Methylcyclohexane ring-contraction: A sensitive solid acidity and shape selectivity probe reaction

Article abstract of DOI:10.1021/jp040205k

In this paper we describe the utility of an acid-catalyzed isomerization reaction, specifically, ring-contraction of methylcyclohexane to an isomeric mixture of alkylcyclopentanes as a tool for characterizing the acidic properties of a wide range of platinum-loaded solid acids. Methylcyclohexane isomerization is particularly useful as a solid acidity probe reaction since it is a simple molecule containing one six-membered ring and a single methyl group substituent. As a solid acidity probe molecule methylcyclohexane has a number of advantages over cyclohexane. Ring-contraction of cyclohexane produces a single product, methlycyclopentane. Methylcyclohexane ring-contraction, in contrast, yields a richer and thus more informative product mixture including ethylcyclopentane, and five isomeric dimethylcyclopentanes. For the first time it will be shown that variations in the three primary descriptors of solid acids, acid site density, acid site strength, and shape selectivity, within a wide range of amorphous and crystalline solid acids can be simultaneously ranked using a single component probe reaction, namely, methylcyclohexane ring-contraction.

Full text of DOI:10.1021/jp040205k

2222
J. Phys. Chem. B 2005, 109, 2222-2226
Methylcyclohexane Ring-Contraction: A Sensitive Solid Acidity and Shape Selectivity Probe
Reaction†
Gary B. McVicker,* Owen C. Feeley, John J. Ziemiak, David E. W. Vaughan,
Karl C. Strohmaier, Wayne R. Kliewer, and Daniel P. Leta
Corporate Strategic Research Laboratories, ExxonMobil Research and Engineering Company,
1545 Route 22 East, Annandale, New Jersey 08801
ReceiVed: March 9, 2004
In this paper we describe the utility of an acid-catalyzed isomerization reaction, specifically, ring-contraction
of methylcyclohexane to an isomeric mixture of alkylcyclopentanes as a tool for characterizing the acidic
properties of a wide range of platinum-loaded solid acids. Methylcyclohexane isomerization is particularly
useful as a solid acidity probe reaction since it is a simple molecule containing one six-membered ring and
a single methyl group substituent. As a solid acidity probe molecule methylcyclohexane has a number of
advantages over cyclohexane. Ring-contraction of cyclohexane produces a single product, methlycyclopentane.
Methylcyclohexane ring-contraction, in contrast, yields a richer and thus more informative product mixture
including ethylcyclopentane, and five isomeric dimethylcyclopentanes. For the first time it will be shown
that variations in the three primary descriptors of solid acids, acid site density, acid site strength, and shape
selectivity, within a wide range of amorphous and crystalline solid acids can be simultaneously ranked using
a single component probe reaction, namely, methylcyclohexane ring-contraction.
1. Introduction
Solid acid catalysts are extensively used by the petroleum
industry for the production of motor fuels and a wide range of
petrochemicals.1,2 Since a large number of hydrocarbon conver-
sion reactions are involved, it is important to establish reliable
test protocols that correlate the physical and compositional
properties of solid acids with the acidity requirements of the
specific reaction being catalyzed. Well-designed probe reactions
can identify subtle differences in both metal and acid functions,
aid process condition optimization, and provide insight into
catalyst activation, deactivation, and regeneration processes. In
this paper we describe the use of a simple solid acid promoted
hydrocarbon conversion reaction, namely, ring-contraction of
methylcyclohexane (MCH) to a mixture of ethylcyclopentane
(ECP) and dimethylcyclopentane (DMCP) isomers, as a probe
reaction for characterizing the acidity parameters of a wide range
of platinum-loaded solid acids. MCH ring-contraction rates and
product patterns can be used to appraise not only the relative
number and relative strength of acid sites in both amorphous
and crystalline solid acids, but can additionally evaluate subtle
shape selectivity differences in zeolites and microporous materi-
als. Figure 1 summarizes the relationships observed among
MCH conversions and product selectivities and the three key
acidity parameters attributed to solid acids.
