Al-MCM-41: Its acidity and activity in cyclohexene conversion

Article abstract of DOI:10.1039/b106118f

Investigation of cyclohexene conversion over Al-MCM-41 has shown that the concentrations of Bronsted and Lewis acid sites increase with the Al content of the materials. The conversion follows mainly two mechanisms: cyclohexene skeletal isomerisation (CSI) and hydrogen transfer (HT). The products with 6 carbon atoms in a molecule prevail in all cases. Some amounts of products with 1 to 8 carbon atoms are also formed as a result of cracking or alkylation. All these reactions can occur at the acidic centres of the catalysts. The process of conversion, proceeding over the Bronsted acid sites, results in the formation of coke deposits in addition to volatile products. The amount of coke decreases with increasing reaction temperature and decreasing Al content of the material. The deposits cause a decrease in the effective concentrations of both the Bronsted and Lewis acid sites. Thermodesorption of pyridine has shown that (i) the concentrations of the Bronsted and Lewis acid sites before and after the conversion processes differ only slightly and (ii) the acidic strength of the Bronsted sites is almost independent of their concentration and the Si/Al ratio.

Full text of DOI:10.1039/b106118f

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Al-MCM-41: its acidity and activity in cyclohexene conversion
Michal Rozwadowski,*a Jerzy Datka,b Maria Lezanska,a Jerzy Wloch,a Krzysztof Erdmanna
and Jan Kornatowskic
a
Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun, Poland
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland
b
c
Lehrstuhl II fur Technische Chemie, Technische Universitat Munchen, Lichtenbergstr. 4,
È
85747 Garching bei Munchen, Germany
È
È
È
Received 10th July 2001, Accepted 19th September 2001
First published as an Advance Article on the web
Investigation of cyclohexene conversion over Al-MCM-41 has shown that the concentrations of Brꢀnsted and
Lewis acid sites increase with the Al content of the materials. The conversion follows mainly two mechanisms:
cyclohexene skeletal isomerisation (CSI) and hydrogen transfer (HT). The products with 6 carbon atoms in
a molecule prevail in all cases. Some amounts of products with 1 to 8 carbon atoms are also formed as a result
of cracking or alkylation. All these reactions can occur at the acidic centres of the catalysts. The process of
conversion, proceeding over the Brꢀnsted acid sites, results in the formation of coke deposits in addition to
volatile products. The amount of coke decreases with increasing reaction temperature and decreasing Al content
of the material. The deposits cause a decrease in the e€ective concentrations of both the Brꢀnsted and Lewis
acid sites. Thermodesorption of pyridine has shown that (i) the concentrations of the Brꢀnsted and Lewis
acid sites before and after the conversion processes di€er only slightly and (ii) the acidic strength of the Brꢀnsted
sites is almost independent of their concentration and the Si=Al ratio.
postulated mechanisms and the nature of the sites. As the
Introduction
discussed process is accompanied by signi®cant coking,20,28,29
we have tried to gain a more systematic understanding of the
factors, which in¯uence formation of coke in the channels and
on the external surface of the particles. The concentration of
cyclohexene in the feed gas was 5%, which di€ered from that
used in the former investigations (16%).30 The di€erent con-
centration was applied in order to determine the potential
in¯uence of the substrate concentration on the kind and dis-
tribution of products and to obtain the possibility for a more
precise analysis of the products and their distribution at a
lower reaction rate.
Promising properties of mesoporous molecular sieves M41S,
such as uniform mesopores and high surface area, have
resulted in numerous studies on synthesis and characterisa-
tion1±7 or on substitution of various metals into these sieves.8±14
However, limited catalytic applications have been reported for
unmodi®ed MCM-41.15±19 This is likely due to its low
hydrothermal stability and mechanical strength as well as the
lack of acidity of the neutral skeleton of the parent siliceous
MCM-41 materials that are, in e€ect, inert and exhibit no
signi®cant catalytic activity. Incorporation of Al or other
cations of valence lower than 48±14 results in modi®cation of
the chemical properties and, particularly, in the formation of
acid sites. However, it has not been ®rmly established whether
the acidity is due to the Brꢀnsted acidic protons, which are
necessary to balance the negative charges of the skeleton,
created by incorporated Al3‡ ions, or may be attributed to the
formation of extraframework Al species, which cause the
Lewis acidity.
