Mechanisms of reactions of methoxy species with benzene and cyclohexane over H-ZSM-5 zeolites

Article abstract of DOI:10.1039/c5cy00600g

The first-step reaction mechanisms of d₃-methoxy (OCD₃) species on H-ZSM-5 zeolite with benzene and cyclohexane were directly observed by infrared (IR) spectroscopy. Only toluene was produced in the gas phase at the beginning, simultaneous with the consumption of methoxy species on the surface at 473 K. Acidic OH groups were formed in place of d₃-methoxy groups for the reaction with benzene, while OD groups were recovered for the reaction with cyclohexane, similar to the case of light olefins. Thus, the presence of different methylation mechanisms of methoxy species is confirmed; the CD₃ unit reacted with benzene as in the cases of amines and dimethyl ether, but the CD₂ unit reacted with cyclohexane and light olefins, leaving one of the deuterium atoms on the zeolite. The difference of these mechanisms in activation energy is also estimated and used for further discussion on the mechanisms.

Full text of DOI:10.1039/c5cy00600g

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. N. Kondo, H.
Yamazaki, T. Yokoi and T. Tatsumi, Catal. Sci. Technol., 2015, DOI: 10.1039/C5CY00600G.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/catalysis

Page 1 of 6
Catalysis Science & Technology
Different mechanisms operate for reacꢀons of methoxy species
to form C-C bonds with various hydrocarbons: in methyl caꢀon-
like and carbene-like mechanisms, most probably both in
concerted manner.
CD3ꢀ
+
Dꢀ
Dꢀ
+ C6H6ꢀ
C
CD3ꢀ
Oꢀ
Dꢀ
Hꢀ
H
Vꢀ
ꢀ
Bꢀ
Oꢀ
Oꢀ Oꢀ
Oꢀ
Oꢀ
Oꢀ
Siꢀ Siꢀ Alꢀ Siꢀ
Siꢀ Alꢀ Siꢀ
Siꢀ Alꢀ Siꢀ
+ ꢀ
CHDꢁꢀ
Dꢀ
Dꢀ
+ C6H12ꢀ
CD3ꢀ
Oꢀ
Dꢀ
Cꢀ
Dꢀ
Oꢀ
Oꢀ
Oꢀ
Oꢀ
Oꢀ Oꢀ
Siꢀ Alꢀ Siꢀ
Siꢀ Alꢀ Siꢀ
Siꢀ Siꢀ Alꢀ Siꢀ

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 6
View Article Online
DOI: 10.1039/C5CY00600G
Journal Name
ARTICLE
Mechanisms of reactions of methoxy species with benzene and
cyclohexane over H-ZSM-5 zeolites
Received 00th January 20xx,
Accepted 00th January 20xx
J. N. Kondo*, H. Yamazaki, T. Yokoi and T. Tatsumi
The first-step reaction mechanisms of d3-methoxy (OCD3) species on H-ZSM-5 zeolite with benzene and cyclohexane were
directly observed by infrared (IR) spectroscopy. Only toluene was produced in gas phase at the beginning simultaneously
with the consumption of methoxy species on the surface at 473 K. Acidic OH groups were formed in the place of d3-
methoxy groups for the reaction with benzene, while OD groups were recovered for the reaction with cyclohexane
similarly to the case of light olefins. Thus, the presence of different methylation mechanisms of methoxy species is
confirmed: CD3 unit reacted with benzene as is the cases of amines and dimethyleter, but CD2 unit with cyclohexane and
light olefins to leave one of the deuterium atoms on the zeolite. Difference of these mechanisms in activation energy is
DOI: 10.1039/x0xx00000x
www.rsc.org/
also
estimated
and
used
for
the
further
discussion
on
the
mechanisms.
the possible intermediate for the reaction of methoxy groups,16
experimental evidences on the intermediate are still limited.
The carbenium cation intermediate was verified at the early
Introduction
A variety of zeolites, a representative family of solid acid
catalysts, is explored for the application to the methanolꢀtoꢀ
olefin MTO reaction in order to selectively obtain propene,
which is a vital starting material for petrochemical products.1ꢀ5
The “hydrocarbon pool mechanism” has been well recognized
stage of the research on MTO by the decrease of the d3
methoxy species, which was observed by IR spectroscopy,
during the reaction with benzene and by the production of d3
ꢀ
ꢀ
toluene in gas phase.9 On the other hand, carbeneꢀlike
intermediate was recently found by IR observation of reactions
of d3ꢀmethoxy species with light olefins, where consumption of
d3ꢀmethoxy species and the concurrent recovery of OD groups
occurred.17 Meanwhile, carbenium cation (CD3 unit)
intermediate was proposed for the reaction of d3ꢀmethoxy
species with dimethylether (DME) to produce propene
directly.18 In this way, the activated structure and the detailed
reaction mechanisms of methoxy species with hydrocarbons
have not been systematically considered. Therefore, by
studying methylation reaction of benzene and cyclohexane by
methoxy species, we aimed at interpreting the detailed nature of
methoxy species and the unique role of Brønsted acidic sites on
zeolites.
