Improving mass-transfer in controlled pore glasses as supports for the platinum-catalyzed aromatics hydrogenation

Article abstract of DOI:10.1039/c4cy01665c

The liquid-phase hydrogenation of toluene and other alkyl substituted benzene derivatives with different critical diameters was investigated over Pt-catalysts supported on spherical controlled pore glasses (CPGs) as model supports at 373 K in the batch mode. The effect of mass-transfer within the catalyst pores was studied by varying the pore width (4, 10, and 80 nm) and average grain size (18-150 μm) of the Pt/CPG catalysts. For toluene hydrogenation, internal mass-transfer limitations were absent (effectiveness factor >90%) only for catalysts with particle sizes below 25 μm and pore widths ≤10 nm or with a pore width of 80 nm and particle sizes around 75 μm, respectively. Effective diffusion coefficients obtained from initial reaction rates via the Thiele concept, e.g., 2.8 × 10^-10 m² s^-1 for toluene over the catalyst with 10 nm pore width, were an order of magnitude lower than when determined by PFG-NMR. This difference was explained in terms of transport resistances such as surface barriers affecting the diffusivity assessment via the Thiele concept, while PFG-NMR measures intraparticle diffusion only.

Full text of DOI:10.1039/c4cy01665c

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Improving mass-transfer in controlled pore
glasses as supports for the platinum-catalyzed
Cite this: DOI: 10.1039/c4cy01665c
aromatics hydrogenation†
a
a
a
a
b
b
M. Goepel, H. Kabir, C. Küster, E. Saraçi, P. Zeigermann, R. Valiullin,
b
a
b
a
C. Chmelik, D. Enke, J. Kärger and R. Gläser*
The liquid-phase hydrogenation of toluene and other alkyl substituted benzene derivatives with different
critical diameters was investigated over Pt-catalysts supported on spherical controlled pore glasses (CPGs)
as model supports at 373 K in the batch mode. The effect of mass-transfer within the catalyst pores was
studied by varying the pore width (4, 10, and 80 nm) and average grain size (18–150 μm) of the Pt/CPG cat-
alysts. For toluene hydrogenation, internal mass-transfer limitations were absent (effectiveness factor
>
90%) only for catalysts with particle sizes below 25 μm and pore widths ≤10 nm or with a pore width of
Received 12th December 2014,
Accepted 9th April 2015
80 nm and particle sizes around 75 μm, respectively. Effective diffusion coefficients obtained from initial
−10
2 −1
reaction rates via the Thiele concept, e.g., 2.8 × 10
m s for toluene over the catalyst with 10 nm pore
width, were an order of magnitude lower than when determined by PFG-NMR. This difference was
explained in terms of transport resistances such as surface barriers affecting the diffusivity assessment via
the Thiele concept, while PFG-NMR measures intraparticle diffusion only.
DOI: 10.1039/c4cy01665c
www.rsc.org/catalysis
9
–11
work was conducted in this field,
systematic experimental
1
Introduction
catalytic investigations are less common. Most of the
12
Hydrogenation reactions play an important role in many
industrial heterogeneously catalyzed reactions such as the
removal of hetero atoms from crude oil fractions, the trans-
formation of unsaturated to saturated (cyclic) hydrocarbons
the defunctionalization of biomass, the reduction of car-
bonyl and nitro groups in organic subtrates as well as the
production of pharmaceuticals, fine and bulk chemicals.
Metal-catalyzed hydrogenation reactions are known to occur
with high intrinsic reaction rates. Thus, the overall reaction
rate is often limited by mass-transfer. Therefore, the
improvement of solid hydrogenation catalysts often depends
on an understanding of the mass-transfer processes with
respect to the catalytic conversion.
Generally, internal and external mass-transfer are distin-
guished. Internal mass-transfer refers to the transport of a
reactant inside the catalyst pores. External mass transfer
relates to reactant transport through the boundary layer
around a catalyst particle, within the bulk phase surrounding
the catalyst particles or across phase boundaries in
multiphase reaction systems. Although a wealth of theoretical
reported work was devoted to gas phase conversions, espe-
cially involving zeolite catalysts within which mass-transfer
limitations may pose a crucial restriction for the reaction
1
2
,3
13
rates.
4
In recent years, materials with hierarchically structured
pore systems (often zeolite-based) generated interest due to
their potential of overcoming mass-transfer limitations in
5
6
7
1
4–16
heterogeneously catalyzed conversions.
These materials
are most attractive due to the combination of the zeolitic
micropores, providing a high surface area, and the larger
pores (typically mesopores, d = 2–50 nm) providing improved
accessibility to the micropores containing the majority of the
catalytically active sites. Thus, the mesopores mainly act to
supply mass transfer routes within the catalyst particles
resulting in an increased effectiveness factor. Several reports
in this field attribute the higher catalytic activity to improved
8
17–19
mass-transport in hierarchically structured pore systems.
It is, however, noteworthy that experimental data on the
mass-transport in these catalytic systems are rarely presented.
Moreover, knowledge about the required pore width for
unhindered mass-transfer in the larger (meso)pores is
needed to arrive at a rational design of hierarchically struc-
tured catalysts.
Particularly few experimental work is available on mass-
transfer limitations of liquid-phase conversions. This is sur-
prising in view of their industrial relevance, especially of
a
Institute of Chemical Technology, Universität Leipzig, Germany.
E-mail: roger.glaeser@uni-leipzig.de
b
Institute of Experimental Physics I, Universität Leipzig, Germany
†
Electronic supplementary information (ESI) available. See DOI:10.1039/
c4cy01665c
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
2
0
catalytic liquid-phase hydrogenations, on the one hand, 2 Experimental section
and in view of the fact that mass-transfer limitations are gen-
2
.1 Synthesis and characterization of the controlled pore
erally more likely to occur because of reduced diffusion rates
in liquid compared to gaseous reaction phases, on the other
hand. Although mass-transfer limitations were observed for
glasses and supported Pt-catalysts
The spherical controlled pore glasses (CPGs) were synthe-
sized according to the VYCOR-Process. Therefore, non-
2
0–22
25
catalytic liquid-phase conversions,
systematic experimen-
tal investigations on how to overcome them are scarce.
porous glass beads (Biosearch Inc. Steinach, grain sizes 100–
200 μm) with a composition of 70 wt.-% SiO , 23 wt.-% B O
2
3,24
Recently, Foley et al.
conducted two studies on platinum
2
2 3
supported on nanoporous carbon as a catalyst for the liquid-
phase hydrogenation of different alkenes. Based on a study
of the influence of mass-transfer, the catalyst design, i.e.,
grain size and porosity, was optimized to overcome internal
mass-transfer limitations. However, in these carbon-based
systems, the pore width cannot be readily adjusted. This
results in broad or less-defined pore width distributions mak-
ing correlations of mass-transfer behavior and pore width
difficult.
