidence that
thermal decomposition of DMC into CO and dimethyl ether
2
proceeds via chelating DMC as a precursor. It is likely that
the alkyl bond cleavage of DMC is favored when DMC is
adsorbed in a chelating fashion. The formation of the chelat-
ing DMC complex with Na` ions is sterically hindered when
phenol is adsorbed on a neighbouring basic oxygen site.
However, no Ðrm conclusion can be drawn as to whether an
Scheme 3
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
991

View Article Online
interact with the p-electron system of phenolate ions and are
able to form a surface ether, O Ph, with phenol.
z
(
2) DMC forms two types of surface complexes with Na`
ions at room temperature: A chelating complex via the ester
oxygen atoms and a monodentate complex via the carbonyl
oxygen atom. The chelating complex reacts with NaÉ É ÉO
z
acidÈbase sites to form dimethyl ether and CO . The decom-
2
position of DMC proceeds stepwise via formation of mono-
methyl carbonate, Na-bonded methoxy groups, NaOCH ,
3
and lattice oxygen-bonded methyl groups, O CH .
(
z
3
3) Phenol inhibits the formation of chelating DMC, thus
retarding the decomposition of DMC. The formation of
anisole proceeds presumably via nucleophilic attack at the
methyl carbon of DMC by the phenol oxygen. This reaction
supposedly takes place on a NaÉ É ÉO acidÈbase site, where
DMC is activated on the Lewis acid site by its carbonyl
z
Scheme 4
oxygen and phenol on an adjacent Lewis base site by H-
bonding. This reaction sets in at ca. 150 ¡C and yields anisole,
additional interaction of phenol with the cation takes place.
Heating to 150 ¡C causes the formation of anisole together
with methanol and hydrogen carbonate ions. In view of these
results, a reaction mechanism is proposed in Scheme 4.
H-bonded methanol and CO .
2
The help of G. Perdereau in the catalytic measurements is
From the negative reaction order in phenol it is inferred
that phenol inhibits the reaction by site blocking. The kinetic
data further suggest that both reactants are adsorbed on the
catalyst surface during the alkylation reaction. If DMC
impinging from the gas phase reacted with adsorbed phenol, a
positive reaction order in phenol would be anticipated. The
gratefully acknowledged. Discussions with Prof. H. Kn oŽ zinger
have been particularly enlightening. I also wish to thank Dr.
M. Briend for her assistance with the mass spectrometry and
Prof. C. Bennett and Prof. Bao-Lian Su for helpful discussions.
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z
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2
3
4
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8
9
Z.-H. Fu and Y. Ono, Catal. L ett., 1993, 21, 43.
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1
1
1
1
2
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6
1
1
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2
2
17 J. David, J. Chem. Soc., 1954, 120.
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2
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1
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Conclusions
29
30
31
(
1) At room temperature, phenol is preferentially adsorbed by
hydrogen bonding to basic oxygen atoms of NaX. Moreover,
phenol is partially deprotonated over basic sites to form zeo-
litic hydroxyl groups and phenolate ions ligated to Na` ions,
NaOPh. The zeolitic hydroxyl groups formed are likely to
992
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3
3
2
3
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4
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Paper 7/06356C; Received 1st September, 1997
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
993