as a general mechanism operating during the steady state of the
MTO reaction.6,
7
The detailed structures of hydrocarbon
species as well as the role of them for the evolution of light
olefins are still under discussions, because they are strongly
dependent of the pore structure of zeolites.8 On the other hand,
less is clarified with respect to the initial elemental steps of the
MTO reaction, which is of importance in order to understand
similarities and differences of Brønsted acidic OH groups on
solid surfaces and protons in homogeneous media. Methoxy
species, which are formed by dehydration of methanol and
acidic OH groups on zeolites, are accepted as the initial
activated species in MTO reaction.9ꢀ11 Methoxy species were
found to function as methylation agent with other hydrocarbon
compounds such as benzene,9,
cyclohexane,
10, 12, 13
10, 11, 13
ethene 10, 11, 15 and so on. The creation of CꢀC bonds between
methoxy species and reacted hydrocarbons was confirmed by
analysis of products and surface species using 13Cꢀlabeled
Experimental
The hydrogen form of ZSMꢀ5 (JRCꢀZ5ꢀ90H, Catalysis Society
of Japan, Si/Al = 45) was pressed into a selfꢀsupporting disk
(20 mm diameter, 60 mg) and placed in an IR cell attached to a
conventional closed gas circulation system. The sample was
pretreated by evacuation at 773 K for 1 h. Benzene (Wako Pure
Chemical Industries, Inc., 99.5 %), d6ꢀbenzene (Merck & Co.
Inc., 99.95 %), clohexane (Tokyo Chemical Industries, Co.
Ltd., 99.5 %), cyclohexene (Tokyo Chemical Industries, Co.
Ltd., 98.0 %), ethene (TAKACHIHO CHEMICAL
INDUSTRIAL Co. Ltd., 99.9 %), dimethyl ether (Kanto
Chemical Industries, Inc., 99.8 %) were used without further
methanol by nuclear magnetic resonance (NMR) or d3
methanol (CD3OH) by infrared (IR) spectroscopy.
ꢀ
On the mechanism of the CꢀC bond formation from
methoxy species, two types of intermediates have been
proposed;5, 11 1) carbenium cation (CH3 unit) and 2) carbeneꢀ
like (CH2 unit) intermediates. While density functional theory
(DFT)ꢀbased calculations excluded carbeneꢀlike species from
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 3 of 6
View Article Online
DOI: 10.1039/C5CY00600G
ARTICLE
Journal Name
purification. For methanol three isotope samples were used; ethene under the same conditions due to the accelerated rate of
CH3OH (Wako Pure Chemical Industries, Inc., 99.8 %), the reaction by increasing the carbon number of light olefins as
CD3OH (Merck & Co., Inc., 99 % isotopic purity) and previously reported.17 Thus, results at short reaction times are
13CH3OH (Cambridge Isotope Laboratories, Inc., 99 % isotopic compared. Nevertheless, C5 (+ C2) compound was already
purity).
The amount of methoxy groups was adjusted to 40 % (8.8
10ꢀ6 mol on 60 mg sample disk) of the acidic OH groups (2.2
observed at 5 min in addition to C4 (+ C1) compound. The
reactivity of benzene with methoxy species was an order of
magnitude higher than that of ethene. Toluene was produced at
×
×
10ꢀ5 mol on 60 mg sample disk). Thus, 60 % of the acidic OH the initial stage with a small amount of xylene, and methylation
groups were coexisting with methoxy species. Then, a quarter proceeded further at 60 min. These results agree well with
amount of hydrocarbon to the amount of methoxy species was previous reports.9ꢀ13 Next, changes of surface species were
supplied from the gas phase. Therefore, the molar ratio of directly observed by IR spectroscopy under the same reaction
acidic OH groups, methoxy species and the reactant was set to conditions as those in Table 1.
