Formation Mechanism of the First Carbon-Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons

Article abstract of DOI:10.1002/anie.201511678

The elementary reactions leading to the formation of the first carbon-carbon bond during early stages of the zeolite-catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C-C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid-catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol^-1 for the formation of the first C-C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms.

Full text of DOI:10.1002/anie.201511678

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201511678
German Edition: DOI: 10.1002/ange.201511678
Catalytic Mechanisms
Formation Mechanism of the First Carbon–Carbon Bond and the First
Olefin in the Methanol Conversion into Hydrocarbons
Yue Liu, Sebastian Müller, Daniel Berger, Jelena Jelic, Karsten Reuter, Markus Tonigold,
Maricruz Sanchez-Sanchez,* and Johannes A. Lercher*
Abstract: The elementary reactions leading to the formation of
the first carbon–carbon bond during early stages of the zeolite-
catalyzed methanol conversion into hydrocarbons were iden-
tified by combining kinetics, spectroscopy, and DFT calcula-
tions. The first intermediates containing a CÀC bond are acetic
The relatively low reactivity of methanol alone on
Brønsted acid sites has raised doubts in the past about the
existence of a direct reaction route from methanol and
dimethyl ether (DME) to the hydrocarbon pool, and some
studies suggested that the hydrocarbon pool originated from
[
3]
acid and methyl acetate, which are formed through carbon-
ylation of methanol or dimethyl ether even in presence of water.
A series of acid-catalyzed reactions including acetylation,
decarboxylation, aldol condensation, and cracking convert
those intermediates into a mixture of surface bounded hydro-
carbons, the hydrocarbon pool, as well as into the first olefin
leaving the catalyst. This carbonylation based mechanism has
impurities in the reactant feed or in zeolite. The approx-
imately 20 different proposals for the direct formation of
a first carbon–carbon bond from C species documented in
1
the literature were all eventually discarded by a comprehen-
[4]
sive theoretical analysis using density functional theory.
From an industrial point of view, the early elementary steps
forming the hydrocarbon pool seem irrelevant, because the
usual industrial realization uses recycling of unwanted
products. Then, the hydrocarbon pool is hypothesized to be
readily generated via alkylation, bypassing the first CÀC bond
À1
an energy barrier of 80 kJmol for the formation of the first
CÀC bond, in line with a broad range of experiments, and
significantly lower than the barriers associated with earlier
proposed mechanisms.
formation. However, our recent investigations on the nature
of the active and deactivating carbon species formed inside
the zeolite showed that strongly deactivating species are
formed at the early stages of MTH making it very important
C
atalytic conversion of methanol into hydrocarbons (MTH)
on zeolites such as H-ZSM-5 and SAPO-34, has become an
alternative pathway to crude-oil-derived hydrocarbons, as
a basis for preparing gasoline (MTG) and light olefins
[
5]
to understand the elementary steps involved. The fact that
the composition of these deactivating species is different from
the typical compounds of the hydrocarbon pool as well as
from carbon deposits led us to revisit the methanol chemistry
involved in the formation of first CÀC bond.
[1]
(
MTO). Despite of the industrial success of this process,
the full reaction mechanism is still under debate. It is accepted
that the steady-state reaction network involves a mixture of
hydrocarbons with long residence time in the zeolite pores,
the so-called hydrocarbon pool, which remains in pseudo
steady-state by a dynamic equilibrium between the carbon
chain growth via alkylation with methanol and the product
elimination by cracking and dealkylation. A dual cycle
mechanism involving one catalytic cycle with long olefins as
intermediates, and another cycle with aromatic intermediates
has been well established and its role in controlling the
The study aims, therefore, not only to reach a better
understanding of the reaction mechanism during the initial
stages of methanol conversion, but also to explore the
possible link to a potentially strong and hitherto unidentified
deactivation of the zeolite catalysts used for the process. The
mechanistic exploration of the reactions during the early
stage of MTH has been unsuccessful so far because of the low
concentrations of intermediates containing a CÀC bond and
[
2]
product distribution is nowadays well understood. However,
the mechanism for the formation of a first carbon–carbon
bond and the initiation of the hydrocarbon pool remains
unclear.
their fast transformation through the subsequent autocata-
lytic reactions with methanol. Using leads from products
during the initial stages of methanol conversion, we were able
to trace these reactions unequivocally. The feasibility of the
reaction pathways is corroborated by density functional
theory (DFT) calculations.