Figure 1. MCH ring-contraction patterns discern three key solid acidity
characteristics. (1) Overall conversions, under a given set of reaction
conditions, are proportional to the number of effective acid sites for
both amorphous and crystalline solid acids. (2) ECP selectivities, at a
particular conversion, reflect relative differences in acid strengths. (3)
The trans-1,2-DMCP/trans-1,3-DMCP product ratio discriminates
product shape selectivity differences in microporous and zeolitic
materials.
catalyst “A” contains twice as many effective acid sites as
catalyst “B”. Second, relative strengths of solid acids can be
estimated by comparing selectivity differences shown toward a
particular ring-contraction product. At low MCH conversion
levels (10-20%) weaker acid strength solid acids typically
exhibit >70% selectivities toward the nonbranching ECP isomer
while stronger acid strength solid acids produce larger quantities
(50-70%) of the doubly branched DMCP isomers. The relative
ease of forming ECP is rationalized as resulting from the facile
rearrangement of a tertiary carbocation.3 Formation of DMCP
isomers, which demands stronger acid sites and furthermore
cannot be formed by a tertiary carbocation route, is assumed to
occur via parallel, nonclassical carbocation (ionic) routes
utilizing protonated cyclopropane transition states (see Figure
2).4 Postulated carbocation transition states for the various MCH
ring-contraction products are listed in the order of decreasing
stabilities.5
First, the relative number of effective acid sites can be
ascertained by comparing overall ring-contraction activities
under a given set of reaction conditions. For example, if catalyst
“A” under a specific set of reaction conditions selectively
converted 50% of MCH to ring-contraction products, while
catalyst “B” under identical conditions selectively converted
only 25% of MCH catalyst it is reasonable to suggest that
Increasing acid strength should enhance the extent to which
the individual DMCP isomer concentrations approach equilib-
rium.6 Based upon this postulate, ECP selectivity, at a given
MCH conversion, can be used as a relative measure of acid
strength. Last, the trans-1,2-DMCP/trans-1,3-DMCP isomer
ratio was found to vary between 0.1 and 2.4 over a large number
† Part of the special issue “Michel Boudart Festschrift”.
* To whom correspondence should be addressed. E-mail: gary.b.
mcvicker@exxonmobil.com.
10.1021/jp040205k CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/14/2004

Methylcyclohexane Ring-Contraction Reaction Probe
J. Phys. Chem. B, Vol. 109, No. 6, 2005 2223
TABLE 1: MCH Conversion Patterns Over Solid Acids are
Well Behaved
rates (g/h/g)b
% ECP
catalysta
ZSM-22
g
WHSV total
RC
ECP selectivity
0.25
1.5
62
10
1.39 1.33 0.82
1.38 1.28 0.62
62
48
c
SiO2-Al2O2
0.25
2.0
62
8
0.75 0.73 0.38
0.70 0.70 0.34
52
49
equlibrium Value
10
a Catalysts contain 0.5 wt % Pt. b Conditions: 553 K, 1475 kPa, 20
cm3 MCH/h, H2/MCH ) 5, MCH conversions <15%, RC ) ring-
contraction rate. c Amorphous.
FAU (USY)-materials. Those interested in obtaining detailed
structural information on the various zeolitic and microporous
materials employed in these studies should consult ref 8. Metal
analyses were in all cases in good agreement with nominal value
of 0.5 wt % Pt. Routine X-ray diffraction measurements on
zeolitic and microporous materials yield sample crystallinity and
lattice parameter information that in the case of zeolites provide
an estimate of the Si/Al molar ratio. Solid-state MAS NMR
measurements furnished a direct measure of framework Al
content, while elemental analyses supplied bulk Si/Al values.
Additional catalyst preparation and characterization particulars
can be found in refs 9-11.
Figure 2. Possible nonclassical carbocation (ionic) routes for the ring-
contraction of methylcyclohexane to ethylcyclopentane and isomeric
dimethylcyclopentanes.
