Bifunctional mesoporous organic-inorganic hybrid silica for combined one-step hydrogenation/esterification

Article abstract of DOI:10.1016/j.apcata.2010.01.015

Bifunctional mesoporous organic-inorganic hybrid silicas with platinum and propylsulfonic acid group (Pt/SBA15-PrSO₃H) were synthesized and tested for the combined one-step hydrogenation/esterification (OHE) reaction, which was employed as a model reaction for catalytic upgrading of biomass-derived bio-oil. The model reagents used were acetic acid and acetaldehyde. Different catalyst synthesis procedures were investigated and compared by varying the functional group incorporation method, platinum loading, reducing agent, etc. The textural and chemical properties of the catalysts made by the different synthesis procedures were characterized and compared with reactivity results. The need to create the organic acid sites prior to platinum incorporation was demonstrated. The efficacy of the bifunctional catalyst system for combined hydrogenation/esterification was demonstrated. Interestingly, the bifunctional Pt/SBA15-PrSO₃H catalyst exhibited superior esterification activity with about twice the acetic acid turnover number relative to that with the monofunctional SBA15-PrSO₃H catalyst. By combining metallic Pt nanoparticles with strong acid sites, this bifunctional mesoporous hybrid catalyst improved OHE activity and, therefore, has potential for application in the catalytic upgrading of bio-oil.

Full text of DOI:10.1016/j.apcata.2010.01.015

Applied Catalysis A: General 375 (2010) 310–317
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Bifunctional mesoporous organic–inorganic hybrid silica for combined one-step
hydrogenation/esterification
a
Yang Tang a, Shaojun Miao b, Brent H. Shanks b, , Xiaoming Zheng
*
a Key Lab of Applied Chemistry of Zhejiang Province, Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China
b Department of Chemical and Biological Engineering, Iowa State University, 2119 Sweeney Hall, Ames, IA 50011, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Bifunctional mesoporous organic–inorganic hybrid silicas with platinum and propylsulfonic acid group
(Pt/SBA15-PrSO3H) were synthesized and tested for the combined one-step hydrogenation/esterification
(OHE) reaction, which was employed as a model reaction for catalytic upgrading of biomass-derived bio-
oil. The model reagents used were acetic acid and acetaldehyde. Different catalyst synthesis procedures
were investigated and compared by varying the functional group incorporation method, platinum
loading, reducing agent, etc. The textural and chemical properties of the catalysts made by the different
synthesis procedures were characterized and compared with reactivity results. The need to create the
organic acid sites prior to platinum incorporation was demonstrated. The efficacy of the bifunctional
catalyst system for combined hydrogenation/esterification was demonstrated. Interestingly, the
bifunctional Pt/SBA15-PrSO3H catalyst exhibited superior esterification activity with about twice the
acetic acid turnover number relative to that with the monofunctional SBA15-PrSO3H catalyst. By
combining metallic Pt nanoparticles with strong acid sites, this bifunctional mesoporous hybrid catalyst
improved OHE activity and, therefore, has potential for application in the catalytic upgrading of bio-oil.
ß 2010 Elsevier B.V. All rights reserved.
Received 2 October 2009
Received in revised form 29 December 2009
Accepted 10 January 2010
Available online 15 January 2010
Keywords:
Bifunctional catalyst
Mesoporous organic–inorganic hybrid silica
Propylsulfonic acid
Platinum
One-step hydrogenation/esterification
Bio-oil
Catalytic upgrading
1. Introduction
ed previously that the combination of hydrogenation and
esterification into one-step is feasible and may be an effective
Bio-oil as produced from the fast pyrolysis of biomass is
acknowledged as a promising energy alternative to nonrenewable
crude oil-based fuels [1,2]. However, there are a number of
technological challenges to the efficient use of bio-oil as a
transportation fuel resulting from the properties and complexity
of its chemical constituents [3,4]. One of these is the corrosive
nature of the bio-oil due to the high content of organic acids and
another is its reactive nature due to the presence of many
unsaturated components, such as aldehydes [1]. Upgrading of the
bio-oil is crucial for its application, and a number of catalytic
approaches have been investigated [4]. The most desirable
approach is to remove most of the oxygen atoms and form
molecules with improved combustion properties [5]. The key
reactions used individually or in combination to reach this goal
include hydrogenation, hydrodeoxygenation, cracking, reforming,
dehydration, isomerization, aldol condensation, oxidation and
esterification [6–12]. Among these reactions, hydrogenation and
esterification are important to stabilize the corrosive and reactive
components of bio-oil, which are beneficial to the storage,
transportation and combustion of bio-oil. It has been demonstrat-
approach for bio-oil upgrading by stabilizing two of the primary
reactive components, aldehydes and organic acids, simultaneously
[13].