Analysis data of the volatile products formed during the
conversion of cyclohexene over Al-MCM-41 with a di€erent
substrate concentration and characterisation of the acidic
centres of Al-MCM-41 before and after coking constitute new
information.
In a previous paper,20 we have reported the preparation of
Al-MCM-41. The direct synthesis of these aluminosilicate
materials o€ers an easy way of tailoring the acidic properties.
Therefore, it was interesting to identify the active sites present
in these materials and to recognise their acidic, basic, or
cationic nature.
Experimental
Materials
Cetyltrimethylammonium chloride and tetraethylammonium
hydroxide were used as templates for the hydrothermal
synthesis of the Al-MCM-41 materials. The products were
calcined at 773 K successively under nitrogen and air streams.
More details on the synthesis are given elsewhere.20 The
samples are designated as Al-MCM-41(n) where n denotes the
Si=Al ratios in the reaction gel, equal to 15, 30, and 60. Atomic
absorption spectroscopy (AAS) analysis of the calcined sam-
ples revealed Si=Al molar ratios of 12.2, 19.9, and 36.8 for
Al-MCM-41(15), Al-MCM-41(30), and Al-MCM-41(60),
respectively. This suggests that the reaction yield is not 100%
in relation to the amount of silica added to the gels.
Conversion of cyclohexene has been accepted21±27 as a sui-
table reaction for testing the catalytic behaviour of various
materials. We have applied this process to the Al-MCM-41
molecular sieves in order to characterise their properties after
modi®cation by coking,20 to determine sorption mechanisms
for water, benzene, and nitrogen,28 and to investigate the coke
deposits29 and the volatile products of the conversion.30 In the
present study, we aim at (i) determination of the probable
mechanism of the relevant conversion reactions, (ii) char-
acterisation of the active sites, and (iii) correlation between the
5082
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DOI: 10.1039/b106118f

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Table 1 Structural propertiesa of original and coked Al-MCM-
41(n)20,28
Table 2 Concentrations and acidic strength of Brꢀnsted and Lewis
acid sites in original and coked Al-MCM-41(n)
1
n
Tc=K
DW (%)
d=nm
a=nm
wd=nm
SBET=m2
g
Content
of carbona
(wt%)
Brꢀnsted Lewis
Acidic strength
of Brꢀnsted
sites Ades=A0
sites=
mmol g
sites=
mmol g
1
1
Sample
n
15
30
60
15
15
15
15
15
15
30
60
N=A
N=A
N=A
483
513
543
573
603
663
573
N=A
N=A
N=A
48.81
33.80
23.24
28.38
18.85
13.46
13.54
6.92
4.01
3.56
3.35
n.d.
n.d.
n.d.
3.94
n.d.
n.d.
3.68
3.35
4.64
4.11
3.86
n.d.
n.d.
n.d.
4.55
n.d.
n.d.
4.25
3.86
3.69
3.42
3.11
n.d.
n.d.
n.d.
3.57
n.d.
n.d.
3.60
3.11
1031
1177
1219
386
598
705
720
740
783
original 15 N=A
30 N=A
60 N=A
157
147
88
527
473
211
0.53
0.64
0.69
coked
15 3.7
30 5.7
60 6.1
152
123
61
520
425
198
0.52
0.62
0.56
a
1073
1235
The coked samples were thermally treated at 620 K under nitrogen
(1 Pa) for 12 h and subsequently activated at 630 K under vacuum for
1 h prior to the IR measurements. N=A means not applicable.
573
a
n designates the Si=Al ratio in the synthesis gel, Tc the conversion
temperature, DW the weight gain upon coking for 12 h, d the (100)
interplanar spacing, a the distance between pore centres, wd the
average pore diameter (XRD), SBET the BET speci®c surface area.
N=A: not applicable and n.d.: not determined.