2.2. Reactor System. MCH ring-contraction experiments
were carried out in a 25-cm3 stainless steel, fixed-bed, downflow
reactor capable of operating pressures to 5500 kPa and tem-
peratures to 823 K. The reactor was heated by a fluidized sand
bath, which provided isothermal operation throughout the reactor
zone. Neat, high-purity MCH was fed without pretreatment to
the reactor by a dual-barrel Ruska feed pump. The upper portion
of the reactor contained inert mullite beads, which served as a
preheat zone and also minimized dead space volume. Total
reactor effluent was analyzed online by a Wasson engineered
GC system equipped with a 60-m capillary column having a
methylsilicone stationary phase. All reaction products, ranging
in molecular weight from methane through the isomeric xylenes,
were verified by supplementary GC/MS analyses. The reactor
was charged with 0.25-2.0 g of catalyst. Catalysts were
prereduced at 773 K under a flow of 500 cm3/min of H2 under
1400-2900 kPa total pressure. Initial reaction temperatures were
determined by the reactivity of the particular solid acid
employed. Liquid MCH was fed to the reactor at 10-20 cm3/
h. The H2 flow was adjusted to provide the desired H2/MCH
mole ratio. After steady-state conversion was achieved the
reaction temperature was increased by 10-25 K increments.
The reaction temperature was routinely returned to an interme-
diate value to establish the extent of catalyst deactivation. No
significant deactivation was noted in any of the solid acid
systems tested.
Figure 3. Equilibrium distribution of MCH ring-contraction products.
Symbol codes: 9 MCH, 0 ECP, [1,1-DMCP, ] cis-1,2-DMCP, 2
trans-1,2-DMCP, 4 cis-1,3-DMCP, and b trans-1,3-DMCP.
of representative solid acids. The substantial variability exhibited
by this ratio is consistent with a product shape selectivity
dependence resulting from differences in the transport capabili-
ties of the 1,2- and 1,3-DMCP isomers. The less bulky 1,3-
DMCP isomer would likely experience a smaller transport
inhibition than the bulkier 1,2-DMCP isomer through the
intracrystalline channels and pore mouths of 10- and 12-ring
zeolites and microporous materials.7 Ten-ring zeolitic and
microporous materials typically display trans-1,2-DMCP/trans-
1,3-DMCP ratios in the 0.1 to 0.5 range, while larger pore size
12-ring zeolites and amorphous solid acids exhibit much higher
values for this isomer ratio, ranging from 1.2 to 2.4. Over the
rather broad temperature range utilized throughout these studies
the equilibrium value for the trans-1,2-DMCP/trans-1,3-DMCP
ratio remains near 1.8-1.9 (see Figure 3).
3. Results and Discussion
3.1. MCH Ring-Contraction is a Well-Behaved Reaction.
MCH conversion patterns over both amorphous and crystalline
zeolitic and microporous solid acids are reproducible and readily
measured by conventional GC techniques. To illustrate the
reliability of the ring-contraction reaction, overall MCH rates
exhibited by ZSM-22, a crystalline 10-ring zeolite, and an
amorphous SiO2-Al2O3 solid acid were found to remain
constant over a 6-8-fold change in weight hourly space velocity
(Table 1).
2. Experimental Section
2.1. Catalysts. Metal-loaded catalysts were prepared by
conventional incipient wetness impregnation of well-character-
ized amorphous and crystalline zeolitic and microporous acidic
supports with a standardized platinum solution (0.10 g Pt/cm3).
Representative solid acids employed in these studies include
halided-Al2O3, amorphous SiO2-Al2O3, and crystalline zeolitic
and microporous materials including Beta, MOR, ZSM-5,
SAPO-11, ZSM-22, EU-1, ZSM-23, CLINO, and high-silica
Selectivities toward ring-contraction (defined as RC rate/total
rate) remain above 90% over the conditions employed. Selec-

2224 J. Phys. Chem. B, Vol. 109, No. 6, 2005
McVicker et al.