For hydrogenation reactions, many supported metal catalysts,
such as ruthenium, rhodium, palladium, platinum, nickel, and
cobalt have been reported in the literature [11,14–16]. Among
them, platinum exhibits exceptional hydrogenation activity as
most functional groups can be hydrogenated under relatively
milder conditions over platinum than with other metals [14].
Additionally, platinum has been shown to be more favorable for
the hydrogenation of C55O groups than other metals, e.g.,
selectivities of as high as 98% and yields of 90–100% in the
hydrogenation of saturated or unsaturated aldehydes to the
corresponding alcohols [14,17]. Coupling platinum with a high
surface area support yields excellent catalysts that have been
commonly used in hydrogenation reactions, such as aldehyde
hydrogenation to alcohol [14].
In contrast, esterification reactions are generally acid-catalyzed
with homogeneous mineral acids, such as H2SO4 and HCl, being
commonly used [18]. The disadvantages of these catalysts are their
miscibility with the reaction medium, which causes separation
problems and potential equipment corrosion when used at higher
concentrations [18]. Therefore, heterogeneous solid acid catalysts
are more desirable, such as zeolites, zirconium sulfate, resins,
* Corresponding author. Tel.: +1 515 294 1895; fax: +1 515 294 2689.
E-mail address: bshanks@iastate.edu (B.H. Shanks).
0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.01.015

Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
311
sulfonated carbon composites and mesoporous silicas functiona-
lized with organic acid groups [18–28]. Zeolites have been widely
applied [22], but they are limited to reactions involving small
molecules in the microporous range with limited accessibility for
large reactants, such as those possible with biorenewables. Due to
their high surface area and tunable pore sizes, mesoporous
organic–inorganic hybrid silicas synthesized by supramolecular
assembly have received increasing attention [24,25]. The incorpo-
ration of organosulfonic acid groups onto mesoporous materials
has been successfully applied in strong acid-catalyzed reactions,
such as esterification [26,27]. In our previous work, organosulfonic
acid-functionalized mesoporous silica (SBA15-PrSO3H) was used
in the esterification of acetic acid with methanol and it exhibited
good catalytic activity with as high as 70% acetic acid conversion at
50 8C in 500 min [26].
In this work, the attributes of supported platinum for
hydrogenation and organosulfonic acid groups for esterification
were combined on mesoporous silicas to catalyze the one-step
hydrogenation/esterification (OHE) reaction, which has potential
application in the upgrading of bio-oil derived from biomass fast
pyrolysis. Catalyst synthesis procedures were investigated and
evaluated for their performance as bifunctional organic–inorganic
hybrid catalysts in the OHE reaction.
2.1.1. Synthesis of Pt(X)/SBA15-PrSO3H (R2)
This process consisted of two steps including first condensing
the propylsulfonic acid groups during the synthesis of SBA15
followed by loading Pt through reduction and deposition in the
aqueous phase. The first step was the same as the synthesis of
SBA15-PrSO3H described previously [28]. For the second step, a
complex reducing agent was synthesized by mixing ethylene
glycol and sodium borohydride at a mole ratio of 1:0.1 at room
temperature for 1 h, as discussed by Hwang et al. [29]. The
reducing agent (10 ml) was added dropwise at 60 8C into the
aqueous suspension of SBA15-PrSO3H and PtCl4 with vigorous
stirring (1 g SBA15-PrSO3H with the corresponding amount of Pt in
30 ml of de-ionized water). The suspension was stirred for 24 h at
60 8C during which its color changed from yellow to black
indicating the reduction of Pt4+ to Pt0 [29,30]. After cooling to
room temperature, the suspension was filtered and washed with
de-ionized water. The wet sample was re-suspended in 1 wt%
H2SO4 (ꢁ0.2 M, 50 ml) for 4 h for acidification followed by filtering,
washing and vacuum drying.
2.1.2. Synthesis of Pt(X)/SBA15-PrSO3H (C2, SO3H)
This procedure was the same as above except the Pt was
supported on the organic–inorganic hybrid material using a Pt
colloid rather than using reduction and deposition. A Pt colloid was
prepared as described previously [30,31]. Typically, 0.1 ml of the
PtCl4 aqueous solution was mixed with 10 ml of ethylene glycol,
which produced a yellowish solution. Then the temperature of the
solution was raised to 150 8C accompanied with the color change
from yellow to black indicating the formation of metallic Pt colloid.
The colloid solution was cooled to room temperature and then was
added to the aqueous suspension of SBA15-PrSO3H (1 g SBA15-
PrSO3H with the corresponding amount of Pt) with stirring for 24 h
at 40 8C, followed by filtering, washing, acidification and vacuum
drying.