Results and discussion
The XRD powder patterns recorded for the original materials,
those coked at 573 K, and calcined after coking showed that
coking and subsequent calcination did not signi®cantly a€ect
the structure of the materials.20 The 27Al MAS NMR spectra
(Fig. 1) were measured for the original Al-MCM-41(15) sam-
ple and this material coked at 513, 573, and 663 K. The spectra
exhibit no changes in the positions and relative intensities of
the two characteristic peaks (at ca. 0 and 55 ppm), which
shows that coking did not in¯uence the state of aluminium in
the studied sample. Thus, the results of the XRD and NMR
measurements indicate that the materials exhibit good chemi-
cal stability during the investigated conversion process. Some
structural and adsorption properties of the original and coked
materials, determined previously,20,28 are presented in Table 1.
The conversion of cyclohexene, over the studied materials,
proves the presence of strong Brꢀnsted acid sites. As found for
Al-MCM-41(15), the conversion increases with the reaction
temperature (Fig. 2). It is relatively low (ca. 52%) at 483 K, then
increases rapidly to ca. 96% at 543 K, and then remains prac-
tically constant up to 663 K. The yield increases with the Al
content of the catalysts, which is seen from the decreasing
amount of unreacted cyclohexene (Fig. 3). The catalytic activity
decreases slightly with the reaction time for Al-MCM-41 with
n ˆ 15 and 30, but rapidly is for Al-MCM-41(60) (Fig. 3).
Process of cyclohexene conversion and analysis of the products
The conversion was performed at several temperatures
between 483 and 663 K (Table 1) in a vertical ¯ow micro-
reactor coupled with a gas chromatograph (INCO GC 505M).
Cyclohexene vapour was passed as a mixture with a carrier gas
(5% of C6H10 , 95% of He) at a rate of 0.5 g of C6H10 per hour
and gram of catalyst for 12 h. More details are reported else-
where.29,30
The volatile products of the conversion were determined
from gas chromatography (GC), using a column packed with
a mixture of Chromosorb PAW, DC 550 oil (15%), and
stearic acid (3%). The light products with 1 or 2 carbon atoms
per molecule were analysed using a column with Chromosorb
102. An additional analysis of the collected reaction products
was carried out using a GC-MS apparatus (Hewlett-Packard
5890) with a fused silica column covered with a 0.25 mm thick
HP-5 ®lm. More details can be found elsewhere.30 The
amounts of carbonaceous deposits were evaluated from the
weight di€erence between the coked samples and the same
samples calcined at 773 K under oxygen for 5 h. The detailed
description of this procedure has been presented else-
where.20,31
IR and NMR measurements
The characterisation of the materials, except for the results of
IR and NMR, has been reported in the previous papers.20,28±30
The IR spectra were recorded with a Bruker 48 PC spec-
trometer equipped with an MCT detector. The samples pressed
into wafers were activated in situ at 630 K under vacuum for
1 h and then exposed at 450 K to pyridine (POCh, Poland,
dried over KOH) taken in excess in relation to all the acid sites.
Then, physisorbed pyridine was removed by evacuation at the
same temperature for 30 min. Subsequently, desorption at
570 K under vacuum was performed. The spectra were
recorded after evacuation at both 450 and 570 K.
Prior to the activation, the coked samples were heated at
620 K under nitrogen (1 Pa) for 12 h. The contents of carbon
remaining after the heat treatment and activation of the sam-
ples were determined by elemental analysis, using a EURO-
VECTOR analyser (Table 2).
27Al MAS NMR spectra of the original and coked sam-
ples were recorded using a Bruker AMX300 WB spectro-
meter operating at a resonance frequency of 78.2 MHz. The
samples were placed in a 7 mm od zirconia rotor spun at
7 kHz.
Fig. 1 27Al MAS NMR spectra of Al-MCM-41(15) samples: a,
original; b, c, and d, coked at 513, 573, and 663 K, respectively.
Phys. Chem. Chem. Phys., 2001, 3, 5082±5086
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Fig. 2 Conversion of cyclohexene over Al-MCM-41(15) vs. reaction
temperature. Reaction time: 12 h.
Fig. 4 Contributions of the C6 compounds to the volatile products of
cyclohexene conversion over Al-MCM-41(15) vs. reaction time for
reaction temperatures: 1, 483; 2, 513; 3, 543; 4, 573; 5, 603; 6, 633 and
7, 663 K.
Fig. 3 Fractions of unreacted substrate (Xs) after cyclohexene con-
version at 573 K over Al-MCM-41(15) (a), Al-MCM-41(30) (b), and
Al-MCM-41(60) (c) vs. reaction time.