[
*] Dr. Y. Liu, Dr. S. Müller, Dr. D. Berger, Dr. J. Jelic, Prof. Dr. K. Reuter,
Dr. M. Sanchez-Sanchez, Prof. Dr. J. A. Lercher
Department of Chemistry and Catalysis Research Center
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
E-mail: m.sanchez@tum.de
We show herein that the decomposition of methanol to
formaldehyde and CO takes place readily under typical
reaction conditions already on inert surfaces, such as silicalite,
Na-ZSM-5, or g-Al O3 (Figure S1–S3 in the Supporting
2
Information). Carbon monoxide is involved subsequently in
several acid-catalyzed reactions, including carbonylation of
methanol, leading to the formation of the first CÀC bond. The
Johannes.lercher@ch.tum.de
Dr. M. Tonigold
Clariant Produkte Deutschland GmbH
Waldheimer Strasse 13, 83052 Bruckmühl (Germany)
initial methanol conversion was studied at 4508C in a quartz
tubular reactor. To investigate the chemistry of methanol
during the initial stages, the feed was passed over silicalite.
Methane, formaldehyde, and CO were detected, together
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201511678.
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with H , indicating that dehydrogenation and disproportio-
nation of methanol took place in the absence of Brønsted acid
site (Figure S1–S3). This observation is in line with the
This hypothesis has its basis in experiments detecting
methyl acetate and acetic acid as reaction intermediates
(Figure 1) and the fact that the reaction rate increased with
CO cofeeding during methanol conversion (Figure 2). For
these experiments, methylal (CH OCH OCH ) was used as
feed, because it reacts on zeolites to form DME, HCHO,
and CO, simulating the partial decomposition of methanol
during the MTH reaction, but with a higher ratio of HCHO
and CO to DME. The reaction of methylal was conducted on
H-ZSM-5 at 4008C (Figure 1A). Initially, full conversion was
obtained, with olefins as the main product. Other products
included paraffins, aromatics, CO, and water (not shown).
With time on stream, the olefin yield decreased, and increas-
ing concentrations of MeOH and DME were detected in the
2
commonly observed high selectivity to CH at very short
4
[
5,6]
contact times of methanol in zeolites.
3
[
2
3
12]
It is interesting to note that the formation of HCHO and
CO in the MTH reaction has been widely reported, but often
disregarded as an undesired byproduct arising from the
presence of metal impurities in the zeolite, catalyzing
[3,7]
methanol dehydrogenation.
The formation of methane,
formaldehyde, and CO in an all quartz reactor system in the
absence of catalyst indicates that these C1 molecules are
present in the reactant stream when passing through the
catalyst bed, making them potential reactants in the catalytic
pathway to the first CÀC bond (Scheme 1).
product stream. These are the stable C products from the
1
The C atom in MeOH, DME, and HCHO is electrophilic,
while the carbon atom in CO is nucleophilic (Scheme 1). This
leads readily to the formation of a CÀC bond between
decomposition of methylal, and their appearance indicates
the deactivation of the catalyst. Methyl acetate and acetic acid
were detected shortly before methanol/DME breakthrough
and they reached a maximum yield of less than 1%. Methyl
acetate and acetic acid were primary products, while olefins
appeared as secondary products (Figure 1B). This is inde-
electrophilic and nucleophilic carbon. Acidic zeolites are able
to catalyze such a CÀC bond formation, that is, carbonylation
[8]
[9]
[10]
of olefins, alcohols, methanol, and dimethyl ether. In the
present case, acetyl species were formed as intermediates at
very low conversions, as described in detail below. Thus, we
hypothesize that the first CÀC bond is generated through
pendent of the concentration of C reactants and of the zeolite
1
type, that is, methyl acetate and acetic acid were also observed
during methanol or methyl formate conversion on H-MOR
and H-BEA (Figure S5). The hypothesis that the first CÀC
carbonylation of methoxy groups on zeolite acid sites, leading
to a surface acetyl group, which subsequently dissociates into
methyl acetate (MeOAc) or acetic acid (HOAc; Scheme 1).
Theoretical calculations stringently supported the ther-
modynamic feasibility of the proposed initial reactions. For
carbonylation, postulated as the crucial step of the first CÀC
bond is formed via carbonylation of MeOH or DME was
further evidenced by converting MeOH on H-MOR with
linearly increasing temperature (Figure 1C). DME appeared
at about 1808C and approached equilibrium with MeOH
above 3008C, while olefin and aromatic production started
only from approximately 3208C and 3408C (Figure S6).
Methyl acetate and acetic acid were observed transiently in
very low concentrations from 2008C to 3008C.
À1
bond formation, an energy barrier of 80 kJmol (Figure S4)
À1
was calculated, in line with literature reports (54 kJmol
and 81 kJmol
À1 [10c]
). This energy barrier is over 60% lower
than those calculated for alternative mechanisms proposed
Olefin formation was promoted by exposing methanol
adsorbed on H-ZSM-5 to CO at increasing temperatures
(Figure 2). When heated under He flow, MeOH desorbed at
1208C and 2008C, DME at 2508C and 4008C, while olefins
were not detected below 5008C (Figure 2A). In contrast,
under CO flow olefins formed from 3608C on (Figure 2B).