6 : 4 : 1, in order to extract the information on the initial
reaction. IR spectra were observed at certain times, followed by methoxy species on HꢀZSMꢀ5 with benzene at 473 K are shown
the immediate collection of the gaseous molecules for analysis. in Figure 1. background spectrum measured after
Time courses of IR spectra measured during reaction of
A
IR spectra were obtained at a resolution of 4 cmꢀ1 using a Jasco pretreatment of HꢀZSMꢀ5 was subtracted from the spectrum of
4100 FTꢀIR spectrometer equipped with a mercury cadmium methoxyꢀformed sample (spectrum (a)) and those measured
telluride (MCT) detector. A total of 64 scans were averaged for after the supply of benzene (spectra (b)ꢀ(f)). Thus, a negative
each spectrum. The IR spectra of the clean disk were recorded band of acidic OH groups at around 3610 cmꢀ1 is attributed to
under evacuation at various temperatures as background consumed OH groups by the formation of methoxy species
spectra. Backgroundꢀsubtracted IR spectra showing adsorbed upon dehydration. Bands attributed to methoxy species were
species are presented throughout this paper. The gaseous observed at 2950ꢀ2800 cmꢀ1 in spectrum (a) (CH3 stretching
components were analyzed by a GC (GCꢀ14B, SHIMADZU bands of methyl group). Time course of spectra were measured
Corporation) and a GCꢀmass (Agilent Technologies, GCꢀ until 30 min after the reaction started (spectra (b)ꢀ(f)). Since
7890Q and MSꢀ5975C with TripleꢀAxis Detector) spectrometer adsorptions of benzene superimposed the spectral change at 1
with a HP plot column.
min, subtracted spectrum of (b) from (f) is arrayed on top in
order to find the differences of the surface species between 30
and 1 min after the reaction. The adsorption of a part of benzen
is found by weak CH stretching bands above 3000 cmꢀ1 (dotted
curve in Figure 1). In difference spectrum (f)ꢀ(b) the increase of
acidic OH groups and the decrease of methoxy species in
amount are confirmed by an upward OH band at 3610 cmꢀ1 and
downward CH3 stretching bands of methoxy species,
respectively. Therefore, acidic OH groups were recovered on
the site of methoxy species after the reaction with benzene.
Above observation is in consistent with the results of reactions
Results and discussion
Before observing the change of surface species during the
reaction, the catalytic performance of the reaction of methoxy
species with benzene over HꢀZSMꢀ5 at 473 K are compared
with those with ethane and propene in Table 1. Note that 40 %
of the total Brønsted acidic sites were exchanged to methoxy
species, and that only a quarter molar amount of reactant with
respect to methoxy species was introduced. Thus, introduced
hydrocarbon molecules were free to be adsorbed on the isolated
acidic OH groups and reacted with methoxy species several
times in the time course.
he reaction of ethene was slow at 473 K and simple at the
initial time to form only propene, followed by further
methylation by methoxy speices to produce C4 compound,
because the amount of methoxy species was 4 times of that of
supplied reactant. Propene reacted much more extensively than
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 6
View Article Online
DOI: 10.1039/C5CY00600G
Journal Name
ARTICLE
shown in Table 1.
observed. In the time ourse of the reaction, the gradual recovery
For the understanding of the detailed reaction mechanism, of OD band at around 2650 cmꢀ1 is clearly observed
d3ꢀmethoxy (OCD3) species were employed for the same accompanied with the decrease od CD bands of d3ꢀmethoxy
reactions (Figure 2). Spectra in Figure 2 are backgroundꢀ species, which is the evidence of the reaction of d3ꢀmethoxy
subtracted as described in Figure 1. The presence of d3
ꢀ
methoxy species is recognized by their CD3 stretching bands
appearing at 2300ꢀ2000 cmꢀ1. A small amount of adsorbed
benzene was found in spectra (b)ꢀ(f) by the CC stretching band
at around 1500 cmꢀ1 in addition to CH stretching bands
observed clearer than those in Figure 1 due to the absence of
CH3 stretching bands of methoxy species in Figure 2. In the
subtracted spectrum of (b) from (f) consumption of methoxy
species and adsorbed benzene are confirmed by the decrease of
stretching bands of CD3 (2300ꢀ2000 cmꢀ1) and CC (around
1500 cmꢀ1) to produce toluene and so on (Table 1). The
regeneration of the OH band assigned to the acidic OH groups
of HꢀZSMꢀ5 was also clearly observed in the spectrum of (f)ꢀ
(b). While residual OD groups were observed in spectra (a)ꢀ(f)
in Figure 2, no OD groups were formed during the reaction of
d3ꢀmethoxy groups with benzene as found in the top spectrum
in Figure 2. Thus, methoxy species reacted as CD3 units to form
CꢀC bonds with benzene molecules, which left H atoms from
one of their CH bonds. This result supports the reported
mechanism, which was evidenced by the observation of CD3ꢀ
toluene formed from benzene (C6H6) and d3ꢀmethanol
Fig. 3 Time course of IR spectra measured during the reaction of d3-methoxy
species with ethene at 473 K. Spectrum (a) was measured before introduction of
ethene, and (b) 5 min, (c) 10 min, (d) 30 min, (e) 40 min, (f) 50 min, (g) 60 min
after starting the reaction. Top spectra were obtained by subtraction of (a) from
(g)
species as CD2 unit with ethene.17
Methylation of cyclohexane with methoxy species over
zeolites are reported to proceed under similar reaction
11, 13
conditions.10,
Although we attempted the simultaneous
analysis of gaseous molecules and surface species, the dilute
molecules in gas phase prohibited any precise quantifications.