2
and 7 wt.-% Na O were used to prepare CPGs with average
pore widths of 4 (CPG(4)) and 10 nm (CPG(10)), respectively.
To obtain a CPG with an average pore width of 80 nm
(CPG(80)), an initial glass with a different composition, i.e.,
62 wt.-% SiO , 30 wt.-% B O and 7 wt.-% Na O and 1 wt.-%
2
2
3
2
−
1
Al O , was used. First, an annealing (heating rate 10 K min )
2
3
was carried out for 24 h at different temperatures according
to the desired pore widths, i.e., at 793 K for CPG(4), at 803 K
for CPG(10), and at 863 K for CPG(80). Then, the solids were
extracted, first with aqueous 3 N hydrochloric acid for 15 h at
363 K followed by treatment with an aqueous sodium hydrox-
ide solution. While the extraction for CPG(4) and CPG(10)
was carried out with 0.5 N sodium hydroxide solution for 1 h
at room temperature, a three-step extraction was applied for
CPG(80), i.e., first for 4 h with 0.6 N, then twice with 0.4 N
sodium hydroxide solution at 303 K for 2 h, respectively.
The CPG beads were loaded with platinum via electrostatic
adsorption (EA). For that purpose, 10 g of the CPG beads
In the present study, catalysts based on controlled pore
glass (CPG) are presented as interesting alternative supports
for metal-based hydrogenation catalysts. Controlled pore
glasses are versatile catalyst support materials with tunable
surface area, pore volume and narrow pore width distribu-
2
5
tions between 1–1000 nm. They were already successfully
2
2,26
used as catalyst supports
and are available as monoliths
2
7
with different shapes including spheres. They are, thus,
interesting model supports for mass-transfer investigations.
Consequently, the aim of this study was to systematically
investigate the influence of pore width and grain size of
CPG-supported noble metal catalysts on the initial reaction
rate in the hydrogenation of alkylated benzene derivatives as
a model hydrogenation reaction. In particular, the mass-
transfer properties and, ultimately, the pore width needed
for unhindered mass-transfer during noble metal-catalyzed
liquid-phase hydrogenations were determined. Thus, CPG
spheres with pore widths between 4 and 100 nm were applied
as supports for platinum as the catalytically active hydrogena-
tion component.
3
were suspended in 50 cm deionized water. This suspension
was kept at a pH of 10 via addition of 1 M aqueous ammonia
solution (prepared using 32 wt.-% aqueous ammonia solu-
tion (≥30 wt.-% NH
3
content, Merck)) for 2 h under stirring.
Afterwards, an aqueous solution of 0.1 M PtIJNH ) Cl ·2H O
3
4
2
2
3 4 2 2
(3.52 g PtIJNH ) Cl ·2H O (99.0 Ma.-%, ABCR) dissolved in
3
100 cm deionized water) was added dropwise under stirring.
After stirring for an additional hour, the solid was separated
3
by filtration and washed three times with 20 cm deionized
water. The complete removal of excess PtIJNH
support was proven by the absence of chloride ions in the
washing water by addition of AgNO -solution. Finally, the
3 4 2
) Cl from the
Furthermore, the effective diffusion coefficients Deff of the
alkyl benzene reactants were determined from the effective-
ness factor based on the Thiele concept. The thus obtained
3
solid was dried at 353 K overnight under an air atmosphere
and reduced at 623 K for 4 h in a stream of 20 vol.-% H
in N (10 cm min ). The resulting catalysts were labelled
2
2
3
−1
Deff values were, then, compared to an effective diffusion
coefficient obtained by pulsed field gradient nuclear mag-
netic resonance (PFG-NMR) spectroscopy for a selected cata-
lytic system. PFG-NMR is a well-established tool to directly
Pt/CPG(4), Pt/CPGIJ10), and Pt/CPGIJ80), respectively, according
to their average pore width. For the investigation of internal
mass-transfer, the catalyst beads of Pt/CPG(4) and Pt/CPG(10)
were crushed and sieved to obtain the grain size fractions of
0–50 μm, 51–71 μm and 72–100 μm for Pt/CPG(4) and of
0–36 μm, 37–50 μm, 51–71 μm, 72–100 and 101–200 μm for
Pt/CPGIJ10), respectively.
2
8–31
measure effective diffusion coefficients,
also applied for
the selective measurement of the diffusivities of the indi-
3
2,33
vidual components during catalytic reactions.
However,
studies conducted under conditions similar to those of cata-
3
4
lytic experiments (temperatures ≥ 373 K) are rare. Thus,
another aim of this study was to directly compare the diffu-
sion coefficient obtained via PFG-NMR with the diffusion
coefficient calculated using the Thiele concept applied to a
catalytic conversion. This presents an attempt to experimen-
tally verify the validity of the Thiele concept for determining
effective diffusion coefficients under conditions of catalytic
conversions.
The characterization via N -sorption was conducted at
2
77 K using a micromeritics ASAP 2010 (for the catalysts
Pt/CPG(4) and Pt/CPGIJ10)) and a Quantachrome Autosorb iQ
(for the catalyst Pt/CPGIJ80)). Hg-intrusion was carried out
using a Thermo Scientific, Pascal 440 Series instrument.
Elemental analysis was achieved via optical emission spec-
trometry with inductively coupled plasma (ICP-OES, after
microwave-assisted digestion of the samples in a mixture of
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
3
3
2
3
.0 cm HF (48%, Roth), 3.0 cm HNO3 (69%, Roth) and
2.3 Pulsed field gradient nuclear magnetic resonance
measurements
3
.0 cm HCl (35%, Roth)) using a Perkin Elmer Optima 8000
instrument. Scanning electron microscopy (SEM) images of
the catalysts were recorded on an E-O-GmbH CamScan CS44.