TABLE 2: MCH Conversion Patterns are not Strongly
Dependent Upon Platinum Dispersion
wt % @ 593 Kc
ECP selectivity
catalysta Si/Al H/Ptb conv. RC cracking @ 10% conv.
ZSM-22
ZSM-22
EU-1
27
27
25
25
0.20
0.67
0.09
0.90
22.9 22.1
19.2 18.2
42.5 38.0
37.8 32.9
0.48
0.65
4.0
67
62
23
22
EU-1
4.5
equilibrium Value
8-11
a Catalysts contain 0.5 wt % Pt. b H/Pt determined by hydrogen
chemisorption. c Conditions: 473-613 K, 1475 kPa, WHSV ) 31, H2/
MCH ) 5.0, RC ) ring-contraction.
Figure 4. Arrhenius plots for MCH ring-contraction over representative
amorphous and crystalline solid acids. Run conditions: 2850 kPa, H2/
MCH ) 5, WHSV ) 31, catalysts contain 0.5% Pt. Symbol codes: 0
Pt/Al2O3, b Pt/SAPO-11, [ Pt/SiO2-Al2O3, O Pt/USY (95), 2 Pt/
USY (40), g Pt/USY (12), 9 Pt/USY (5.1); (number in parentheses)
) Si/Al mole ratio.
TABLE 3: Relative MCH Ring-Contraction Rates can be
Used to Estimate Acid Site Densitiesa
ring-contraction
rateb
relative number
of acid sites
catalyst 0.5% Pt
Si/Al
Pt/Al2O3
0.05
1.0
1.4
3.9
25
105
132
126
1.0
20
28
78
500
2100
2640
2520
an activity comparable to that of the most active USY (Si/Al )
5.1) catalyst. This suggests that at comparable framework Al
contents the acid sites present in a Beta zeolite are likely stronger
and therefore more effective for MCH ring-contraction than
those of USY. Arrhenius plots for a variety of both amorphous
and crystalline solid acids (see Figure 4) established that
apparent activation energies associated with MCH ring-contrac-
tion fall within the range of 28-32 kcal/mol (117-134 kJ/mol).
This is an important observation since a common activation
energy allows catalysts with markedly differing activities to be
compared (ranked) at a given temperature.
Pt/SAPO-11
Pt/SiO2-Al2O3
Pt/1.5%F-Al2O3
Pt/USY
Pt/USY
Pt/USY
0.05
3.4
37
8.3
5.1
15
Pt/Beta
a Conditions: WHSV ) 31, H2/MCH ) 5, 2850 kPa, 548-598 K.
b Rates in mol/h/g × 1000, at 548 K.
tivities toward the ECP isomer decreased, as expected, with
increasing conversion. Side reactions such as acid site cracking
and metal site dehydrogenation were minimized by the choice
of reaction conditions. Acid site cracking becomes important
only at higher temperatures and then only with the use of more
strongly acidic catalysts. Dehydrogenation of MCH to toluene,
as well as to olefins, is limited by the relatively high hydrogen
partial pressure (1230 kPa) and low temperatures used. Higher
hydrogen partial pressures also significantly reduce catalyst
deactivation rates. During the course of these studies it was
observed that ring-contraction of MCH over a wide range of
platinum-loaded solid acids was not highly dependent upon the
activity of the platinum component. As shown in Table 2
platinum dispersions differing by a factor of 10 had only a
limited effect on the ring-contraction and cracking activities and
ECP selectivities of ZSM-22 and EU-1 based hydroisomeriza-
tion catalysts. This observation suggests that only small quanti-
ties of platinum are required to generate equilibrium quantities
of olefins, which in turn are readily transformed into ionic
intermediates on the acidic ZSM-22 and EU-1 supports.12
3.2. Relative MCH Ring-Contraction Rates can be used
to Estimate Acid Site Densities. MCH ring-contraction rates
for a wide variety of solid acids are compared in Table 3. Ring-
contraction rates normalized to the least active catalyst Pt/Al2O3
catalyst provide a relative scale of acid site densities that spans
3 orders of magnitude. Ring-contraction rates for Al2O3 based
catalysts are increased 80-fold upon addition of 1.5 wt %
fluorine, indicating a substantial increase in the number of
effective acid sites. This large increase in the number of effective
acid sites suggests that upon fluoridation initially inactive Al2O3
hydroxyl groups are transformed, presumably by electron
induction effects, into Brønsted acid sites of sufficient strength
to more efficiently catalyze ring-contraction.13 For a series of
ultrastable faujasites (USYs) the relative number of effective
acid sites increases smoothly with decreasing zeolite Si/Al ratios.