On the other hand, IR observation of surface species provided
important information on the reaction mechanism from clear
spectral changes. Therefore, IR results are shown for the
reaction of methoxy species with cyclohexane. d3ꢀMethoxy
species were also reacted with cyclohexane under the same
conditions. The time course of spectra measured during the
reaction of d3ꢀmethoxy species with cyclohexane at 473 K is
(CD3OH) over HꢀZSMꢀ5.9
The reaction mechanism of methoxy species with benzene,
which was confirmed by observation of surface species, is
different from that with light olefins. In the case of the reaction
of d3ꢀmethoxy species with ethene, acidic hydroxy groups were
recovered as OD, indicating that methoxy species reacted with
ethene as CD2 unit. IR spectra observed during this reaction
uncer the same conditions as those in Table 1 and Figures 1 and
2 are shown in Figure 3. No bands of ethene were observed due
to very small amount of adsorbed species at 473 K, but bands
of d3ꢀmethoxy species and consumed acidic OH groups were
Fig. 4 Time course of IR spectra measured during the reaction of d3-methoxy
species with cyclohexane at 473 K. Spectrum (a) was measured before
introduction of benzene, and (b) 1 min, (c) 5 min, (d) 10 min, (e) 20 min and (f) 30
min after starting the reaction. Top spectra were obtained by subtraction of (b)
from (f).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 5 of 6
View Article Online
DOI: 10.1039/C5CY00600G
ARTICLE
Journal Name
shown in Figure 4. In the case of cyclohexane, intense bands
attributed to CH2 stretching of adsorbed cyclohexane were
observed at 3000ꢀ2800 cmꢀ1 with a weak CH2 deformation band
at around 1450 cmꢀ1. The adsorption of cyclohexane is in
equilibrium with gaseous molecules, since the adsorbed
cyclohexane was completely removed by evacuation. The top
spectrum in Figure 4, indicating the change of adsorbed
species, is only explained for simplicity. The decrease of CH2
stretching bands in intensity due to the decrease of the amount
of cyclohexane is clear in the spectrum in addition to negative
bands of d3ꢀmethoxy species. In the light of the recovery of the
acidic sites, they appeared as OD groups and the acidic OH
band at 3610 cmꢀ1 was silent; d3ꢀmehotxy species reacted in
CD2 units with cyclohexane leaving one of their D atoms on the
acid sites similarly to the case of the reaction with light
olefins.17 Since only 13Cꢀlabelled methanol was used as a
methylation agent of cyclohexane for NMR analysis11 so far,
this new information has been found for the first time by using
d3ꢀmehotxy species for IR analysis.
be noted that the activation energy of methylation of
cyclohexane with methoxy species is similar to that of ethene
(111 ± 4 kJ·molꢀ1). While ethene, propene and cyclohexane
form CꢀC bond with methoxy species through the same
mechanism (carbeneꢀlike intermediate), the reactivity of
propene was much higher than those of the others, with the
activation energy of 41 ± 1 kJ·molꢀ1 (unpublished data). This
indicates that the rate determining step for the CꢀC bond
formation of hydrocarbons with methoxy species through
carbeneꢀlike mechanism cannot be simply attributed to the CꢀH
bond cleavage of methoxy species. Therefore, it is further
implied that the role of the reactant hydrocarbon for the CꢀH
bond activation of methoxy species is significant in the
activated complex.
The reaction mechanisms of methoxy species with benzene
and cyclohexane are compared by illustration in Figure 5.