Transmission electron microscopy (TEM) images were taken
on a Philips CM-200 with an acceleration voltage of 200 kV.
The detection limit for Pt-crystallites of the TEM instrument
used is about 1 nm. The catalyst density was determined
using a micromeritics multivolume pycnometer 1305. X-ray
powder diffraction (XRD) patterns were recorded on a Sie-
mens D5000 diffractometer operating with Cu-Kα radiation
For the pulsed field gradient nuclear magnetic resonance
−1
(
PFG-NMR) experiments, a solution of 1.60 mol l toluene
dissolved in d-14 n-hexane (Merck Millipore, >99% deuter-
ated) was added to the reduced Pt/CPG(10) in a glass ampule.
The amount of the liquid (d-14 n-hexane and toluene) was
chosen to provide complete filling with the capillary-
condensed liquid of the mesopore space only. Therefore, an
excess amount of the d-14 n-hexane–toluene mixture was
added to Pt/CPG(10) into the glass ampule. The ampule was
then shaken and left to equilibrate for 5 min. Afterwards, the
excess solution was removed using a syringe. The inter-
particle space remained filled with the vapor phase. Thereaf-
ter, the glass tube was sealed keeping the bottom part of the
tube containing the catalyst particles at liquid nitrogen
temperature.
(
0.154 nm). The scanning range was 5–80° (2θ) using a step
scan mode with the step of 0.05° (2θ). Hydrogen (Air Liquide,
9.999%) chemisorption experiments were conducted using a
9
Quantachrome Autosorb iQ in the vacuum volumetric mode
at 313 K. The isotherms for the dispersion calculation are
given in the ESI,† Fig. S5 and S6.
1
The H diffusion measurements were performed with an
NMR spectrometer operating at a proton resonance frequency
of 400 MHz, equipped with a home-built pulsed field gradi-
2
.2 Catalytic experiments
The catalytic hydrogenation of aromatics was carried out in
the batch mode using a stainless steel autoclave (Berghof,
BR-200, volume = 200 cm , tetrafluoroethylene-coated) at
−
1 35
ent unit with field gradient amplitudes up to 35 T m . This
means that only toluene molecules were traced in the experi-
ments. For minimizing disturbing effects due to internal field
gradients and eddy currents, the 13-interval stimulated-echo
3
−
1
3
73 K, 1300 min stirring speed and a static hydrogen pres-
sure of 50 bar. As a reactant solution, a mixture of the aro-
36
pulse sequence with bipolar gradients was applied.
−
1
−1
matic (1.60 mol l ) and n-octane (0.19 mol l , as internal
standard for GC analysis, ≥98%, Sigma-Aldrich) dissolved in
n-hexane (≥95%, VWR) was used. As aromatic reactants, tolu-
ene (≥99.5%, Sigma Aldrich), m-xylene (99%, Sigma-Aldrich),
mesitylene (≥98.0, Merck Schuchardt) and 1,3,5-triisopropyl-
benzene (TIPB, 95% Sigma-Aldrich) were used without fur-
ther purification. The catalysts were reduced prior to use in
Typical experimental parameters were δ = 400 μs for the
gradient pulse width, 1.2 ms for the pulse separations within
a pair of bipolar gradient pulses. The observation time, i.e.,
d
the separation t between the two gradient pulse pairs, was
varied between 10 and 100 ms. The primarily measured spin-
echo diffusion attenuation functions, which were obtained by
2
varying the gradient strength g keeping all other parameters
an H
2 2
/N flow as described above. A typical catalytic experi-
constant, were found to have exponential shape. In addition,
ment was performed by heating up the reactant solution
2
when plotted versus IJγδg) t
d
, where γ is the gyromagnetic ratio
3
(75 cm ) and the suspended catalyst (150 mg) in the reaction
for protons, the echo attenuations were found to be indepen-
autoclave up to 373 K. The reaction was, then, started by
applying a static hydrogen pressure of 50 bar. Samples
dent of the observation time t . These two facts revealed that,
d
in this way, no disturbing boundary effects were present dur-
ing measurements and the genuine intraparticle diffusivities
were measured.
3
(0.5 cm ) were taken periodically from the liquid reaction
mixture through a sampling tube and the catalyst was sepa-
rated by filtration. The product composition was analyzed
using a GC (Shimadzu, 14 A) equipped with an FID (separa-
tion column: Macherey-Nagel, Optima 5, length: 30 m, inner
diameter: 0.25 mm, film thickness: 0.25 μm).
3
Results and discussion
3.1 Pt catalysts supported on CPG beads
The conversion of the aromatic reactants Xaromatic was cal-
culated based on the aromatics concentration at time t
Three catalysts containing comparable Pt contents on con-
trolled pore glasses (CPGs) were prepared. The three catalysts
possess comparable specific surface areas, but different pore
widths and, thus, different specific pore volumes as can be
seen from the nitrogen sorption isotherms (Fig. 1). A rela-
tively narrow pore width distribution with a maximum at
4 nm and 10 nm was found for the catalysts Pt/CPG(4) and
Pt/CPGIJ10), respectively (Fig. 2). The pore width distribution
of the catalyst Pt/CPG(80) as determined via Hg-intrusion is
broader with a well pronounced maximum at ca. 80 nm
(Fig. 3). The pore width maximum calculated from the nitrogen
sorption isotherm is, however, somewhat lower, i.e., around
70 nm (Fig. 2). Inaccuracies of the pore size characterization
(caromatic,t) and the initial aromatics concentration at time
0
(caromatic,0) using the equation

caromatic,0  c
aromatic t
,

100%.
Xaromatic

(1)
caromatic,0
The experimental accuracy of the conversion data is ±8%
of the reported value. The carbon balance was closed within
8%. No coke formation or side products were observed.