2. Experimental
2.1. Catalyst preparation
The catalysts in this work were organic–inorganic hybrid
SBA15 materials, which were contained both platinum and
propylsulfonic acid groups. Tetraethoxysilane (TEOS, 98%,
Aldrich) was used as the silica precursor, and (3-mercaptopro-
pyl)trimethoxysilane (MPTMS, 85%, Acros) was used without
further purification as the organosulfonic acid source. Pluronic
P123 (BASF),
a triblock copolymer of polyethylene oxide–
2.1.3. Synthesis of Pt(X)/SBA15-PrSO3H (C2, SH)
polypropylene oxide–polyethylene oxide with the molecular
structure PEO20–PPO70–PEO20 (Mw = 5800), was used as pur-
chased to tailor the textural properties of the mesoporous
materials. An aqueous solution of platinum(IV) chloride (PtCl4,
Acros, 0.01 g Pt/ml) was used as the platinum precursor. Sodium
borohydride (NaBH4, Fisher) with ethylene glycol ((CH2OH)2, 99%,
Acros), hydrazine monohydrate (N2H4ꢀH2O, Fisher), and formal-
dehyde (HCHO, 37% by weight, aqueous solution, Fisher) were
used as reducing agents.
As shown in Fig. 1, the different synthesis procedures were
classified according to the treatment for platinum, which were also
be divided into either a one-step or two-step procedure in terms of
incorporating the platinum and propylsulfonic acid jointly or
separately. X represented the nominal weight percent of platinum
in the catalysts. The molar ratio of organosulfonic acid precursor/
TEOS was kept at 10% for all the samples in our study.
In this two-step procedure, propylthiol groups were incorpo-
rated onto SBA15 using the co-condensation method, which was
the same as the preparation of SBA15-PrSO3H except without
addition of H2O2. The second step was the deposition of the Pt
colloid prepared as above in an aqueous suspension of SBA15-
PrSH, with stirring for 24 h at 40 8C. Following filtering and
washing, the oxidation of PrSH to PrSO3H was performed by
mixing the sample with 8 g H2O2 (30%, Fisher), 30 ml methanol
(Fisher) and 30 ml 1.9 M HCl and stirring for 24 h at 40 8C. The
sample was then acidified and vacuum dried.
2.1.4. Synthesis of Pt(X)/SBA15-PrSO3H (C2, Pt)
For this two-step procedure, platinum was supported during
the synthesis of SBA15 and the propylsulfonic acid groups were
post-grafted. The Pt colloid prepared as discussed above was added
to a solution of HCl (1.9 M), P123 and TEOS during the preparation
of SBA15 to synthesize Pt/SBA15 in the first step. The resulted
sample was treated by removing the template with ethanol reflux,
washing and drying. Secondly, MPTMS was post-grafted (added to
a toluene suspension of Pt/SBA15 with stirring and reflux for 24 h
at 120 8C). The thiol groups were then oxidized as the sample was
added to 8 g H2O2 (30%, Fisher), 30 ml methanol (Fisher) and 30 ml
1.9 M HCl and stirred for 24 h at 40 8C. Finally it was filtered,
washed, acidified and vacuum dried.
2.1.5. Synthesis of Pt(X)/SBA15-PrSO3H (C1)
The platinum and propylthiol groups were jointly loaded in
one-step and then the PrSH groups were oxidized to PrSO3H with
H2O2. Post-oxidation was required as addition of H2O2 in the first
step would lead to oxidation and dissolution of the platinum
Fig. 1. Classification of the Pt(X)/SBA15-PrSO3H synthesis procedures.

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Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
particles during the aging process required in synthesizing SBA15.
The Pt colloid and MPTMS were added to a solution of HCl (1.9 M),
P123 and TEOS to synthesize Pt/SBA15-PrSH. After template
removal with ethanol reflux, washing and drying, Pt/SBA15-PrSH
was obtained and then H2O2 was added to the 1.9 M HCl and
methanol suspension of Pt/SBA15-PrSH with stirring for 24 h at
40 8C to oxidize the PrSH groups to PrSO3H. Finally the material
was filtered, washed, acidified and vacuum dried.
3. Results and discussion
3.1. Catalyst characterization results
3.1.1. Textural properties of the catalysts
The N2 adsorption–desorption isotherms of the catalyst
samples are shown in Fig. 2 and the textural properties of the
synthesized organic–inorganic hybrid materials are summarized
in Table 1. The median pore diameter (MPD) of the samples was
determined by the BJH method using the adsorption branch of the
N2 adsorption–desorption isotherm. As seen in Table 1, loading
with platinum decreased the median pore size and the surface area
relative to SBA15-PrSO3H. All of the samples had the same type IV
isotherms as shown in Fig. 2, which was consistent with
mesoporous materials tailored by a nonionic surfactant [32,33].