Benzene substitutes a H atom with more electrophilic (cationic)
methyl groups to generate CD3ꢀtoluene probably using two sites
17
(Scheme A), similarly to reactions of amines11,
and
dimethyleter.18 This is in analogous to soꢀcalled carbenium
cation route, where CD3 groups in d3ꢀmethoxy species are
maintained. On the other hand, the same methoxy species form
CꢀC bonds with cyclohexane to evolve methylcyclohexane
(C6H11ꢀCHD2) and OD groups through carbeneꢀlike
intermediate most likely in a concerted manner (Scheme B). In
the case of cyclohexene (cyclic olefin), the reaction followed
Scheme B leaving OD groups from d3ꢀmethoxy species,
Conclusions
Different mechanisms operate for reactions of methoxy species
to form CꢀC bonds with various hydrocarbons: in methyl
cationꢀlike (Figure 5 Scheme A) and carbeneꢀlike (Figure 5
Scheme B) mechanisms, most probably both in concerted
manner. While the hydrocarbon pool intermediates and their
reactions inside the cageꢀtype pores of zeolites should be the
major processes of MTO reaction, the details of elementary
pathways of WTO reaction would be also of importance. The
mechanism of activation and the reactivity of methoxy species,
one of the elementary intermediates, are markedly affected by
the interaction with reactant molecules.
Notes and references
1
Song, W.; Fu, H.; Haw, J. F. J. Am. Chem. Soc., 2001, 123,
4749-4754.
because cyclohexene is less electroꢀdonative than benzene.
The rate of the reaction was measured by using the increase
of recovery of the integrated peak intensity of the acidic OH
band in the time course. The reaction of methoxy species with
both benzene and cyclohexane followed first order kinetics with
respect to gas phase pressure of the reactants and the amount of
methoxy species on HꢀZSMꢀ5. The activation energies were
estimated form 473 to 523 K to be 54 ± 4 and 115 ± 3 kJ·molꢀ1
for benzene and cyclohexane, respectively (Figure 6). It should
2
Svelle, S.; Joensen, F.; Nerlov, J.; Olsbye, U.; Lillerud, K.-P.;
Kolboe, S.; Bjørgen, M. J. Am. Chem. Soc., 2006, 128, 14770-
14771.
Bjørgen, M.; Joensen, F.; Lillerud, K.-P.; Olsbye, U.; Svelle, S.
Catal. Today, 2009, 142, 90-97.
Stöcker, M. Microp. Mesop. Mater., 1999, 29, 3-48.
Bjørgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.;
Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U. J. Catal., 2007,
249, 195-207.
3
4
5
6
Dahl, I.; Kolboe, S. J. Catal., 1994, 149, 458-464.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 6 of 6
View Article Online
DOI: 10.1039/C5CY00600G
Journal Name
ARTICLE
7
8
Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem.
Res., 2003, 36, 317-326, and the references therein.
Olsbye, U.; Svelle, S.; Bjørgen, M.;Beato, P.; Janssens, T. V.
W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Angew. Chem. Int.
Ed., 2012, 51, 5810-5831.
9
Ono, Y.; Mori, T. J. Chem. Soc., Faraday Trans. 1, 1981, 77
2209-2221.
,
10 Forester, T. R.; Howe, R. J. Am. Chem. Soc., 1987, 109, 5076-
5082.
11 Wang, W.; Hunger, M. Acc. Chem. Res., 2008, 41, 8, 895-904.
12 Rakoczy, J.; Romotowski, T. Zeolites, 1993, 13, 256-260.
13 Forester, T. R.; Howe, R. J. Chem. Soc.,Chem. Commun., 1986,
1611-1613.
14 Campbell, S.; Jiang, X.; Howe, R. Microp. Mesop. Mater.,
1999, 29, 91-108.
15 Cui, Z.-M.; Liu, Q.; Ma, Z.; Bain, S.-W.; Song, W.-G. J. Catal.,
2008, 258, 83-86.
16 Lesthaeghe, D.; Van Speybroeck, V.; Martin, G. B.;
Waroquier, M. Angew. Chem. Int. Ed., 2006, 45, 1714-1719.
17 Yamazaki, H.; Shima, H.; Imai, H.; Yokoi, T.; Tatsumi T.;
Kondo, J. N. Angew. Chem. Int. Ed., 2011, 50, 1853-1856.
18 Yamazaki, H.; Shima, H.; Imai, H.; Yokoi, T.; Tatsumi, T.:
Kondo, J. N. J. Phys. Chem. C, 2012, 116, 24091-24097.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins