Structure-activity studies of dodecatungstophosphoric acid impregnated bentonite clay catalyst in hydroxyalkylation of p-cresol

Article abstract of DOI:10.1016/j.catcom.2010.04.008

Bentonite clay impregnated with dodecatungstophosphoric acid (20% DTP/BNT) showed an excellent activity, selectivity and stability [95% product yield with 94% selectivity to 2, 2′-methylenebis (4-methylphenol), DAM] for the hydroxyalkylation of p-cresol with formaldehyde at 353 K and for a mole ratio of 5. Ammonia-TPD results showed that an increase in total concentration of acid sites from 4.9 of parent bentonite to 11.6 micromoles per surface area NH3 (μmolS^-1 NH₃) of 20% DTP/BNT was due to a strong interaction of protons of bulk DTP with surface hydroxyl groups of BNT as evidenced by 31P NMR studies.

Full text of DOI:10.1016/j.catcom.2010.04.008

Catalysis Communications 11 (2010) 942–945
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Structure–activity studies of dodecatungstophosphoric acid impregnated bentonite
clay catalyst in hydroxyalkylation of p-cresol
⁎
A.C. Garade, V.S. Kshirsagar, A. Jha, C.V. Rode
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Bentonite clay impregnated with dodecatungstophosphoric acid (20% DTP/BNT) showed an excellent activity,
selectivity and stability [95% product yield with 94% selectivity to 2, 2′-methylenebis (4-methylphenol), DAM]
for the hydroxyalkylation of p-cresol with formaldehyde at 353 K and for a mole ratio of 5. Ammonia-TPD
results showed that an increase in total concentration of acid sites from 4.9 of parent bentonite to
11.6 micromoles per surface area NH3 (μmolS−1 NH3) of 20% DTP/BNT was due to a strong interaction of
protons of bulk DTP with surface hydroxyl groups of BNT as evidenced by 31P NMR studies.
© 2010 Elsevier B.V. All rights reserved.
Received 1 February 2010
Received in revised form 7 April 2010
Accepted 13 April 2010
Available online 24 April 2010
Keywords:
Hydroxyalkylation
Surface modification
Bentonite
p-Cresol
31P CPMAS NMR
1. Introduction
of the solid acid catalysts which is a fundamental property governing
the activity and selectivity pattern in hydroxyalkylation reac-
Activity of catalysts concomitant with their structures is frequently
tuned by modifying their surface properties [1–3]. Surface properties
and thereby activity of the catalysts for a particular reaction can be
tailored by adopting different methods of preparation and post-
synthesis treatments [4–6]. Among various organic transformations,
hydroxyalkylation of phenols is an industrially important reaction
whose end product e.g. dihydroxydiarylmethane is widely used
as chemical intermediate in plastic and rubber industries [7]. Several
solid acid catalysts have been reported by researchers [8–11] for
replacing the conventional toxic and corrosive reagents which often
create serious environmental and operational problems in the
hydroxyalkylation of phenols. Among various catalysts, zeolites are
widely used for the hydroxyalkylation reaction of various aromatics
compounds [12,13]. A major problem associated with these catalysts is
their small pore size which restricts diffusion of reactants and products
through their voids. Secondly, bulky and high molecular weight
condensation products formed during the progress of a reaction
poison the catalyst, thereby decreasing its activity [14]. Recently, we
have also reported dodecatungstophosphoric acid (DTP) impregnated
on various supports, as catalysts for the hydroxyalkylation of phenol
where we observed variation in catalyst activity as a function of
supports under similar DTP loadings [15,16]. This was mainly due
to the difference in textural properties of supports and the extent
of interactions of various supports with bulk DTP affecting the acidity
tions [17,18]. Supports like silica interact with DTP and forms
−
(SiOH2)+(H2PW12O40
)
species which enhanced activity of silica for
various organic transformations while other supports like MgO and
Al2O3 tend to decompose heteropolyacids [19,20] hence the proper
choice of support for impregnation of DTP is very crucial. In this
context, we thought that bentonite clay is a good alternative due to
its inherent acidity which can be further modified by DTP impregna-
tion to create highly active acidic centers [21]. In the present study we
report for the first time, surface modified bentonite (BNT) impreg-
nated with DTP as a highly active, selective and reusable catalyst
for the hydroxyalkylation of p-cresol to form 2, 2′-methylenebis (4-
methylphenol) [DAM] as shown in Scheme 1. Activity comparison
with bulk DTP, H-β-zeolite (SiO2/Al2O3 =25) and montmorillonite
KSF/O was also studied for the hydroxyalkylation of p-cresol. All
the catalysts were characterized by BET surface area measurements,
NH3-TPD, and by 31P-CPMAS NMR. The reusability and stability of the
catalyst were studied by catalyst recycle experiments.
2. Experimental
2.1. Materials
p-Cresol, formaldehyde, toluene and DTP were purchased from
Loba Chemie, Mumbai, India. Bentonite was obtained from Ashapura,
India. Montmorillonite KSF/O was purchased from Fluka, India. H-β-
zeolite (SiO2/Al2O3 =25) was available from Catalysis Pilot Plant,
National Chemical Laboratory, Pune India.
⁎
Corresponding author. Tel.: +91 20 2590 2349; fax: +91 20 2590 2621.
E-mail address: cv.rode@ncl.res.in (C.V. Rode).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.04.008

A.C. Garade et al. / Catalysis Communications 11 (2010) 942–945
943
Scheme 1. Hydroxyalkylation of p-cresol to 2, 2′-methylenebis (4-methylphenol).
2.2. Catalyst preparation
Catalyst recycle experiments were carried out as follows. After the
first hydroxyalkylation run, the used catalyst was filtered and washed
several times with dichloromethane. Then the catalyst was dried at
383 K for 2 h and reused for the subsequent run. The procedure was
followed for three subsequent hydroxyalkylation experiments.
Surface modified bentonite impregnated with 20% dodecatung-
stophosphoric acid (20% DTP/BNT) catalyst was prepared by a
wet impregnation method. Four g of BNT was weighed and added
slowly to the solution of 1 g of DTP in 50 mL methanol with constant
stirring over a period of 20 min. The slurry was stirred for 8 h at room
temperature and the solvent was evaporated under vacuum. The
catalyst formed was dried in an oven at 393 K for 2 h and then
calcined at 573 K for 5 h.
3. Results and discussion
3.1. Catalyst characterization
Both parent BNT and 20% DTP/BNT samples showed XRD pattern
which is a characteristic of amorphous material. 20% DTP/BNT sample
did not show any crystalline phase of bulk DTP, indicating an excellent
dispersion of DNT on BNT after impregnation.
Table 1 presents BET surface areas and NH3-TPD results of various
solid acid catalysts. As can be seen from Table 1, BET surface area
of bulk DTP obtained was 8 m2/g while that of BNT was 242 m2/g
which decreased to 151 m2/g after impregnation of 20% DTP. This
confirmed that DTP was well dispersed (non-porous material) on BNT
support after impregnation. The BET surface areas of various catalysts
decreased in the following order: H-β-zeoliteNBNTN20% DTP/
BNTNmontmorillonite KSF/ONDTP.
Fig. 1(a–c) shows the NH3-TPD profiles of BNT, 20% DTP/BNT and
bulk DTP catalysts, while FT-IR pyridine adsorption spectra of
various solid acid catalysts are presented in Fig. 2. Among various
catalysts, bulk DTP showed the highest concentration of acid sites
[163.8 micromoles NH3 per surface area (μmolS−1NH3), Table 1]
having a very sharp desorption peak of strongly chemisorbed
ammonia at 925 K [22]. Since bulk DTP exhibit purely Brønsted
acidity [23], the peak appeared at 925 K having high acid strength
could be assigned to Brønsted acid sites. This was further confirmed
by FT-IR pyridine adsorption spectrum of bulk DTP (Fig. 2c) which
showed a peak at 1545 cm− 1 corresponding to Brønsted acidity.
NH3-TPD of parent BNT also showed two signals appearing as low
(425 K) and high (825 K) temperature peaks, which could be assigned
to Brønsted and Lewis acid sites respectively [24] and total concentra-
tion of acid sites was estimated as 4.9 μmolS−1NH3. Interestingly, the
total concentration of acid sites of parent BNT increased significantly
from 4.9 to 11.6 μmolS−1NH3 after impregnation of 20% DTP loading
2.3. Characterization
X-ray powder diffraction patterns have been recorded on a Rigaku,
D-Max III VC model, using nickel-filtered CuKα radiation. The samples
were scanned in the 2θ range of 1.5–80°. BET surface areas have
been measured by means of N2 adsorption at 77 K preformed on a
Quantachrome CHEMBET 3000 instrument. TPD measurements were
carried out on a Quantachrome CHEMBET 3000 TPR/TPD instrument.
TPD measurements were carried out by: (i) pre-treating the samples
from room temperature to 473 K in a flow of nitrogen; (ii) adsorption
of ammonia at room temperature; (iii) desorption of adsorbed
ammonia at 10 K min−1 starting from the adsorption temperature
to 973 K. Brønsted and Lewis acid sites were determined by ex-situ
FT-IR spectroscopy with chemisorbed pyridine. For this purpose,
catalyst samples were dried at 473 K for 2 h and then saturated with
pyridine vapors in a desiccator containing pyridine. Physically
adsorbed pyridine was removed by heating the samples at 393 K
for 2 h in a continuous flow of nitrogen. FT-IR spectra of the samples
were recorded on a Shimadzu (Model-820 PC) spectrophotometer
under DRIFT (diffuse reflectance infrared Fourier transform) mode.
31P-CPMAS NMR spectra were measured at room temperature on a
Bruker TOPSPIN AVANCE-300 spectrometer equipped with a Bruker
MAS-4 BLCP probe using phosphoric acid as a reference compound for
the calibration in NMR. The probe head, rotor (4 mm diameter), and
sample tubes were made totally moisture free and 0.1 g samples of
catalysts were weighted separately and 300 Mz 31P-CPMAS NMR
was recorded. Data acquisition and processing were done by using
TOPSPIN software provided by Bruker.
2.4. Activity measurement
Table 1
The hydroxyalkylation of p-cresol with formaldehyde was carried
out in a magnetically stirred glass reactor (capacity 50 mL) fitted with
a reflux condenser and an arrangement for temperature control. In a
typical experiment, p-cresol (42.5 mmol), formaldehyde (8.5 mmol),
toluene (12 cm3) and catalyst (0.03 g/cm3) were added to the reactor,
which was then heated to a 353 K for 1 h. The product yield
(sum of DAM and trimer based on formaldehyde) and selectivity
were determined with an HP6890 series GC System (Hewlett
Packard) coupled with FID detector and capillary column (HP-1
capillary column, 30 m length×0.32 mm i.d.). The products were
identified by 1H-NMR, 13C-NMR and by GC-MS.
Textural properties of solid acid catalysts.
Catalysts
SBET
NH3 adsorbed
TPD of NH3 (%)
distribution of acid sites
(m2/g)
(μmolS−1
)
Region I
Region II
(LT-peaks)
(HT-peaks)
BNT
20% DTP/BNT
DTP
Montmorillonite KSF/O
H-β-zeolite
242
151
8
131
650
4.9
11.6
163.8
15.5
38
40
35
25
63
62
60
65
75
37
11.8

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A.C. Garade et al. / Catalysis Communications 11 (2010) 942–945
Fig. 1. Ammonia-TPD profile of (a) BNT (b) 20% DTP/BNT and (c) DTP.
(Figs. 1b and 2b), mainly due to the enhancement of Brønsted acid sites
that was also evident from a very intense peak appearing at 1545 cm−1
as compared to a weak peak at 1445 cm−1 corresponding to Lewis
acidity. This alteration in acidity could be due to the surface modification
of BNT by interaction of bulk DTP with its surface hydroxyl groups
similar to that of silica [25,26] which was further confirmed by 31P-
CPMAS NMR analysis as discussed later.
Montmorillonite KSF/O also showed two types of acid sites having
maxima at 425 K and 675 K with highest total acid sites concentration
as 15.5 μmolS−1NH3 (Table 1). In case of montmorillonite KSF/O,
concentration of acid sites at 675 K was 3 times (75%) higher than
that of weak acid sites (25%) at 425 K. As can be seen from Fig. 2d,
montmorillonite KSF/O showed a single pyridine desorption peak at
1545 cm−1 corresponding to purely Brønsted acid sites. This indicates
that two desorption peaks at 425 K and 675 K observed in ammonia-
TPD plot of montmorillonite KSF/O were due to the Brønsted acid sites
having different acid strengths. H-β-zeolite showed higher concen-
tration of acid sites (63%) in a low temperature region (425 K) having
total concentration of acid sites of 11.8 μmolS−1 NH3. These acidic
sites in a low temperature region could be due to Lewis acidity [27]
which was further confirmed by pyridine-IR analysis of H-β-zeolite
which showed a very intense peak at 1445 cm−1 (Fig. 2e).
Fig. 3. 31P-CPMAS NMR of (a) DTP and (b) 20% DTP/BNT.
co-ordination of PO4 in the Keggin unit [28,29]. Interestingly, 20%
DTP/BNT showed a broad signal which shifted towards downfield
from −15.61 to −13.7 ppm after impregnation of 20% DTP on it. This
broadening and downfield shifting of signal could be due to the strong
interaction of protons of bulk DTP with surface hydroxyl groups of
BNT similar to that observed in case of silica and alumina [30–32]. This
interaction leads to the localization of DTP protons thereby increasing
their proximity to the Keggin anion because of the decrease in proton
mobility [30]. Such surface as well as acidity modification of BNT due
to DTP impregnation enhanced its catalyst activity as discussed below.
Fig. 3(a, b) shows 31P-CPMAS NMR of DTP and 20% DTP/BNT. DTP
showed a sharp signal at −15.61 ppm that corresponds to tetrahedral
3.2. Catalyst activity
In all the activity measurement experiments, the molar ratio of
p-cresol to formaldehyde was kept at 5 which was found to be
optimum, according to our earlier work [18]. Among the various
catalysts studied for the hydroxyalkylation of p-cresol (Fig. 4),
parent BNT showed 49% product yield with 96% and 4% selectivity to
DAM and carbinol respectively, while bulk DTP showed 72% product
yield with 65% selectivity to DAM. The lower DAM selectivity with
bulk DTP was due to its high acidity (163.8 μmolS−1 NH3) as well as
due to presence of only Brønsted acid sites having high strength
(65%) in a high temperature region which facilitates the formation
of a trimer by reaction of initially formed DAM with formaldehyde
followed by p-cresol. Some undetected tarry products were also
observed with bulk DTP which resulted into lowering in the mass
balance (80–85%). Interestingly, surface modified 20% DTP/BNT
catalyst showed the highest product yield (95%) with 94% selectivity
Fig. 2. FT-IR pyridine adsorption spectra of various percentages of various solid acid
catalysts.