However, there were different types of hysteresis loops indicating
different mesopore structures (according to the classification by
IUPAC [34]). The (R2) catalysts maintained the mesopore
morphology of SBA15-PrSO3H as apparent from their shared type
H1 hysteresis loop, which are associated with a narrow pore size
distribution and uniform pore arrays. However, loading of
platinum using the colloid significantly modified the mesopore
structure. The samples synthesized with the (C2, SO3H) and (C2, Pt)
procedures had type H2 hysteresis loops indicating ‘‘ink-bottle’’
pores, which might be due to platinum blocking at the pore mouth
[34]. The desorption curve for sample (C1) suggested incomplete
N2 removal, which might also be caused by blockage of the ‘‘ink-
bottle’’ pores [34]. From these results, it was inferred that loading
platinum by reduction and deposition in the aqueous phase had
little influence on the mesopores texture unlike with the platinum
colloid methods, perhaps due to different Pt particle sizes.
2.2. Catalyst characterization
The textural properties of the catalytic materials were
measured using the nitrogen adsorption–desorption method.
The analysis was carried out at ꢂ196 8C using a Micromeritics
ASAP 2020 system. The surface area was calculated by the BET
method and the pore volume (Vp) and pore size distribution by the
BET and BJH method. All samples were degassed at 100 8C for 5 h
before measurement. The organic composition and thermal
stability of the solids were evaluated by thermogravimetric
analysis (TGA) with a Perkin-Elmer TGA7 instrument using heating
from 50 to 700 8C at a rate of 10 8C/min under air flow. The acid
strength and number of acid sites for the catalysts were
determined by acid-base titration. The relative pKa values of the
sulfonic acid functional groups were estimated using the Gran plot
analysis. In a typical experiment, 0.1 g of the sample was
suspended in 10 ml of distilled water and the resulting mixture
was titrated potentiometrically by continuous addition of 5 mM
KOH in distilled water. The platinum loading on the materials was
quantified by elemental analysis performed on a Thermo Jarrell-
Ash ICP atomic emission spectrometer (Thermo Elemental). The
dispersion of platinum (DPt) on the surface of the support was
determined by H2 chemisorption performed on a Micromeritics
ASAP 2020C system. UV–vis spectra were taken using a UV-
2401(PC) spectrometer for evaluating the level of platinum
reduction in the catalytic materials.
3.1.2. Characterization of organic groups
The thermal decomposition of the tethered organic moieties
in the catalyst samples, as determined by differential thermo-
2.3. Catalytic reaction testing
The Pt(X)/SBA15-PrSO3H catalysts were tested for their activity
in the OHE reaction. The reagents used for the reaction were
acetaldehyde (3.96 g, 99.5%, Acros), acetic acid (5.40 g, Fisher), 1,4-
dioxane (18.00 g, Fisher) as the solvent, and tetrahydrofuran (THF,
2.00 g, Fisher) as the internal standard for the gas chromatography
(GC) analysis (Agilent 7890A, HP-5 column, FID). Standard samples
with different concentrations of acetaldehyde, acetic acid, ethanol,
acetic acetate and THF were prepared for GC analysis to identify
the peak position of each component, and to calibrate their relative
response factors (compared to THF) for quantification. The one-
step reaction being evaluated was:
The OHE reaction was performed in a high-pressure stainless
steelbatchreactor(Parr4598;ParrInstrumentCompany)fittedwith
a mechanical stirrer and a sample outlet. The reaction mixture was
charged to the reactor with 0.1 g of catalyst and the reactor was
pressurized with hydrogen to 15 bar at room temperature. The
reactor was heated to the reaction temperature of 150 8C while
under continuous stirring at a rate of 500 rpm. For comparison,
monofunctional Pt(1)/SBA15 (obtained in the first step of synthe-
sizing (C2, Pt) as described in Section 2.1.4) was tested in the
acetaldehyde hydrogenation reaction at the same conditions used
for the OHE reaction. Previously, SBA15-PrSO3H was also tested in
the esterification reaction at 70 8C and ambient pressure [26].
Fig. 2. N2 adsorption–desorption isotherms of the catalyst samples. (&) Pt(1)/
SBA15-PrSO3H (R2); (&) Pt(5)/SBA15-PrSO3H (R2); (~) Pt(1)/SBA15-PrSO3H (C2,
SO3H); (
$) Pt(1)/SBA15-PrSO3H (C2, SH); (~) Pt(1)/SBA15-PrSO3H (C2, Pt); (ꢃ)
Pt(1)/SBA15-PrSO3H (C1); (+) SBA15-PrSO3H.

Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
313
Table 1
Textural and chemical properties of the catalyst samples.