9
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
of macroporous solids via nitrogen sorption are well known
and are mostly attributed to the capillary condensation occur-
ring close to the saturation pressure of nitrogen. To ensure
comparability of the textural data obtained, Hg-intrusion
measurements were also conducted on the catalysts
Pt/CPG(4) and Pt/CPG(10) (ESI,† Fig. S1 and S2). The data
obtained via Hg-intrusion agree reasonably well with those
from N -sorption. The textural properties of the catalysts are
2
summarized in Table 1. The materials exhibit a spherical
macroscopic shape with a narrow size distributions around
1
5
50–200 μm for the catalysts Pt/CPG(4) and Pt/CPG(10) and
0–100 μm for the catalyst Pt/CPGIJ80), respectively (Fig. 4).
Furthermore, all catalysts possess a similar Pt-loading of
0
Fig. 1 Volume adsorbed (Vads) as a function of relative pressure IJp/p )
from N
2
-sorption for Pt supported on controlled pore glass with
around 2.2 wt.-% (Table 1). No Pt reflections are observed in
the XRD patterns for all catalysts investigated (ESI,† Fig. S3).
Assuming a detection limit of 5 nm for XRD, a Pt dispersion
different pore widths recorded at 77 K (the isotherms are offset by
3
−1
1
00 cm g ).
2
2,37
of >25% is calculated.
In addition, TEM analysis was
conducted for the catalyst Pt/CPG(10) (SEM overview: ESI,†
Fig. S4). However, no Pt particles can be detected by TEM.
Considering the detection limit for Pt-crystallites of the TEM
instrument used (ca. 1 nm), the Pt dispersion is >85%
3
7
assuming ideally spherical particles. These findings are fur-
ther supported by the results from hydrogen chemisorption
for the catalysts Pt/CPG(10) and Pt/CPG(80) (for isotherms
see ESI,† Fig. S5 and S6). Accordingly, a Pt dispersion of 91%
and 87% was determined for Pt/CPG(10) and Pt/CPGIJ80),
respectively. These values correspond to the same Pt dispersion
within experimental accuracy of hydrogen chemisorption.
3.2 Toluene hydrogenation
To investigate for possible mass-transfer limitations, the
CPG-supported Pt catalysts were used in the liquid-phase
hydrogenation of toluene. A clear correlation between the
average pore width (dpore) and the conversion as a function of
time can be seen for the three catalysts (Fig. 5). Thus, the cat-
alyst with the largest average pore width IJPt/CPGIJ80)) shows
the fastest toluene conversion. For the catalysts with the
smaller pore widths, the toluene conversion is slower. As a
measure for the catalyst performance in the hydrogenation
reactions, the initial reaction rate, determined as the slope of
the linear part of the dependence of conversion on time at
the beginning of the experiments, was used. As the linear
dependence of conversion over reaction time extends over
the whole reaction duration studied (up to 200 min, or until
complete conversion was reached), a deactivation of the cata-
lyst during the experiment can be excluded. Since the Pt load-
ing and dispersion are comparable, the higher initial reaction
rate for the catalysts with larger pore widths IJPt/CPGIJ4):
Fig.
2 Relative differential pore volume (Vpore,diff,rel), i.e., the
differential pore volume divided by the maximum pore volume, as
function of pore width (dpore) calculated using the BJH-model on the
desorption branch of the nitrogen sorption isotherm for Pt supported
on controlled pore glass with different pore widths.
−1
−1
−1 −1
0
.032 mmol g s , Pt/CPG(10) 0.060 mmol g
s
and Pt/CPGIJ80):
−
1
−1
0.144 mmol g s ) is an indication for higher internal mass-
transfer rates within the respective catalyst pore systems.
The reaction rate for Pt/CPG(10) can be calculated on
basis of the reaction rate of Pt/CPG(4) using the Thiele con-
cept and assuming internal mass-transfer limitation and
Knudsen diffusion (for detailed calculation see ESI†).
Fig. 3 Relative volume (Vrel) and volume intruded (Vint) as a function
of pore width (dpore) for the Hg-intrusion profile and pore width distri-
bution histogram of Pt/CPGIJ80).
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Table 1 Specific surface area ABET, specific pore volume Vpore, and average pore width dpore determined from N
Paper
2
-sorption at 77 K and from Hg-
intrusion for Pt supported on controlled pore glasses. The platinum content from elemental analysis via ICP-OES is also included
2
−1
3
−1
Sample
A
BET/IJm g
)
V
pore/IJcm g
)
dpore/nm
Pt-content/wt.-%
a
a
Pt/CPG(4)
Pt/CPG(10)
Pt/CPG(80)
95
130
80
0.2
0.4
1.3
4
10
2.1
2.2
2.4
b
60–90
a
b
Determined from N
2
-sorption. Determined from Hg-intrusion.
This calculated reaction rate value for Pt/CPG(10)
3.2.1 Influence of the catalyst grain size. In order to inves-
tigate the role of internal mass-transfer limitation in more
detail, a variation of the average grain size was performed for
the catalysts Pt/CPG(4) and Pt/CPGIJ10). Upon increasing the
average grain size, the observed initial reaction rate decreases
for both catalysts (Fig. 6). This again indicates the occurrence
of internal mass-transfer limitation, since the diffusion path-
way is longer in larger catalyst grains. Note that the reaction
is apparently limited by internal mass transfer over the whole
grain size range studied. Furthermore, for average grain sizes
smaller than 25 μm, the initial reaction rates for Pt/CPG(4)
and Pt/CPG(10) are comparable. This can be understood,
since at this average grain size the influence of the internal
mass-transfer on the reaction rate is negligible and, thus, the
intrinsic reaction on the Pt-particles dominates the observed
initial rate and is independent of mass transfer. Because of
the similar Pt-content and -dispersion, these intrinsic reac-
tion rates are expected to be similar for both catalysts. On
the other hand, the comparable reaction rates over the two
catalysts can be seen as an indication for a comparable local
distribution of the Pt-particles inside the catalyst grains. Note
also that the slope of the rate decrease with increasing aver-
age grain size is steeper for the catalyst Pt/CPG(4) with the
smaller pore width than for Pt/CPGIJ10). This is, again, con-
sistent with the expected limitation of the observed reaction
rate by mass transfer within the catalyst pores.
−
1 −1
(
0.051 mmol g s ) is in good agreement with the one deter-
−
1
−1
mined experimentally (0.060 mmol g s ). Evidently, the
assumption of mass-transfer limitation in case of Pt/CPG(4)
and Pt/CPG(10) for the toluene hydrogenation is valid. Note,
however, that Knudsen diffusion is usually related to mass-
transfer in porous solids with a surrounding gas-phase. It
apparently describes the diffusion under the liquid-phase
conditions of the present reaction systems equally well.
The absence of external mass-transfer limitations, i.e., hin-
dered mass-transfer of hydrogen from the gas into the liquid
phase, inside the bulk liquid phase and from the liquid
phase through the boundary layer to the outer catalyst sur-
face, was investigated in a separate series of experiments
varying the stirring speed (see ESI,† Fig. S7). At values above
−
1
6
00 min , the initial reaction rate of toluene was indepen-
dent of the stirring rate proving the absence of external
mass-transfer limitations. Consequently, all further conver-
−
1
sions were carried out at a stirring speed of 1300 min to
exclude limitations by external mass transfer.
Fig. 5 Toluene conversion XToluene as a function of reaction time (t)
for the hydrogenation of toluene using Pt supported on controlled
pore glasses with different pore widths (T = 373 K, pH2 = 50 bar,
−
1
3
Fig. 4 SEM-micrographs of the catalysts Pt/CPG(4) (top), Pt/CPG(10)
ctoluene = 1.60 mol l , solvent: n-hexane (V = 75 cm ), mcat = 150 mg,
−1
(
middle) and Pt/CPG(80) (bottom).
stirring rate = 1300 min ).
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
dgrain,1
dgrain,2
1

2.
(5)
(6)
Substituting eqn (5) in eqn (3) results in
r d
 coth 1
0
,2 grain,2
2
2

.
r d
d
grain,1
d
grain,1
0
,1 grain,1
2
coth
2 1
dgrain,2
dgrain,2
This equation can be numerically solved to obtain the
Thiele modulus θ . θ can then be calculated from eqn (5).
2
1
0
Fig. 6 Initial reaction rate (r ) as a function of average grain size
(
d
grain
)
for the hydrogenation of toluene using Pt/CPG(4) and
The effectiveness factor η and the effective diffusion coeffi-
cient Deff are calculated using eqn (2) (ρ = 2.1 g cm ).
−1
−3
Pt/CPG(10) (T = 373 K, pH2 = 50 bar, ctoluene = 1.60 mol l , solvent:
n-hexane (V = 75 cm ), mcat = 150 mg, stirring rate = 1300 min ).
3
−1
As Fig. 7 shows, an effectiveness factor of about 90% is
reached for both catalysts Pt/CPG(4) and Pt/CPGIJ10), when
the average grain size is below 25 μm. The differently strong
influence of the mass-transfer limitation on the conversion
over the catalysts Pt/CPG(4) and Pt/CPG(10) is indicated by
the different slopes of the decrease of the effectiveness factor
with average grain size. This decrease is more pronounced
for Pt/CPG(4) than for Pt/CPGIJ10), consistent with the earlier
conclusion drawn from the dependence of the initial reaction
rate of average grain size (see Fig. 6). Consequently, the effec-
3
.2.2 Calculation of Thiele modulus and effectiveness factor.
Based on the results of the grain size variation (Fig. 6), the
Thiele modulus and the effectiveness factor for each catalyst
were calculated according to the literature. Based on the
Weisz–Prater-Criterion (CWP), the Thiele modulus (θ) and the
effectiveness factor (η) can be put in relation with the
3
8
observed initial reaction rate r according to
0
tive diffusion coefficients calculated for Pt/CPG(4) (Deff
=
2
−10
2
−1
r0 d
0.8 × 10
m s ) is about three times lower than that of
2
grain
CWP  
 3

 coth 1

,
(2)
−10
2 −1
Pt/CPG(10) (Deff = 2.8 × 10 m s ).
D c
eff surf
3
.3 Hydrogenation of alkyl substituted benzene derivatives
where ρ is the catalyst density, dgrain is the grain size, Deff is
the effective diffusion coefficient and csurf the toluene con-
centration at the outer catalyst surface. If no external mass-
transfer limitations occur (as proven by the stirring speed
variation, cf. section 3.2 and ESI,† Fig. S7), the toluene con-
centration on the outer surface of the catalyst (csurf) can be
assumed to be equal to the bulk concentration, i.e.,
As another important parameter of the mass-transport inside
a pore network, the dimension (size) of the reactant molecule
needs to be considered. Therefore, the catalytic hydrogena-
tion of alkyl substituted benzene derivatives with different
critical diameters dcrit was studied, i.e., methyl-benzene
−
1
1
.60 mol l . If eqn (2) is applied to two catalytic experiments
with the subscripts 1 and 2), in which only the grain size of
the catalyst is varied, the ratio of these two equations is
(
r d
 coth 1
2 2
0
,2 grain,2

.
(3)
r d
 coth 1
1 1
0
,1 grain,1
Note that the terms csurf, ρ, Deff cancel because they are
assumed to be identical for two catalytic experiments under
3
8
the same conditions. According to the literature, the Thiele
modulus (with rint as the intrinsic, i.e., not mass-transfer lim-
ited reaction rate) can be defined as
r 
int

 dgrain
.
(4)
D c
Fig. 7 Effectiveness factor (η) and Thiele modulus (θ) as a function of
average grain size (dgrain) for the hydrogenation of toluene using
eff surf
−1
Pt/CPG(4) and Pt/CPG(10) (T = 373 K, pH2 = 50 bar, ctoluene = 1.60 mol l ,
Thus, for the ratio of the Thiele moduli for the two experi-
ments, we obtain
3
solvent: n-hexane (V = 75 cm ), mcat = 150 mg, stirring rate =
−
1
1300 min ).
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
3
9
(
toluene, dcrit = 0.67 nm ), 1,3-dimethylbenzene (m-xylene,
3
9
dcrit = 0.74 nm ), 1,3,5-trimethylbenzene (mesitylene, d
.82 nm ) and 1,3,5-triisopropylbenzene (TIPB, d
.95 nm ) (reaction scheme: Fig. 8). The critical diameter
=
=
crit
4
0
0
0
crit
4
1
dcrit is defined as the smallest cylindrical orifice through
4
2
which a molecule can pass.