The desorption rates of MeOH and DME were almost
unaffected, except for the decrease of DME around 3708C
owing to the onset of olefin formation. This difference in
reactivity in presence of CO was also shown by abruptly
replacing He with CO at 4008C (Figure 2C), which caused the
for the formation of the first CÀC bond in MTH, namely the
À1 [11a]
À1 [11b]
ylide type mechanism (320 kJmol ,
330 kJmol
),
À1 [11b]
the carbenium ion alkylation mechanism (210 kJmol ,
À1 [11c]
3
(
(
00 kJmol
),
),
the
or
240 kJmol
trimethyloxonium
mechanism
mechanism
À1[11c]
340 kJmol
the
carbene
). The low energy barrier of
À1 [11a]
À1 [4]
360 kJmol ,
the carbonylation route is attributed to the nucleophilicity of
CO, which allows the circumventing of an energetically
difficult transformation of MeOH into a nucleophile, such as
a carbene or an oxonium ylide.
Scheme 1. First CÀC bond formation in MTH through coupling between nucleophilic and electrophilic carbon atoms. Adsorption of MeOH (or
DME) on a zeolite Brønsted acid site forms a surface methoxy group. The methoxy group undergoes nucleophilic attack by CO, forming a surface
acetyl group which contains the first CÀC bond. The acetyl group dissociates into methyl acetate and acetic acid in the gas phase. Energy barrier of
À1
the carbonylation step is 80 kJmol
.
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Figure 1. HOAc and MeOAc as intermediates in MTH reactions. A) and B) Product formation rate in the reaction of methylal on H-ZSM-5 as
À1
a function of A) time on stream and B) conversion. (Conditions: methylal 150 mbar, He 25 mLmin , 50 mg H-ZSM-5, reaction temperature
À1
4
008C). C) Reaction of MeOH on H-MOR with linearly increasing temperature. (Conditions: MeOH 150 mbar, He 25 mLmin , 10 mg H-MOR,
À1
temperature ramping rate 108Cmin ).
Figure 2. A),B) Temperature-programmed surface reaction of MeOH on H-ZSM-5 under A) He flow and B) CO flow. C) Olefin desorption rate in
À1
the surface reaction of MeOH on H-ZSM-5 under alternate He and CO flow at 4008C. (Conditions: He or CO 25 mLmin , 25 mg H-ZSM-5,
À1
temperature ramping rate 108Cmin ).
nearly immediate formation of olefins as long as CO was
present. IR spectra of H-ZSM-5 surface species taken in situ
showed the existence of acetyl surface groups in the presence
of CO and their transformation into unsaturated carbonyl
groups concurrently with the first detection of olefins in the
gas phase (Figure S7), indicating the formation route of
olefins from carbonyl species. Conversely, under He flow, no
such surface carbonyl species were formed and olefins were
not formed under the chosen reaction conditions.
water decreased the carbonylation rate to 20% of the initial
value, and it further reduced to below 2% when 42 mbar
water was present (Figure S8A). The inhibition was proposed
[10a]
to be due to competitive adsorption of water with CO
or
[10d]
methanol,
water clusters.
and blockage of 8MR pockets in H-MOR by
[10c]
Similar inhibiting effects were also
observed, when converting DME into olefins on H-ZSM-5
(Figure S8B). At low conversion (< 1%), cofeeding 6 mbar
water largely decreased the olefin yield, but it was recovered
after removing the cofed water. Despite the low carbon-
ylation rates to be expected in the zeolite under MTH
conditions (S9, Supporting Information), the traces of methyl
acetate and acetic acid formed via this pathway suffice to start
conversions of the intermediates, triggering the formation of
the hydrocarbon pool and subsequently the fast methanol
conversion into hydrocarbons via alternative methylation and
cracking pathways. Thus, the existence of the induction period
In contrast to the present results, such promotional effects
of CO in MTH reaction were not observed in previous
[13]
studies. Therefore, the participation of CO was discarded
and excluded from the dominant proposals for MTH reaction
mechanism. The lack of this promotion in those experiments
is attributed to the presence of the much higher partial
pressure of water, a byproduct from the methanol dehydra-
[10a,b]
tion to DME, inhibiting such reaction.
In the carbon-
[1b,c]
ylation of DME on H-MOR (reported to be the most active
in methanol conversion at short contact times
is concluded
[10a,b]
zeolite for such reaction
), the presence of only 6 mbar of
to be the consequence of the very low rate of the first step of
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Keywords: carbonylation · CÀC coupling ·
methanol-to-hydrocarbons · olefin · zeolites
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5723–5726
Angew. Chem. 2016, 128, 5817–5820
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Acknowledgements
We acknowledge financial support from Clariant Produkte
Received: December 16, 2015
Revised: February 25, 2016
Published online: April 1, 2016
(
Deutschland) GmbH and beneficial discussion within the
framework of MuniCat.
5
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