Catalysts
Textural properties
Chemical properties
Acid sites (mmol/g)
b
SBET (m2/g)
MPD (A)
Vp (cm3/g)
pKa
Pt contenta (wt%)
DPt (%)
˚
Pt(1)/SBA15-PrSO3H (R2)
Pt(5)/SBA15-PrSO3H (R2)
Pt(1)/SBA15-PrSO3H(C2, SO3H)
Pt(1)/SBA15-PrSO3H (C2, SH)
Pt(1)/SBA15-PrSO3H (C2, Pt)
Pt(1)/SBA15-PrSO3H(C1)
Pt(1)/SBA15(R2)
549
513
350
559
765
261
510
846
54
65
29
53
33
25
68
79
1.00
0.98
0.25
0.77
0.68
0.17
1.12
1.64
0.69
0.57
0.82
0.67
0.56
0.63
–
3.55
3.31
3.50
4.46
4.38
4.31
–
0.62
4.47
0.48
0.12
0.16
0.15
0.80
–
51
9
37
42
40
43
35
–
SBA15-PrSO3Hc
0.89
3.91
a
The Pt contents were detected by ICP element analysis shown as the weight percentage of the catalysts.
The Pt dispersion values of the catalysts, based on the Pt contents detected by ICP element analysis.
This sample was prepared by the co-condensation method.
b
c
gravimetric (DTG) analysis, is shown in Fig. 3. Desorption of
Since metallic platinum was the desired active site, the extent
of reduction for the platinum ions in the catalysts was investigated
using UV–vis. The spectra are shown in Fig. 4 in which the
characteristic absorption of platinum ions (such as [PtCl6]2ꢂ) is at
around 260 nm [35]. For ease of comparison, the spectra were
normalized at 450 nm as Furlong et al. did in their work [36]. As can
be seen from the spectra the (C2, Pt), (C2, SH) and (C1) catalysts still
had significant adsorption at around 260 nm, which was charac-
teristic of the presence of some platinum ions. These results
indicated incomplete reduction of platinum ions to their metallic
form in these samples.
Three types of reducing agents (NaBH4 + (CH2OH)2; N2H4ꢀH2O;
HCHO) were compared in the preparation of the (R2) catalyst
because these reducing agents are commonly used in aqueous
phase reductions [37]. The platinum reduction resulting from the
different reducing agents were compared by UV–vis with the
spectra shown in Fig. 5. The lowest level of absorption at around
260 nm (ascribed to platinum ions) was observed for the catalyst
reduced using NaBH4 + (CH2OH)2. From Fig. 5, the reducing
efficiency of the three agents employed in the current work were
ranked as: NaBH4 + (CH2OH)2 > N2H4ꢀH2O > HCHO. Therefore,
NaBH4 + (CH2OH)2 was utilized as the reducing agent when
comparing the (R2) synthesis procedure with the other procedures.
physisorbed water was observed below 110 8C. The weight loss
peak centered at about 350 8C has been attributed to the
decomposition of the propylthiol groups [25,28] and the peak
centered at about 450 8C has been assigned to the decomposition
of propylsulfonic acid groups [25,28]. The DTG profiles of the (C2,
Pt), (C2, SH) and (C1) catalysts showed less weight loss in the
propylsulfonic acid decomposition region than the other two
catalysts as well as some weight loss around 350 8C, indicating
incomplete oxidation of propylthiol to propylsulfonic acid.
Additionally, the DTG analysis showed that the decomposition
of the propylsulfonic acid groups on the (R2) catalyst were at a
somewhat higher temperature centered at about 480 8C, suggest-
ing higher thermal stability of the acid sites on (R2) than with the
other catalysts.
3.1.3. Characterization of inorganic groups
Table 1 shows the Pt loadings of the catalysts as determined by
ICP analysis and the Pt dispersions from H2 chemisorption. It was
demonstrated that post-oxidation and acidification of the pro-
pylthiol groups after Pt loading significantly reduced the Pt loading
amounts (see the (C2, Pt), (C2, SH) and (C1) catalysts), which
suggested the unsuitability of these synthesis procedures for the
catalysts in the current work. Compared with the Pt colloid loading
methods, the aqueous reduction and deposition of Pt ((R2), with
NaBH4 + (CH2OH)2 as the reducing agents) yielded the highest
amount of Pt on the catalysts (0.62 wt%) and led to the highest Pt
dispersion (51%), suggesting that (R2) was the best method of Pt
incorporation.
3.1.4. Thiol–platinum interaction and [PtnꢂH]+ adduct effect
As discussed above, the grafted propylthiol groups and
platinum ions were not fully transformed into the corresponding
propylsulfonic acid groups and metallic platinum for some of the
synthesis approaches. A possible explanation for this observation
Fig. 3. Differential thermogravimetric (DTG) analysis of the different Pt(1)/SBA15-
PrSO3H materials. (&) (R2); (ꢃ) (C2, SO3H); (&) (C2, SH); (–) (C2, Pt); (~) (C1).
Fig. 4. UV–vis spectra with normalized absorbance for the different Pt(1)/SBA15-
PrSO3H. (—) (R2); (–) (C2, SO3H); (-ꢀ-ꢀ-ꢀ) (C2, SH); (ꢀꢀꢀꢀꢀ) (C2, Pt); (- - -) (C1).

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Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
number of sulfonic acid sites and metallic platinum sites on the
catalysts.
The incomplete reduction of platinum for the thiol-containing
materials could be observed visually from the color of the prepared
catalysts which were not black as with metallic platinum but
instead were somewhat yellowish as is the case with platinum
cations. It was observed that when exposing thiol group (SBA15-
PrSH) to the PtCl4 solution, the solid phase became yellowish while
the aqueous phase became colorless and the color did not change
obviously to black during the following platinum reduction step.
Therefore, these observations provided visual support to the
indication of the strong adsorption of the platinum cations caused
by thiol–platinum interaction, which subsequently inhibited the
platinum reduction.
Since the acidic sites were critical for the esterification reaction,
the number of acid sites and the strength of those acid sites (pKa
values) were determined by acid–base potentiometric titration
using the Gran plot technique [28]. As can be seen in Table 1, the
number of acid sites diminished when the materials were loaded
with platinum, which suggested that the platinum loading process
covered some of the acid sites. However, it is interesting that lower
pKa values were found for the (R2) and (C2, SO3H) catalysts
compared with SBA15-PrSO3H (Table 1), which indicated that in
the presence of platinum the strength of the acid sites were
increased. The (C2, SO3H) catalyst was found to have both more
and stronger acid sites than the (R2) catalyst as shown in Table 1.
The improved acidity in the presence of platinum suggested
that there was some interaction between platinum and the
sulfonic acid groups. Such interactions have been investigated
previously where the existence of attraction between these two
types of clusters has been suggested [41–43]. Several studies with
Pd [21,44,45] reported similar results where the presence of
metallic Pd improved the acidic strength of an acid resin, which
was ascribed to the formation of a ‘‘[PdnꢂH]+ adduct.’’ Considering
the similar properties of Pt and Pd (both are the Group VIII
elements with similar outer electronic configurations), and their
analogous effects caused by their apparent interactions with
sulfonic acid groups, it is reasonable to attribute the Pt promotion
effect observed in the current study to a similar mechanism as that
proposed for Pd. As illustrated in Fig. 6, the acid strength might be
enhanced with the change in electron density created by forming a
possible ‘‘[PtnꢂH]+ adduct,’’ with oxygen atoms in sulfonic acid
groups and their protons bonded to the electron-deficient Pt
particles. Inspired by the model suggested by Miller et al. [46], we
propose that there was a Coulombic interaction between the Pt and
the oxygen atom in the sulfonic acid groups that caused electron
withdrawal from the Pt particles to the oxygen atom. According to
EXAFS results [46], it was concluded that, as the electron density of
the oxygen atom increased, the bonding energy of the Pt valence
orbitals decreased leading to a weakening of the Pt–H bond.
Therefore, the protons on the Pt particles would become less
restricted and more acidic. More detailed characterization of the
catalysts will be done to investigate this acid–metal interaction in
our future work.
Fig. 5. UV–vis spectra with normalized absorbance of the Pt(1)/SBA15-PrSO3H (R2)
using different reducing agents. (—) NaBH4 + (CH2OH)2; (–) N2H4ꢀH2O; (ꢀꢀꢀꢀꢀ) HCHO.
was a thiol-bonding effect that created an interaction between Pt
and S [38–40], as illustrated in Fig. 6. For three of the catalyst
synthesis procedures, (C2, Pt), (C2, SH) and (C1), the ordering of the
platinum addition and thiol oxidation created the possibility that
thiol groups could interact with platinum prior to their oxidation
to sulfonic acid groups. The results suggested that an interaction
between platinum and thiol groups inhibited the formation of both
the metal and acid sites. The formation of thiolate–platinum
adducts was likely since thiolate ions are capable of providing a
strong binding affinity owing to their better
s-donating ability.
This effect has been investigated in the area of drug synthesis in
heavy metal poisoning of DNA and proteins [39] and it has also
been implicated in the high dispersion of Pt on carbon supports
used in catalyst preparation [40]. The apparent thiol-bonding
effect seemed to be quite strong during our synthesis work such
that it inhibited both the oxidation of the thiol groups and the
reduction of platinum, thereby leading to suppression in the
3.2. Catalytic testing results
The catalytic materials synthesized above were tested in the
one-step hydrogenation/esterification (OHE) of acetic acid with
acetaldehyde in the presence of hydrogen. Differing from previous
work [13], 1,4-dioxane was used as the solvent instead of just the
pure reagents. In this solvent-diluted system no significant
byproducts from acetaldehyde condensation were observed.
Additionally, the possibility of hydrogenating the acetic acid into
alcohol was ruled out by performing a controlled reaction using Pt/
SBA15-PrSO3H in the absence of acetaldehyde. From these baseline
Fig. 6. Speculative schematic representation of the bifunctional interactions,
including thiol-bonding, Pt covering effect and possible [PtnꢂH]+ effects.

Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
315
studies, it was apparent that acetic acid was only involved in the
esterification reaction without any hydrogenation or cracking, and
acetaldehyde was only involved in the hydrogenation to ethanol
under the reaction conditions used in this work. Therefore, the
reaction performance of the catalysts was evaluated by determin-
ing the conversion of acetic acid and acetaldehyde. The conversion
of acetaldehyde represented hydrogenation activity, while the
conversion of acetic acid (HAc) represented the esterification
activity as well as the combined OHE activity. Conversions were
determined using the following equations, in which ‘‘M’’ repre-
sented the molar amounts of the reactants or products in each
sample collected during the reactions.
activity. The common feature of the (R2) and (C2, SO3H) materials
was that the thiol groups had been loaded and oxidized to sulfonic
acid group prior to Pt loading, which avoided the possibility of a
thiol–platinum bonding effect. The lower activity of the (C2, Pt),
(C2, SH) and (C1) catalysts was consistent with the formation of
thiol–platinum interactions that inhibited the oxidation of the
thiol groups to acidic sites and the reduction of platinum ions to
metallic Pt.
The catalytic results combined with DTG and UV–vis charac-
terization confirmed that improved reduction of the platinum ions
and oxidation of thiol groups are beneficial for the OHE reaction
and demonstrated that the apparent thiol–platinum interaction
inhibited the oxidation of the thiol groups and the reduction of
platinum ions. Of the synthesis procedures used, the most
successful procedure for introducing the two functional groups
was the two-step procedure that first introduced thiol groups and
oxidized them to sulfonic acid groups followed by platinum
loading through aqueous reduction and deposition as used in the
(R2) synthesis.
M
ðethyl acetateÞ
HAc conv: ð%Þ ¼
ꢃ 100%
M
þ M
ðHAcÞ
ðethyl acetateÞ
M
acetateÞ þ M
ðethyl
ðethanolÞ
acetaldehyde conv: ð%Þ ¼
M
acetateÞ þ MðethanolÞ þ M
ðethyl
ðacetaldehydeÞ
ꢃ 100%
3.2.2. Effect of platinum loading amount and reducing agent on OHE
activity
3.2.1. Effect of catalyst synthesis procedures on OHE activity
OHE reaction results catalyzed by the different Pt(1)/SBA15-
PrSO3H catalysts are shown in Fig. 7. It was found that for the (R2)
and (C2, SO3H) catalysts, both the aldehyde conversion (hydro-
genation) and the acid conversion (esterification) were higher than
when using the other three catalysts, indicating better OHE
Two additional synthesis factors that were examined relative to
final reactivity were the platinum loading and the choice of
reducing agent. Results in Fig. 8 show that there were significant
improvements in OHE activity when the platinum nominal loading
was increased from 1 to 5 wt%. However as clearly shown in
Table 1, the higher loading resulted in a much lower dispersion of
platinum and less available acidic sites. The results indicated that
the higher platinum loading was favorable for the improvement of
overall OHE activity, although it decreased the efficiency of the Pt
utilization due to lower dispersion and sacrificed some acidic sites.
The reactivity results for the Pt(1)/SBA15-PrSO3H (R2) catalysts
using different reducing agents (NaBH4 + (CH2OH)2, N2H4ꢀH2O,
HCHO) for the platinum are shown in Fig. 9. These results were in
good agreement with the degree of reduction given by the UV–vis
results. The improved reduction with NaBH4 + (CH2OH)2 was
clearly seen during the synthesis by the color change of the
suspension. The suspension turned from yellow to black just 1 or
2 min after the introduction of NaBH4 with (CH2OH)2, but required
several hours when N2H4ꢀH2O or HCHO was used. It should be
noted Ma et al. [37] have proposed that improved reactivity for
Fig. 8. Acetaldehyde and acetic acid conversion in the OHE reaction catalyzed by
Pt(X)/SBA15-PrSO3H with different platinum loadings. (^ —) Pt(1)/SBA15-PrSO3H
(R2) HAc conv.; (~ – – –) Pt(1)/SBA15-PrSO3H (R2) acetaldehyde conv.; (^ —)
Pt(5)/SBA15-PrSO3H (R2) HAc conv.; (~ – – –) Pt(5)/SBA15-PrSO3H (R2)
acetaldehyde conv.
Fig. 7. OHE reaction activities catalyzed by different Pt(1)/SBA15-PrSO3H: (a)
acetaldehyde conversion; (b) acetic acid conversion. (^ —) (R2); (~ – – –) (C2,
SO3H); (~ꢀꢀꢀꢀ) (C2, SH); (& —) (C2, Pt); (& – ꢀ – ꢀ) (C1).

316
Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
Fig. 10. Acetaldehyde hydrogenation catalyzed by monofunctional and bifunctional
catalysts. a(& —) Pt(1)/SBA15-PrSO3H (in hydrogenation); b(& –ꢀ–ꢀ–ꢀ) Pt(1)/
SBA15(in hydrogenation); c(~ —) Pt(1)/SBA15 (in hydrogenation with the
presence of ethanol); d(~ – – –) Pt(1)/SBA15 (in hydrogenation with the
presence of acetic acid); e(^ ꢀꢀꢀꢀ) Pt(1)/SBA15-PrSO3H (in OHE).
be expected to diminish the acetaldehyde hydrogenation reaction
as was seen in the current work. The hydrogenation reaction was
enhanced by the presence of acetic acid since its esterification with
ethanol, either self-catalyzed by the acetic acid or catalyzed by acid
groups of Pt(1)/SBA15-PrSO3H, diminished the concentration of
ethanol in the reaction system thereby increasing the hydrogena-
tion reaction rate.
Esterification activity was dependent on the number of acid
sites and the acidic strength of those sites (pKa values). Fig. 11
compares the esterification activity of acetic acid with ethanol
expressed as the cumulative turnover number of acetic acid
molecules per acid site. The bifunctional catalysts Pt/SBA15-
PrSO3H exhibited superior activity for esterification with up to
twice the acetic acid turnover number (when nominal platinum
loading reached 5 wt%) as with that over the monofunctional
sample SBA15-PrSO3H. The results were in accordance with the
acid strength order (pKa values) in Table 1. Both the characteriza-
tion and catalytic testing results pointed to the promoting effect of
platinum on the acidic strength of propylsulfonic acid groups,
Fig. 9. OHE reaction activities by Pt(1)/SBA15-PrSO3H(R2) using different reducing
agents: (a) acetaldehyde conversion; (b) acetic acid conversion. (^ —)
NaBH4 + (CH2OH)2; (~ꢀꢀꢀꢀ) N2H4ꢀH2O; (~ – – –) HCHO.
their Pt catalyst reduced by NaBH4 + (CH2OH)2 was due to boron
doping improving the dispersion of Pt [37].
3.2.3. Mutual influence of hydrogenation and esterification
The final issue explored with the bifunctional catalyst system
was whether there existed an interaction between the acid and
metal functional groups, which influenced the activity for the
hydrogenation and/or esterification reaction. To evaluate this
possibility, monofunctional and bifunctional catalysts were
compared in catalytic tests of ethanol esterification with acetic
acid and acetaldehyde hydrogenation separately. The monofunc-
tional catalysts tested were Pt(1)/SBA15 for hydrogenation and
SBA15-PrSO3H for esterification.
The activity of the Pt(1)/SBA15-PrSO3H (R2) and monofunc-
tional Pt(1)/SBA15 were found to be very similar in the
acetaldehyde hydrogenation reaction (Fig. 10), which indicated
that the presence of the propylsulfonic acid groups did not
significantly influence the hydrogenation activity of the platinum
sites. Hydrogenation experiments were also performed in the
presence of ethanol and acetic acid with the monofunctional Pt(1)/
SBA15 catalyst. As seen in Fig. 10, the presence of ethanol inhibited
acetaldehyde hydrogenation, whereas, the presence of acetic acid
promoted the hydrogenation reaction. Previous work has demon-
strated that acetaldehyde hydrogenation/ethanol dehydrogena-
tion reactions are reversible and quasi-equilibrated, particularly
over Pt catalysts [47–49]. Therefore, the addition of ethanol should
Fig. 11. Acetic acid cumulative turnover number in the esterification reaction
catalyzed by different monofunctional and bifunctional catalysts. (& —) Pt(5)/
SBA15-PrSO3H (R2); (& – – –) Pt(1)/SBA15-PrSO3H (C2, SO3H); (~ —) Pt(1)/SBA15-
PrSO3H (R2); (~ ꢀꢀꢀꢀ) SBA15-PrSO3H.

Y. Tang et al. / Applied Catalysis A: General 375 (2010) 310–317
317
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