The most active USY (Si/Al ) 5.1) catalyst studied is at least
2600 times more active than Al2O3. A Beta zeolite having a
significantly lower framework Al content, Si/Al ) 15, displayed
3.3. Acid Strength Information can be Abstracted from
Ring-Contraction Product Patterns. Thermodynamics favor
five-membered rings over six-membered rings at temperatures
typically used in petroleum processing.14 As shown in Figure
3, five-membered ring isomers become favorable above 500
K. Figure 3 also shows that ECP and 1,1-DMCP are less favored
than the combined dibranched cis- and trans-1,2-DMCP and
cis- and trans-1,3-DMCP isomers. As described in section 3.2.,
overall MCH ring-contraction rates are a reliable gauge of the
relative number of acid sites strong enough to catalyze ring
contraction. Strength variation within these acid sites can be
estimated by comparing selectivity patterns accompanying ring
contraction. Although among the least thermodynamically stable
isomers, ECP is the initial MCH ring-contraction product formed
in measurable quantities and is the majority product over lower
acid strength catalysts at conversions less than 10%. At higher
reaction temperatures and MCH conversions greater than 50%
an equilibrium distribution of DMCP isomers is approached over
higher acid strength catalysts. The equilibrium concentration
of ECP remains between 8 and 11% over the temperature range
employed (473-613 K). Figure 5 illustrates the combined effect
of acid strength and conversion level on ECP selectivity.
At lower conversions over a weak to moderate acid strength
Pt/SAPO-11 catalyst ECP is the predominate product. As
conversion continues to rise, selectivity to ECP decreases with
increasing formation of the more thermodynamically favored
DMCP isomers. Over the higher acid strength Pt/USY(Si/Al )
5.1) catalyst, ECP is initially formed well above its equilibrium
level, however, the DMCP isomers predominate the product mix.
At higher MCH conversion levels product distributions trend
toward equilibrium values. Comparison at 10% MCH conversion
clearly shows that the lower acid strength Pt/SAPO catalyst
exhibits an ECP selectivity of 55% while the ECP selectivity
shown by the higher acid strength Pt/USY (5.1) catalyst is much
lower (30%). High ECP selectivities measured at 10-20% MCH
conversion, indicating weak to moderate acid strengths, are

Methylcyclohexane Ring-Contraction Reaction Probe
J. Phys. Chem. B, Vol. 109, No. 6, 2005 2225
Figure 6. Dimethylcyclopentanes are appropriately sized to detect
small porosity differences in 10-ring zeolites and microporous materials.
While trans-1,3-DMCP can pass through the 5.7-Å channel opening
of ZSM-22, trans-1,2-DMCP will experience a tighter squeeze.
TABLE 5: MCH Conversion Selectivites Reveal Porosity
Differences Among Amorphous and Crystalline Solid Acids
Figure 5. Branching patterns of MCH ring-contraction products reflect
differences in acid site strength.
1,2-DMCP/
wt % @ 593 Kb
1,3-DMCPc
TABLE 4: MCH Conversion Activities are Responsive to
Zeolite Framework to Si/Al Ratios
pore
catalysta
USY
MOR
Beta
SAPO-11
ZSM-22
ZSM-5
ZSM-23
CLINO
SiO2-Al2O3
ZSM-22/
SiO2-Al2O3
Si/Al size (Å) conv. RC cracking @ 593 K
wt % @ 593 Kb
ECP selectivityc
33
167
71
7.4 × 7.4 14.2 14.1 0.04
7.0 × 6.5 10.0 9.9 0.04
7.6 × 6.4 60.9 59.4 1.3
2.21
2.29
1.23
0.34
0.16
0.02
0.51
1.94
2.16
1.15
catalysta
Si/Al
conv.
RC
cracking
@10% conv.
USY
USY
ZSM-5
ZSM-5
EU-1
EU-1
66
171
12
69
18
47.3
9.6
37.4
3.8
59.7
26.7
46.6
9.5
30.6
3.4
49.7
23.7
0.71
0.03
6.3
0.4
9.8
54
56
34
36
30
29
0.05 6.3 × 3.9 8.2 8.1 0.1
27
110
49
5.7 × 4.4 19.2 18.2 0.7
5.6 × 5.3 21.1 21.0 0.1
5.2 × 4.5 13.1 12.8 0.2
5.2 7.6 × 3.0 6.1 5.9 0.1
35
2.9
15.8 15.7 0.05
equilibrium Value
8-11
18.2 18.1 0.1
d
a Catalysts contain 0.5 wt % Pt. b Conditions: 473-613 K, 1475
kPa, WHSV ) 31, H2/MCH ) 5.0. c Increasing % ECP values indicate
decreasing acid strengths.
equilibrium Value
1.8-1.9
a Catalysts contain 0.5 wt % Pt. b Conditions: 473-613K, 1475kPa,
WHSV ) 31, H2/MCH ) 5.0. c Increasing trans-1,2DMCP/trans-
1,3DMCP ratios suggest decreasing in-pore constraint. d 50/50 wt %
mixture.
observed for amorphous SiO2-Al2O3, halided-Al2O3, highly
dealuminated USY, ZSM-22, and microporous materials such
as SAPO-11. Lower ECP selectivities at 10-20% MCH
conversion, which reflect a closer approach to an equilibrium
isomer distribution, are exhibited by higher acid strength zeolitic
materials such as ZSM-5, low Si/Al USY, mordenite, EU-1,
and Beta.
using van der Waals’ radii for a ZSM-22 zeolite shown in Figure
6. While the smaller trans-1,3-DMCP isomer formed within the
channels of ZSM-22 should readily exit through the 5.7-Å
channel opening of ZSM-22, the larger trans-1,2-DMCP isomer
which is also produced in the channels of the zeolite will
experience a tighter squeeze which will likely hinder its egress
from the 10-ring ZSM-22 crystal. Highly constraining 10-ring
zeolites and 10-ring microcrystalline materials such as SAPO-
11 consistently display trans-1,2-DMCP/trans-1,3-DMCP ratios
of 0.5 or less. Larger pore size 12-ring zeolites and amorphous
solid acids exhibit values ranging from 1.2 to 2.3 for this ratio.
The trans-1,2-DMCP/trans-1,3-DMCP ratios for a number of
crystalline 10- and 12-ring zeolites and microporous materials
and an amorphous SiO2-Al2O3 catalyst are compared in Table
5. An amorphous SiO2-Al2O3 solid acid exhibits a trans-1,2-
DMCP/trans-1,3-DMCP ratio of about 2.2 which is close to
the equilibrium value of 1.8-1.9. This close approach to
equilibrium suggests that the surface of amorphous SiO2-Al2O3
does not restrict the transport of the various DMCP isomers.
USY and mordenite zeolites displayed trans-1,2-DMCP/trans-
1,3-DMCP ratios of 2.2 and 2.3, respectively, indicating that
these large-pore, 12-ring zeolites, like the open structure of
amorphous SiO2-Al2O3, do not impede the generation of
equilibrium quantities of the four DMCP isomers. The 12-ring
Beta zeolite, which has a pore size similar to those of USY and
mordinite, exhibited a substantially lower trans-1,2-DMCP/
trans-1,3-DMCP ratio (1.2) indicating a greater degree of in-
channel or pore mouth constraint. Markedly smaller trans-1,2-
DMCP/trans-1,3-DMCP ratios, typically 0.5 or less, are observed
for 10-ring zeolitic and microporous solid acids. For a given
3.4. MCH Conversion Activities are Responsive to Zeolite
Framework Si/Al Ratios. The effect of framework Si/Al ratio
upon the MCH conversion patterns of three representative
zeolites, namely USY, ZSM-5 and EU-1, is summarized in Table
4. As would be expected, increasing Si/Al ratios by lowering
the concentration of framework Al species (proportionally
lowers acidic hydroxyl content) substantially reduces both ring-
contraction and acid cracking activities for all three zeolites.