The results of the hydrogenation of these reactants using
the catalysts Pt/CPG(10) and Pt/CPG(80) are displayed in
Fig. 9. As already discussed above (cf. Fig. 5), the hydrogena-
tion of toluene proceeds with an about three-fold higher ini-
tial reaction rate when using the catalyst with the larger aver-
age pore width of 80 nm IJPt/CPGIJ80)) instead of the one with
1
0 nm pore width IJPt/CPGIJ10)). This higher initial reaction
rate was attributed to faster internal mass transfer within the
Pt/CPG(80) catalyst compared to the catalyst Pt/CPGIJ10).
Moreover, the initial reaction rate for toluene hydrogenation
over Pt/CPG(80) is essentially the same as that for the cata-
lysts with the smaller pore widths, i.e., Pt/CPG(10) and
Pt/CPG(4), and an average grain size of 25 μm (Fig. 6, ca.
Fig. 9 Initial reaction rate (r
m-xylene, mesitylene and 1,3,5-triisopropyl benzene as a function of
their critical molecule diameter (dcrit) over the catalysts Pt/CPG(80)
0
) for the hydrogenation of toluene,
and Pt/CPGIJ10), respectively (T = 373 K, pH2 = 50 bar, caromatic
=
−
1
3
1
.60 mol l , solvent: n-hexane (V = 75 cm ), mcat = 150 mg, stirring
−1
rate = 1300 min ).
−
1
−1
0
.15 mmol g s ). This indicates that the reaction rate of
toluene is not limited by internal mass transfer on Pt/CPGIJ80).
As mentioned above, the similar reaction rates provide fur-
ther evidence for a comparable local distribution of the Pt-
particles inside the catalyst grains. Also for the other alkyl
substituted benzenes, the initial reaction rate is generally
higher by a factor of 2 to 4 over Pt/CPG(80) than over
Pt/CPGIJ10). Note, however, that the pore width of both cata-
lysts is larger by at least on order of magnitude than the criti-
cal molecule diameter of all reactants studied.
both catalysts, with the initial rate for TIPB conversion over
Pt/CPG(80) being the only exception. This clearly reflects the
growing impact of mass transfer on the observed reaction
rate with increasing critical molecule diameter. For toluene
and m-xylene no, or only small mass-transfer limitations are
expected. With increasing critical molecule diameter, the
influence of mass transfer becomes more pronounced,
severely limiting the initial reaction rate. Interestingly, the
slopes for the linear decrease of the initial reaction rate with
increasing critical molecule diameter is similar for both cata-
lysts (Fig. 9). This indicates that the same mode of mass
transfer takes place in the pores of both catalysts. It is, how-
ever, surprising that the slope is not steeper for the catalyst
with the smaller pores IJPt/CPGIJ10)) which might be expected
for stronger limitations of the mass transfer within the grains
of this catalyst. A possible explanation for this finding is that
the initial reaction rates for Pt/CPG(10) are closer to the
threshold reaction rate for an effectiveness factor (η) → 0 cor-
responding to the conversion taking place at the outer sur-
face of the catalyst only. Thus, the reaction rate is less
strongly affected by slower mass-transport of the bulkier
molecules.
The decrease in initial reaction rate with increasing criti-
cal molecule diameter of the reactant is almost linear for
When considering the decrease of the initial reaction rate
of the alkyl substituted benzenes with increasing critical
diameter of the reactant molecules, the intrinsic reaction rate
needs to be taken into account. The adsorption of aromatics
on noble metal surfaces is known to occur via π-bonding
involving electron transfer from the aromatic ring to the
4
3
unoccupied d-orbitals of the noble metal atoms. Since the
π-electron density in aromatics increases with increasing
degree of alkyl-substitution, the aromatic reactants are
expected to bind more strongly. This was confirmed by
4
4
Gallezot et al. for the adsorption of benzene and toluene on
4
5
Pt(111) surfaces. Smith et al. observed similar reaction rates
for the hydrogenation of toluene, m-xylene and mesitylene
Fig. 8 Reaction scheme for the hydrogenation of alkyl substituted
benzenes over Pt supported on controlled pore glasses (Pt/CPG).
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
using a platinum catalysts under similar reaction conditions
genuine pore space close to the surface and the pore space
concentration in equilibrium with the bulk liquid-phase reac-
tant concentration. The relevance of these surface perme-
(
liquid phase, solvent: acetic acid, aromatics concentration
−
1
49
0
.2 mol l , 303 K). However, it is to be expected that a higher
reaction rate results for more electron rich aromatics, i.e.,
with higher degree of alkylation. Thus, the decreasing reac-
tion rates with increasing molecular diameter of the aromatic
reactants as observed here (Fig. 8) are in line with an increas-
ing mass-transfer limitation within the catalysts pores.
abilities concerning mass-transfer processes was proven in
5
0,51
different studies
review.
and was recently summarized in a
4
9
When Deff is determined on the basis of the Thiele con-
cept, it is assumed, that the deviation of the observed initial
reaction rate from the intrinsic reaction rate is only based on
the slower rate of mass-transfer through the pore system with
respect to the reaction at the catalytically active sites. If, how-
ever, the surface permeability of the investigated system is
low, i.e., if mass transfer through the surface (not the exter-
nal boundary layer, cf. section 3.2 and ESI,† Fig. S7) is hin-
dered, the observed reaction rate will also be decreased. The
Thiele concept does not take into account surface permeabil-
ities, and, thus, the decrease in the initial reaction rate
observed is only attributed to a slower mass-transfer inside
the pore system. This results in an underestimation of the
diffusion coefficient. However, in PFG-NMR, the experimental
conditions (observation time and grain size) are chosen so
that the diffusion of the molecules of interest are only
observed within the pore system and not entering or leaving
the pore system. As a consequence, the Deff determined by
PFG-NMR will not be falsified by surface permeability effects.