A.C. Garade et al. / Catalysis Communications 11 (2010) 942–945
945
necessary for achieving highest selectivity to DAM than Lewis acid
sites.
Catalyst recycle experiments were carried out at 353 K with
p-cresol to formaldehyde mole ratio of 5 using 20% DTP/BNT catalyst
and the results obtained are presented in Fig. 5. The catalyst was
found to retain its activity even after the third recycle experiment
showing 95% product yield. The selectivity to DAM (94%) remained
constant for all the three recycle experiments.
4. Conclusions
Among various solid acid catalysts studied, surface modified
BNT (20% DTP/BNT) catalyst showed an excellent activity (95%
product yield with 94% selectivity to DAM) for the hydroxyalkylation
of p-cresol. The increase in total acid sites concentration especially
Brønsted acid sites of 20% DTP/BNT from 4.9 to 11.6 μmolS−1 NH3 was
found to be responsible for its high activity and selectivity. 31P
(CPMAS) NMR studies showed the broadening and downfield
shifting of signal from −15.61 to −13.7 ppm after impregnation of
20% DTP on BNT due to a strong interaction of protons of bulk DTP
with surface hydroxyl groups of BNT. The 20% DTP/BNT catalyst was
found to retain its original activity even after 3rd recycle experiment.
Fig. 4. Catalyst screening for hydroxyalkylation of p-cresol. Reaction conditions: p-cresol,
42.5 mmol; formaldehyde, 8.5 mmol; mole ratio of p-cresol to formaldehyde, 5; catalyst
concentration, 0.03 g/cm3; temperature, 353 K; time, 1 h; solvent, toluene (12 cm3).
Acknowledgment
One of the authors (ACG) thanks the University Grant Commission
(UGC) New Delhi, for the award of a senior research fellowship.
to DAM. The highest product yield with 20% DTP/BNT catalyst was
due to the increase in the total concentration of acid sites especially
that of Brønsted acid sites, from 4.9 to 11.6 μmolS−1 NH3 and the
stabilization of intermediate carbocation formed during the reaction
of heteropolyanion of DTP that also facilitates the formation of DAM
[33]. Montmorillonite KSF/O showed 47% product yield, with a lower
selectivity (76%) to DAM due to the predominant formation of a
trimer (24%) facilitated by its high acidity (15.5 μmolS−1 NH3) and
the presence of Brønsted acid sites (75%) in a high temperature
region. Although, H-β-zeolite showed the total acid sites concen-
tration (11.8 μmolS−1 NH3) similar to 20% DTP/BNT, however lower
product yield (78%) with 89% selectivity to DAM was observed with
a substantial formation of carbinol (8%) due to the presence of more
Lewis acid sites (63%) in a low temperature region. Catalyst activity
results showed that Brønsted acid sites in optimal concentration are
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Fig. 5. Catalyst recycle experiment. Reaction conditions: p-cresol, 42.5 mmol; formal-
dehyde, 8.5 mmol; mole ratio of p-cresol to formaldehyde, 5; catalyst concentration,
0.03 g/cm3; temperature, 353 K; time, 1 h; solvent, toluene (12 cm3).