ECP selectivity remained essentially constant for each zeolite
regardless of the extent of dealumination. This finding strongly
suggests that for the moderately to highly dealuminated zeolites
compared here the relative strength of acid sites present in each
individual zeolite is not appreciably altered upon additional Al
removal.15 It is possible that more aluminum-rich USY and EU-1
zeolites (Si/Al ratios < 10), which would have an increased
number of nearest neighbor aluminum interactions, might exhibit
modified ECP selectivities indicating, thereby, changes in acid
strength. Based upon the ECP selectivities at 10% conversion,
the relative acid strengths of the three zeolites listed in Table 4
decrease in the order EU-1 > ZSM-5 > USY.
3.5. MCH Conversion Selectivities Detect Differences in
Solid Acid Porosities. The trans-1,2-DMCP/trans-1,3-DMCP
product selectivity ratio was found to be highly dependent upon
the pore size of a wide range of zeolitic and microporous solid
acids. The basis for this pore size dependency can for the most
part be accounted for by considering the space-filling model

2226 J. Phys. Chem. B, Vol. 109, No. 6, 2005
McVicker et al.
TABLE 6: MCH Conversion Selectivity Patterns Readily
Reflect Changes in the Crystallite Size of Zeolites and
Microporous Materials
In the case of ZSM-5 acid site cracking yield over the smaller
crystal size material is also substantially increased relative to
that of ring-contraction. Additionally, as indicated by increased
trans-1,2-DMCP/trans-1,3-DMCP isomer ratios, the extent of
exterior surface versus in-channel activity for both SAPO-11
and ZSM-5 is markedly enhanced by decreasing crystallite size.
Thus, modifying the crystallite size of 10-ring zeolites and
microporous materials can significantly alter the relative propor-
tion of in-channel acid site catalysis.
1,2-DMCP/
1,3-DMCPc
catalysta Si/Al size (µ) conv. RC cracking @ 593 K
wt % @ 593 Kb
crystallite
SAPO-11 0.05 0.1-1.0
8.2
8.1
0.1
0.02
3.9
0.34
1.33
0.19
0.45
SAPO-11 0.05 0.01-0.2 20.0 19.9
ZSM-1
ZSM-1
13
15
2.0
14.1 10.1
0.01-0.2 43.2 22.2
20.8
equilibrium Value
1.8-1.9
4. Conclusion
a Catalysts contain 0.5 wt % Pt. b Conditions: 473-613 K, 1475
kPa, WHSV ) 31, H2/MCH ) 5.0. c Increasing trans-1,2DMCP/trans-
1,3DMCP ratios suggest decreasing in-pore constraint.
Under well-defined test conditions, MCH ring-contraction has
been established not only as a reliable measure of the relative
number and relative strength of acid sites for both amorphous
and crystalline solid acids, but it has utility in assessing shape
selectivity differences among a wide range of 10-ring and 12-
ring zeolites and microporous materials.
10-ring material it is possible to modify its trans-1,2-DMCP/
trans-1,3-DMCP ratio by a variety of selectivation treatments.16
The extremely low isomer ratio of 0.02 shown by ZSM-5 is
not consistent with its pore size which is similar to those of
both ZSM-22 and ZSM-23. It is possible that ZSM-5 (and
perhaps Beta as well) exhibits a lower trans-1,2-DMCP/trans-
1,3-DMCP ratio than expected as the result of the formation of
an in-channel and/or pore mouth constricting carbon overlayer
that rapidly forms on this zeolite during the initial on oil line
out period.17 In the case of a CLINO-based catalyst the extreme
elliptical nature of the pore mouth of this 10-ring zeolite is so
restrictive that MCH conversion occurs primarily on the outer
surface, this significantly reduces conversion and produces a
trans-1,2-DMCP/trans-1,3-DMCP ratio value typical of a non-
constraining, open surface. Finally, a 50/50 by weight mixture
of ZSM-22 with an amorphous SiO2-Al2O3 demonstrates a
trans-1,2-DMCP/trans-1,3-DMCP ratio that is the average value
of the two components. In cases where the constituent parts
have largely different activities the composite isomer ratio
closely scales with relative activities. The ability to monitor
subtle changes in the relative amounts of in-channel and outer
surface activities of neat as well as bound 10- and 12-ring
zeolites and microporous materials is an important and highly
informative feature of the MCH ring-contraction probe reaction.
3.6. Crystallite Size Modifications in Zeolites and Micro-
porous Materials are Readily Reflected in MCH Conversion
Selectivity Patterns. As shown in Table 6, reducing crystallite
size by an order of magnitude results in 2-3-fold activity
enhancements for both SAPO-11 and ZSM-5.
References and Notes
(1) Rabo, J. A. Catal. Today 1994, 22, 201.
(2) Hoelderich, W. F.; Heitmann, G. Catal. Today 1997, 38, 227.
(3) Schulz, H. F.; Weitkamp, J. H. Ind. Eng. Chem. Prod. Res. DeV.
1972, 11, 46.
(4) Alvarez, F.; Montes, A.; Perot, G.; Guisnet, M. In Zeolites: Facts,
Figures, Future; Jacobs, P. A., vanSanten, R. A., Eds.; Elsevier: Amsterdam,
1989; p 1367.
(5) Brouwer, D. M.; Hogeveen, H. In Progress in Physical Organic
Chemistry; Streitwieser, A., Jr., Taft, R. W., Eds.; Wiley-Interscience: New
York, 1972; Vol. 9, p 179.
(6) Weitkamp, J.; Jacobs, P. A.; Ernst, S. In Structure and ReactiVity
of Modified Zeolites; Jacobs, P. A, Jaeger, N. I., Jiru, P., Schulz-Ekloff,
G., Kazansky, V. B., Eds.; Elsevier: Amsterdam, 1984; p 279.
(7) Weisz, P. B. Pure Appl. Chem. 1980, 52, 2091.
(8) Meir, W. M.; Olson, D. H. Atlas of Zeolite Structure Types;
Butterworth-Heineman: Boston, 1992.
(9) McVicker, G. B.; Baker, R. T. K.; Garten, R. L.; Kugler, E. L. J.
Catal. 1980, 65, 207.
(10) Beyerlein, R. A.; McVicker, G. B.; Yacullo, L. N.; Ziemiak, J. J.
J. Phys. Chem. 1988, 92, 1967.
(11) McVicker, G. B.; Touvelle, M. S.; Hudson, G. W.; Vaughan, D.
E. W.; Daage, M.; Hantzer, S.; Klein, D. P.; Ellis, E. S.; Cook, B. R.; Feeley,
O. C.; Baumgartner, J. E. U.S. Patent 5,763,731, June 9, 1998.
(12) Weisz, P. B. Chemtech 1973, 498.
(13) Ghosh, A. K.; Kydd, A. Catal. ReV.- Sci. Eng. 1985, 27, 539.
(14) Stull, D. R.; Westrum, E. F.; Sinke, G. C. The Chemical
Thermodynamics of Organic Compounds; Wiley: New York, 1969.
(15) Haag, W. O.; Lago, R. M.; Weisz, P. B. Nature (London) 1984,
309, 589.
(16) Halgeri, A. B.; Das, J. Catal. Today 2002, 73, 65.
(17) Ashton, A. G.; Batmanian, S.; Dwyer, J.; Elliott, I. S.; Fitch, F. R.
J. Mol. Catal. 1986, 34, 73.