Selective dehydrosulfurization of 3-mercaptopropionic acid to acrylic acid on silicalite catalyst

Article abstract of DOI:10.1016/j.catcom.2009.11.022

Dehydrosulfurization of 3-mercaptopropionic acid has been investigated in a fixed bed reactor using a silicalite catalyst in order to recover the hydrocarbon core. Under reductive conditions, acrylic acid was formed in high yield at 350 °C and atmospheric pressure as the desired product along the toxic H₂S, whereas, carrying out the reaction in the presence of air, safer dihydrogenpolysulfides resulted as the co-product.

Full text of DOI:10.1016/j.catcom.2009.11.022

Catalysis Communications 11 (2010) 456–459
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Selective dehydrosulfurization of 3-mercaptopropionic acid to acrylic acid
on silicalite catalyst
*
Cristina Della Pina, Ermelinda Falletta, Michele Rossi
Dipartimento di Chimica Inorganica, Metallorganica e Analitica ‘‘Lamberto Malatesta”, Università degli Studi di Milano via G. Venezian, 21, 20133 Milano, Italy
CNR – Institute of Molecular Science and Technologies (ISTM) via C. Golgi 19, 20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 October 2009
Received in revised form 19 November 2009
Accepted 23 November 2009
Available online 26 November 2009
Dehydrosulfurization of 3-mercaptopropionic acid has been investigated in a fixed bed reactor using a
silicalite catalyst in order to recover the hydrocarbon core. Under reductive conditions, acrylic acid
was formed in high yield at 350 °C and atmospheric pressure as the desired product along the toxic
H2S, whereas, carrying out the reaction in the presence of air, safer dihydrogenpolysulfides resulted as
the co-product.
Ó 2009 Elsevier B.V. All rights reserved.
Dedicated to the humanity and scientific
creativity of Prof. Mauro Graziani in the
occasion of his 75th birthday.
Keywords:
Acrylic acid
3-Mercaptopropionic acid
Acid catalysis
Dehydrosulfurization
Silicalite
1. Introduction
Beside H2S, environmental pollution of sulfur is mainly related
to the emission of gaseous SO2 and SO3 because these latter com-
Decomposition products of organosulfur compounds and chlo-
rinated organic compounds have been found to be toxic to humans
[1]. Petroleum and natural gas contain various organosulfur com-
pounds and their removal by hydrodesulfurization (HDS) over het-
erogeneous catalysts (reaction (1)) produces decontaminated
hydrocarbons along with H2S [2–10], the latter to be transformed
into safe elemental sulfur by the Claus process (reactions (2) and
(3)) [11,12].
pounds have a strong impact on the health of animals and vegeta-
bles. Reducing emissions of sulfur compounds by selective
catalytic reduction (SCR) and flue gas desulfurization (FGD) sys-
tems by basic compounds (the ‘‘scrubbers”) remains a crucial com-
ponent of strategy to clean air and, consequently, international
agencies are developing several control programs.
Considering the fate of sulfur in chemical wastes, the deep oxi-
dation produces SOx compounds which are removed by the above
cited gas desulfurization process according to the common practice
whereas less care has been paid to selective processes allowing
hydrocarbons recovery similarly to the petrol-HDS process.
Considering the importance of chemicals recycling, we have
investigated a process for recovering the hydrocarbon core of
3-mercaptopropionic acid (3-MPA), selected as a model compound
by using silicalite as a metal free dehydrosulfurization catalyst. 3-
Mercaptopropionic acid and its derivatives are currently employed
as ion exchange catalysts, PVC-stabilizers, chain-transfer and cross-
linking agents and UV-curable formulations. From literature data
we know that this compound can be desulfurized by means of stoi-
chiometric reactions. In particular, by using 2-chloro-1,3-dimethy-
limidazolium chloride the valuable commercial compound acrylic
acid was produced [19], whereas, to our knowledge, no catalytic pro-
cesses have been investigated for removing sulfur from this molecule.
CxHyS þ 2H2 ! CxHyþ2 þ H2S
2H2S þ 3O2 ! 2SO2 þ 2H2O
2H2S þ SO2 ! 3S þ 2H2O
ð1Þ
ð2Þ
ð3Þ
Although soluble metal derivatives, particularly aluminium,
vanadium and nickel, are able to promote the desulfurization of
some simple organosulfur compounds [13] the present industrial
HDS process is based on alumina-supported Mo and Co catalysts
[14–17], whereas recent interest has also been devoted to the
use of gold catalysts [18].
* Corresponding author.
E-mail address: michele.rossi@unimi.it (M. Rossi).
1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.022

C.D. Pina et al. / Catalysis Communications 11 (2010) 456–459
457
Although silicalites are widely used as acid catalysts for indus-
trial processes, particularly in petroleum refining (hydrocracking,
isomerization, alkylation and reforming) [20,21], minor interest
has been devoted to laboratory-scale catalytic applications. In this
paper we show that silicalite can be advantageously applied in the
absence of heavy metals as a catalyst for removing sulfur from
3-mercaptopropionic acid according to a selective process, pre-
serving the hydrocarbon core.
of a catalyst (silicalite), either in the absence or in the presence of
dioxygen.
The experimental apparatus is represented in Scheme 1.
Fig. 1 shows the results obtained in the tests carried out under
N2 stream.
During these experiments, the main product, acrylic acid, was
collected in the trap containing a weakly acid aqueous solution
(H3PO4 0.01 M) cooled at 0 °C, whereas H2S was collected in a sec-
ond trap containing NaOH 10 M solution. According to the KMnO4
analysis in final trap, almost no oxidable products were present in
the vent gas.
2. Experimental
2.1. Reagents and instruments
Starting from 200 °C and increasing the temperature we noted
that the dehydrosulfurization process started at ca. 250 °C forming
acrylic acid and H2S. No other products have been detected except
non-quantified amounts of polyacrylic acid solidified at the exit of
the reactor. Owing to this loss of acrylic acid, yields below 100%
have been observed in trap A by HPLC determination: however, ac-
rylic acid was recovered up to 76.5% by converting 3-MPA at
370 °C. In any case, total conversion of 3-mercaptopropionic acid
occurred by passing the feed on the catalyst at 350 °C and over this
temperature, as the reagent resulted absent in the products. Iodi-
metric titration of H2S collected in trap B showed its stoichiometric
equivalence to the converted 3-mercaptopropionic acid according
to reaction (4):
3-Mercaptoproprionic acid (Fluka, P99.0%), NaOH (Fluka,
P98%), KMnO4 (Fluka, P99.0%), H2SO4 95% (Prolabo), CH3OH (Flu-
ka, >99.5%), CH3I (Fluka, P99.5%), Na2SO4 anhydrous (Fluka, >99%).
N2 and air (SIAD, 99.99%), silikalite-1 pellets (1 mm ꢀ 4 mm size,
Si = 44%, Al < 0.01%, Fe < 0.005%, micropore volume = 0.18 ml/g
were provided by BASF).
Acrylic acid was identified by the 1H NMR and 13C NMR spectra
recorded on a Bruker 300 MHz instrument and quantified by HPLC
method using a Shimadzu LC-10 instrument equipped with a
refractive index detector Shimadzu RID-10A, a Varian MetaCarb
H Plus column (300 mm ꢀ 7.8 mm) and using aqueous 0.01 M
H3PO4 as the eluant. XRD spectra were recorded using a Rigaku D
O
III-MAX horizontal-scan powder diffractometer with Cu Ka radia-
O
SH
tion. SEM images were collected by using a LEO 1430 microscope.
The polysulfides were identified by UV–vis spectroscopy
(230 nm) on a Hewlett Packard 8453 instrument using chloroform
as solvent and quantified by a LECO SC-132 Sulfur Determinator
instrument.
H S
+
2
HO
OH
ð4Þ
2.2. Catalytic tests and analysis for the reactions under N2 flow
The reactions were carried out in a fixed bed vertical glass reac-
tor (l = 25 cm, diameter = 3.5 cm) fitted with a porous septum car-
rying the catalyst (2.5 g) and provided with an electronically
controlled furnace. The gas stream (N2 = 268 mmol/h) was con-
trolled by a Brooks mass-flow instrument and the liquid reagent
(18.4 mmol/h) was supplied through an automatic syringe pump
ensuring that liquid vaporization occurred on the reactor wall prior
to the catalytic bed. The condensable reaction products were col-
lected for 1 h by bubbling the effluent into trap A containing an
aqueous solution of H3PO4 0.01 M, cooled in an ice-water bath.
The non-condensable products were collected in the second trap
B containing NaOH 10 M. In order to detect further reducible gas-
eous products, a trap C containing KMnO4 1 M was connected at
the end of the system.
Scheme 1. Apparatus used for the experiments.
2.3. Catalytic tests and analysis for the reaction in the presence of air
100
80
60
40
20
0
These tests were carried out as above but using a mixture of gas
stream (N2 = 224 mmol/h, air = 54 mmol/h) along with the liquid
3-MPA stream (18.4 mmol/h) in the range of temperature 250–
370 °C. Acrylic acid, collected in trap A as aqueous solution, was
analysed by HPLC chromatography, while polysulfides were col-
lected at the bottom of the same trap as a dark brown insoluble li-
quid. This fraction was methylated by reacting with CH3I in
methanol [20] and analysed by UV–vis spectroscopy in order to
identify the products and quantified by a standard combustion
method [21]. No products were collected in the second trap B
and in the last trap C.
Conv
Sel
Yield
220
250
280
310
340
370
400
3. Results and discussion
Temperature (°C)
The catalytic dehydrosulfurization of 3-MPA was investigated in
Fig. 1. Catalytic test in nitrogen stream. Reaction conditions: 3-MPA = 18.4 mmol/h,
a range of temperature between 200 °C and 370 °C, in the presence
N2 = 268 mmol/h, catalyst amount = 2.5 g. Yield and selectivity as acrylic acid.

458
C.D. Pina et al. / Catalysis Communications 11 (2010) 456–459
As reported in Fig. 2, in the presence of a small excess of O2
Conv
Sel
100
80
60
40
20
0
(O2/S = 0.6) 3-mercaptopropionic acid was completely converted
to acrylic acid with fairly good selectivity (>90%) at 300–330 °C.
In this case, no H2S was collected in trap B whereas an insoluble
oily material was collected in trap A along with the acrylic acid
solution. It was identified as dihydrogenpolysulfide by means of
UV–vis spectroscopy after methylation [22] and quantified by
infrared spectroscopy [23]. Its formation is due to the incomplete
oxidation of 3-mercaptopropionic acid to H2S which is able to react
with the formed sulfur according to reaction (5):
Yield
H2S þ nS ! H2Snþ1
ð5Þ
250
280
310
340
370
400
Temperature (°C)
The amount of sulfur transformed into polysulfides in the
experiments of Fig. 2 accounts for 81% of the converted 3-mercap-
topropionic acid.
The results of the tests carried out in air stream are important as
the process allows either the recovery of a great part of acrylic acid
or the removal of total sulfur in form of harmful material; for the
economic balance, it is interesting to note that polysulfides are
commercially valuable products [24].
Fig. 2. Catalytic test in air stream. Reaction conditions: 3-MPA = 18.39 mmol/h,
N2 = 224 mmol/h, air = 54 mmol/h, catalyst amount = 2.5 g. Yield and selectivity as
acrylic acid.
100
Finally, a preliminary test was performed at 370 °C for evaluat-
ing the catalyst life under reductive dehydrosulfurization condi-
tions. According to Fig. 3, 100% conversion was preserved for
24 h, while the yield of acrylic acid gradually decreased from 80%
to 50% owing to formation of non-characterised products, probably
polyacrylates, collected in the aqueous solution. No attempts were
done either to regenerate the catalyst under oxidative conditions
or to optimize typical engineering aspects (diffusion problems
and catalyst bed dilution).
The XRD spectra and SEM pictures of the fresh silicalite and the
used silicalite are recorded in Figs. 4 and 5, respectively: no signif-
icant changes of the silica framework appear after 24 h on stream,
to be related to the observed decline of selectivity.
Conv
80
Sel
Yield
60
40
20
0
0
5
10
15
20
25
30
Time (h)
In the absence of specific mechanistic investigations we believe
that the dehydrogenation mechanism of the 3-mercaptopropionic
acid on silicalite should be similar to that reported for hydroxy-
compounds dehydrogenation [25,26]. Accordingly, the starting
activation occurs at the silica surface where strained Si–O–Si-
bridges are taken part in dissociative adsorption preceding hydro-
gen abstraction. In the case of alcohols, competition between C1
and C2 abstraction leads to the more favoured carbonyl compound,
whereas in the case of thiol it leads selectively to the olefinic com-
pound (Scheme 2).
Fig. 3. Evaluation of the catalytic decay. Experimental conditions: T = 370 °C the
rest as in Fig. 1.
A blank test in the absence of catalyst showed the thermal sta-
bility of 3-mercaptopropionic acid up to 370 °C. Considering that
the dangerous H2S is commonly converted to harmless elemental
sulfur by the Claus process, we have investigated the dehydrosul-
furization process in the presence of air in order to promote reac-
tions (2) and (3).
In the presence of oxygen, partial oxidation of sulfur occurs pro-
ducing the polysulfides according to reaction (5).
B
A
2000
1500
1000
500
0
2000
1500
1000
500
0
10
20
30
40
2 theta
50
60
70
80
10
20
30
40
2 theta
50
60
70
80
Fig. 4. XRD spectra of fresh silicalite (A) and 24 h on stream silicalite (B).

C.D. Pina et al. / Catalysis Communications 11 (2010) 456–459
459
Fig. 5. SEM pictures of fresh silicalite (A) and silicalite used for 24 h (B).
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4. Conclusions
Silicalite has been evaluated as a catalyst in the reductive and
oxidative dehydrosulfurization of 3-mercaptopropionic acid in or-
der to recover a valuable product, namely acrylic acid. Under N2
flow, sulfur can be totally removed as H2S. The possibility to couple
this process to a Claus type process has been demonstrated by
reacting 3-mercaptopropionic in the presence of air: under these
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Acknowledgements
We are grateful to Dr. Valter Mantelli, Dr. Gianfranco Bagnara
and Dr. Giorgio Mirabelli of IPLOM S.p.a. Busalla (Genova) for ana-
lytical determinations. BASF is also acknowledged for supplying
the silicalite catalyst.
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