Fig. 5 Contributions of various compounds to volatile products of
cyclohexene conversion over Al-MCM-41(15) vs. reaction tempera-
ture; 1, C7; 2, C4; 3, C8; 4, C5 and 5, C3. Reaction time: 12 h.
The volatile products of the reaction include compounds
containing 1 to 8 carbon atoms (C1±C8). They are formed via
cracking and alkylation and their amounts depend on the
reaction temperature. As opposed to the earlier work,30 in which
trace amounts of C9 compounds were detected, none were
found. This might result from the lower concentrations in the
present study and, consequently, a more dicult or slower
alkylation process. The C6 hydrocarbons predominate at each
temperature (Fig. 4 and 5) although their contributions decrease
with temperature (from 95 to 85% between 483 and 633 K,
respectively) and increase slightly with the reaction time.
The Si=Al ratio of the materials does not in¯uence the high
(above 92%) contributions of the C6 compounds at 573 K
(Fig. 6). With the reaction time, the content of the C6 compounds
increases slightly to ca. 95% for n ˆ 30 and60, andto ca. 98%for
n ˆ 15. This indicates a relatively stable catalytic activity and
high selectivity of the samples towards C6 at 573 K. The for-
mation of several C6 products indicates parallel isomerisation
and hydrogenation processes. The major components are
1-methylcyclopentane (MCPA) as well as 1- and 3-methylcy-
clopentene (1 MCPE and 3 MCPE), while at 663 K, cyclohexane
and benzene are also formed in small amounts (Fig. 7 and 8).
The increasing reaction temperature favours the cracking and
alkylation processes and, thus, the C3±C5 and C7±C8 hydro-
carbons reach their maximum contributions of between 1.0 (C3)
and 5.2% (C7) at 663 K (Fig. 5). Hydrocarbons C1 and C2 form
only above 573 K. The GC-MS analysis revealed 2-methyl-
butane, 2-methyl-2-butene, 2-methyl-1-butene, 2-pentene, and
cyclopentene as the C5 compounds and 3,5-dimethylcyclo-
pentane, 3-ethylcyclopentane, 2,3-dimethylpentadiene, and
Fig. 6 Contributions of the C6 compounds to the volatile products of
cyclohexene conversion at 573 K over Al-MCM-41(15) (a), Al-MCM-
41(30) (b), and Al-MCM-41(60) (c) vs. reaction time.
2-methyl-2,4-hexadiene as the C7 and C8 compounds formed at
543 K.
As can be concluded, the conversion of cyclohexene follows
two main concurrent reactions (Fig. 9): hydrogen transfer (HT)
and cyclohexene skeletal isomerisation (CSI). HT leads mainly
to formation of MCPA over all the examined samples, while
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Fig. 9 Scheme for reactions of cyclohexene conversion.
In short, coke forms primarily on the aluminium atoms located
in the pores20,29 and creating the Brꢀnsted acid sites.30
Information on the Brꢀnsted and Lewis acid sites has been
derived from the IR spectra within the region of OH vibra-
tions. The spectrum of the original Al-MCM-41(30) sample
(Fig. 10a) shows a distinct band at 3740 cm 1, characteristic of
silanol OH groups as well as of acidic hydroxy groups in
Al-MCM-41 and in amorphous aluminosilicates.32 The
deposition of coke results in a decrease in intensity of this band
due to the van der Waals interactions between the hydroxy
groups and coke (Fig. 10b).
Fig. 7 Contributions of several C6 compounds to the volatile pro-
ducts of cyclohexene conversion at the reaction temperatures indicated
over Al-MCM-41(15). 1, cyclohexane; 2, 3MCPE; 3, 1MCPE; 4,
MCPA; 5, benzene.