It can furthermore be concluded that, although the Thiele
modulus can be applied to obtain an estimate of Deff, even if
the surface permeability of an investigated catalyst is low, the
3
.4 Pulsed field gradient nuclear magnetic resonance
For the catalyst Pt/CPGIJ10), the effective diffusion coefficient
was determined using PFG-NMR at temperatures between
3
13 K and 373 K (Fig. 10). For 373 K, the temperature of the
catalytic experiments (cf. above), the effective diffusion coeffi-
cient D determined via PFG-NMR amounts to 35 ×
eff
2
−
10
−1
1
0
m s . This is about one order of magnitude larger
than the effective diffusion coefficient calculated using the
Thiele concept from the initial catalytic reaction rates (2.9 ×
−
10
2
−1
1
0
m s ). The difference in D may be attributed to the
eff
fact, that the PFG-NMR measurement was, due to experimen-
tal reasons, not conducted under a static hydrogen pressure
of 50 bar as applied in the catalytic experiments. Because of
4
6
the low solubility of hydrogen in n-hexane, the static pres-
sure of 50 bar mainly results in a compression and, thus, a
density increase of the liquid n-hexane. However, the density
of toluene increases only by 1% upon a pressure increase of
4
7
5
0 bar. In addition, toluene can be assumed to be a hard
4
8
sphere fluid, the diffusivity and density of which are indi-
rectly proportional. Thus, a tenfold decrease of Deff cannot be
explained by the increased pressure.
D
eff calculated using the Thiele concept will always be lower
than the real value.
Consequently, the higher values of the effective diffusiv-
ities determined by PFG-NMR might be related to the exis-
tence of an additional transport resistance at the particle
surface (“surface barriers”), giving rise to a finite surface per-
meability. The surface permeability is defined as the factor of
proportionality between the flux through the surface and the
concentration difference between actual concentration in the
4
Conclusions
Mass-transfer effects in liquid-phase hydrogenation reactions
of alkyl substituted aromatics were investigated over Pt cata-
lysts supported on controlled pore glasses (CPGs) as suitable
model supports. These supports are available as monoliths
with adjustable pore widths in the range of 1–1000 nm. For
three different CPGs with spherical particles of 80 and
200 μm diameter and pore widths between 4 and 80 nm as
well as a Pt-loading of 2 wt.-%, it was shown that aromatics
hydrogenation under typical conditions can be mass-transfer
limited, even for larger mesopores around 80 nm.
While for catalyst particles with 25 μm diameter (and for
80 nm pore width), toluene conversion occurrs with the
intrinsic (kinetic) reaction rate, the conversion of reactants
with bulkier molecules such as m-xylene or mesitylene, is
evidently hindered by internal mass-transfer within the cata-
lyst mesopores. Evidently, an average pore width of at least
8
0 nm is needed to overcome internal mass-transfer limita-
tions in the liquid-phase hydrogenation of toluene (dcrit
.61 nm), corresponding to a ratio of average pore width to
=
0
critical molecule diameter of about 130. This knowledge may
serve as a guide for a rational design of mass-transfer opti-
mized hierarchical hydrogenation catalysts in the future.
Fig. 10 Effective diffusion coefficients (Deff) determined using PFG-
NMR for the catalyst Pt/CPG(10) at temperatures (T) between 313 and
373 K.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Catalysis Science & Technology
Paper
Moreover, a particle size reduction may be applied to
avoid mass-transfer limitations. Thus, for catalysts with
mesopores of 10 nm, to achieve effectiveness factors ≥0.9 in
toluene hydrogenation, the particle size would have to be
below 25 μm, which is in the range of fine powders. If cata-
lyst particles of larger diameters are to be applied, the only
option for prohibiting mass-transfer-limited conditions for
liquid-phase hydrogenations is, thus, to introduce sufficiently
large pores, ideally with widths of 100 nm or beyond.
14 C. H. Christensen, K. Johannsen, E. Toernqvist, I. Schmidt,
H. Topsoe and C. H. Christensen, Catal. Today, 2007, 128,
117.
15 M. S. Holm, E. Taarning, K. Egeblad and C. H. Christensen,
Catal. Today, 2011, 168, 3.
16 J. Perez-Ramirez, C. H. Christensen, K. Egeblad, C. H.
Christensen and J. C. Groen, Chem. Soc. Rev., 2008, 37, 2530.
17 B. Y. Liu, L. M. Zheng, Z. H. Zhu, C. Li, H. X. Xi and Y. Qian,
Appl. Catal., A, 2014, 470, 412.
The effective diffusion coefficient of toluene for the cata-
lysts with 4 and 10 nm pore width was determined from the
initial reaction rates using the Thiele concept. The diffusion
coefficient obtained from PFG-NMR is, however, an order of
magnitude larger. Apparently, the Thiele concept is too sim-
plified to assess the intraparticle transport of the reactants
only. Rather, additional transport hindrances such as surface
barriers may affect the diffusivity determination via the
Thiele concept. It presents a challenge for further studies to
identify more precisely these additional phenomena influenc-
ing diffusivity determination under conditions of catalytic
reactions.
18 J. J. Zheng, Q. H. Zeng, Y. M. Yi, Y. Wang, J. H. Ma, B. Qin,
X. W. Zhang, W. F. Sun and R. F. Li, Catal. Today, 2011, 168,
124.
19 D. J. Wang, Z. N. Liu, H. Wang, Z. K. Xie and Y. Tang,
Microporous Mesoporous Mater., 2010, 132, 428.
20 P. A. Rautanen, J. R. Aittamaa and K. O. I. Krause, Ind. Eng.
Chem. Res., 2000, 39, 4032.
21 N. Wilde, C. Worch, W. Suprun and R. Gläser, Microporous
Mesoporous Mater., 2012, 164, 182.
22 M. Goepel, M. Al-Naji, P. With, G. Wagner, O. Oeckler, D.
Enke and R. Gläser, Chem. Eng. Technol., 2014, 37, 551.
23 M. Peer, A. Qajar, B. P. M. Holbrook, R. Rajagopalan and
H. C. Foley, Carbon, 2013, 57, 485.
2
4 B. P. M. Holbrook, R. Rajagopalan, K. Dronvajjala, Y. K.
Choudhary and H. C. Foley, J. Mol. Catal. A: Chem.,
2013, 367, 61.