‡
The intensities of the IR bands of pyridinium ions, HPy , at
1
1545 cm (Brꢀnsted sites) and of pyridine molecules bonded
to the Lewis sites, PyL, at 1455 cm allowed us to calculate
1
the concentrations of the acid sites. The molar absorption
coecients necessary for this calculation were determined from
the spectra of pyridine adsorbed on zeolite HY and on dehy-
droxylated zeolite HY, containing almost exclusively Brꢀnsted
and Lewis acid sites, respectively;33,34 they were equal to 0.070
‡
cm2 mmol 1 for HPy and 0.100 cm2 mmol 1 for PyL. The acid
strength of the Brꢀnsted sites was determined from thermo-
desorption of pyridine at 570 K (cf. Experimental). The ratio
of the band intensities Ades=A0 (where A0 and Ades are the
‡
intensities of the HPy band upon evacuation at 450 K and
desorption at 570 K, respectively) was taken as a measure of
the acid strength.
As found, the concentrations of the Brꢀnsted and Lewis acid
sites (Table 2) increase with the Al content and are twice as
high in Al-MCM-41(30) with Si=Al ˆ 19.9 as in Al-MCM-
41(60) with Si=Al ˆ 36.8. A further increase in Al content to
Si=Al ˆ 12.2 in Al-MCM-41(15) causes only a slight increase in
the concentrations of the acid sites of both types. Probably, Al
atoms in the latter sample partially form clusters that create
neither Brꢀnsted nor Lewis sites. Another possibility is that a
relatively higher amount of Al in this sample is present inside
the material walls. The acid strength of the Brꢀnsted sites
appears to be practically independent of both the concentra-
tion of these sites and the Si=Al ratio.
Fig. 8 Contributions of several C6 compounds to the volatile
products of cyclohexene conversion at 573 K over Al-MCM-41(n)
with n indicated on the horizontal axis. 1, 3MCPE; 2, 1MCPE; 3,
MCPA.
a trace amount of cyclohexane (CHA) is produced at 663 K only
over Al-MCM-41(15) (Fig. 7). CSI yields 1 MCPE and 3 MCPE
with 1 MCPE prevailing in the case of n ˆ 15 and 30 (Fig. 8).
The presented results indicate that changing the concentra-
tion of cyclohexene from 16%30 to 5% caused no signi®cant
change in the kinds of products and in the high selectivity
toward the C6 compounds except for the formation of small
amounts of C9 products at the higher concentration. However,
the distribution of the products varies, i.e., the contents of C1±
C5 are lower and the contents of C7±C8 are higher at the lower
concentration of the feed gas.
The process of conversion, which requires strong Brꢀnsted
acid sites, results in the formation of carbonaceous deposits on
the catalysts. The amount of coke increases with the Al
amount in the materials and with decreasing reaction tem-
perature (Table 1). The detailed discussion on the parameters
determining formation of coke has been presented elsewhere.29
Fig. 10 IR spectra of OH groups in original (a) and coked (b) Al-
MCM-41(30) activated at 630 K.
Phys. Chem. Chem. Phys., 2001, 3, 5082±5086
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Deposition of coke leads to a decrease in the concentrations of
both the Brꢀnsted and Lewis sites though the e€ect seems to be
not very important (Table 2). The smallest e€ect has been
observed for the Al-MCM-41(15) sample exhibiting the lowest
amount of coke remaining after the thermal treatment.
A decrease in the concentration of the Brꢀnsted sites has also
been observed in the case of coked dealuminated HY zeo-
lites,33,34 but, as opposed to our Al-MCM-41 results, the for-
mation of a comparable amount of coke did not in¯uence the
properties of the Lewis sites in HY. The results of the thermo-
desorption of pyridine suggest that deposition of coke practi-
cally does not in¯uence the acid strength of the Brꢀnsted sites.
The relatively small di€erences between the acid site con-
centrations determined before and after the conversion of
cyclohexene indicate that pyridine can penetrate to the acid
centres in spite of the surface being blocked by the deposits. The
ease of this penetration depends presumably on the deposit
composition. It has to be taken into account that the thermal
treatment of the coked samples at 620 K under nitrogen and the
subsequent activation at 630 K under vacuum may easily con-
tribute to a further partial removal of the coke deposits from the
examined samples.
Brꢀnsted sites. Thus, pyridine seems to be able to penetrate to
the acid centres in spite the surface blocking by the carbo-
naceous deposits. This penetration depends presumably on the
deposit composition and likely on a possible further partial
removal of coke during the pretreatment of the samples before
the adsorption of pyridine.
Acknowledgement
Thanks are due to Mr. Pawel Baran (Jagiellonian University,
Krakow) for the IR measurements.
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