Acknowledgements
The authors would like to thank the “Deutsche
Forschungsgemeinschaft” for funding within the Interna-
tional Research Training Group GRK 1056 “Diffusion in
Porous Materials”.
25 D. Enke, F. Janowski and W. Schwieger, Microporous
Mesoporous Mater., 2003, 60, 19.
26 A. Stolle, C. Schmoger, B. Ondruschka, W. Bonrath, T. F.
Keller and K. D. Jandt, Cuihua Xuebao, 2011, 32, 1312.
2
7 T. Munkelt, C. Kuster, C. Hamel, D. Enke and A. Seidel-
References
Morgenstern, Chem. Ing. Tech., 2013, 85, 1686.
1
2
C. S. Song, Catal. Today, 2003, 86, 211.
H. U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner and
M. Studer, Adv. Synth. Catal., 2003, 345, 103.
M. C. Willis, Chem. Rev., 2010, 110, 725.
A. D. Sutton, F. D. Waldie, R. L. Wu, M. Schlaf, L. A. Silks
and J. C. Gordon, Nat. Chem., 2013, 5, 544.
H. Bonnemann, W. Brijoux, A. S. Tilling and K. Siepen, Top.
Catal., 1997, 4, 217.
X. B. Lou, L. He, Y. Qian, Y. M. Liu, Y. Cao and K. N. Fan,
Adv. Synth. Catal., 2011, 353, 281.
B. Chen, U. Dingerdissen, J. G. E. Krauter, H. G. J. L.
Rotgerink, K. Mobus, D. J. Ostgard, P. Panster, T. H.
Riermeier, S. Seebald, T. Tacke and H. Trauthwein, Appl.
Catal., A, 2005, 280, 17.
28 D. Mehlhorn, R. Valiullin, J. Kärger, K. Cho and R. Ryoo,
Materials, 2012, 5, 699.
29 C. Chmelik and J. Karger, Chem. Soc. Rev., 2010, 39, 4864.
30 J. Kärger, D. M. Ruthven and D. N. Theodorou, Diffusion in
Nanoporous Materials, Wiley - VCH, Weinheim, 2012.
31 J. Kärger and R. Valiullin, Chem. Soc. Rev., 2013, 42, 4172.
32 U. Hong, J. Kärger, B. Hunger, N. N. Feoktistova and S. P.
Zhdanov, J. Catal., 1992, 137, 243.
33 H. B. Schwarz, S. Ernst, J. Kärger, B. Knorr, G. Seiffert, R. Q.
Snurr, B. Staudte and J. Weitkamp, J. Catal., 1997, 167, 248.
34 W. Heink, J. Karger, H. Pfeifer, K. P. Datema and A. K.
Nowak, J. Chem. Soc., Faraday Trans., 1992, 88, 3505.
35 P. Galvosas, F. Stallmach, G. Seiffert, J. Kärger, U. Kaess and
G. Majer, J. Magn. Reson., 2001, 151, 260.
3
4
5
6
7
8
9
S. Tadepalli and A. Lawal, Int. J. Chem. React. Eng., 2008, 6.
S. M. Rao and M. O. Coppens, J. Phys. Chem. C, 2012, 116,
36 D. H. Wu, A. D. Chen and C. S. Johnson, J. Magn. Reson.,
Ser. A, 1995, 115, 260.
2
6816.
0 G. Wang and M. O. Coppens, Ind. Eng. Chem. Res., 2008, 47,
847.
37 P. Gallezot and G. Bergeret, in Handbook of Heterogenous
Catalysis, ed. G. Ertl, H. Knözinger, F. Schüth and J.
Weitkamp, Wiley-VCH, Weinheim, Germany, 2008.
38 Elements of Chemical Reaction Engineering, ed. H. S. Fogler,
Prentice Hall PTR, Westford, 2006, 4th edn.
1
1
1
1
3
1 G. Wang, E. Johannessen, C. R. Kleijn, S. W. de Leeuwa and
M. O. Coppens, Chem. Eng. Sci., 2007, 62, 5110.
2 T. D. Tang, C. Y. Yin, L. F. Wang, Y. Y. Ji and F. S. Xiao,
J. Catal., 2007, 249, 111.
39 V. R. Choudhary, V. S. Nayak and T. V. Choudhary, Ind. Eng.
Chem. Res., 1997, 36, 1812.
3 C. H. Christensen, K. Johannsen, I. Schmidt and C. H.
Christensen, J. Am. Chem. Soc., 2003, 125, 13370.
40 C. E. Webster, R. S. Drago and M. C. Zerner, J. Phys. Chem.
B, 1999, 103, 1242.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
4
4
1 S. Al-Khattaf and H. de Lasa, Appl. Catal., A, 2002, 226, 139.
2 S. F. Zaman, K. F. Loughlin and S. S. Al-Khattaf, Ind. Eng.
Chem. Res., 2005, 44, 2027.
47 M. O. McLinden and J. D. Splett, J. Res. Natl. Inst. Stand.
Technol., 2008, 113, 29.
48 C. K. J. Sun and S. H. Chen, AIChE J., 1985, 31, 1510.
49 J. Kärger, Microporous Mesoporous Mater., 2014, 189, 126.
50 D. Tzoulaki, L. Heinke, H. Lim, J. Li, D. Olson, J. Caro, R.
Krishna, C. Chmelik and J. Kärger, Angew. Chem., Int. Ed.,
2009, 48, 3525.
51 F. Hibbe, J. Caro, C. Chmelik, A. S. Huang, T. Kirchner, D.
Ruthven, R. Valiullin and J. Kärger, J. Am. Chem. Soc.,
2012, 134, 7725.
4
4
3 J. J. Rooney, J. Mol. Catal., 1985, 31, 147.
4 J. Massardier, J. C. Bertolini, T. M. Tri, P. Gallezot and B.
Imelik, Bull. Soc. Chim. Fr., 1985, 333.
4
4
5 C. P. Rader and H. A. Smith, J. Am. Chem. Soc., 1962, 84,
1
443.
6 W. Z. Gao, R. L. Robinson and K. A. M. Gasem, J. Chem. Eng.
Data, 2001, 46, 609.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2015