Sulfated zirconia: An efficient catalyst for the Friedel-Crafts monoalkylation of resorcinol with methyl tertiary butyl ether to 4-tertiary butylresorcinol

Article abstract of DOI:10.1039/c9nj01311c

Friedel-Crafts alkylation of resorcinol with methyl tertiary butyl ether was carried out over sulfated zirconia (SZ) catalysts in the liquid phase. The SZ catalysts were synthesized by an impregnation method with different sulfur amounts and characterized by XRD, FT-IR, nitrogen sorption, XPS, SEM, pyridine-FTIR, and NH₃-TPD. The effect of the sulfur loading on the total acidity and catalytic activity was investigated. The influence of the nature of the solvents on the alkylation reaction was inspected in terms of their acceptor and donor numbers. The sulfur loading, amount of solvent, temperature, catalyst amount, mole ratio and reusability of the catalyst were examined. The SZ catalyst synthesized by impregnating 1 N sulfuric acid was found to be highly selective for the monoalkylation to 4-tertiary butyl resorcinol (72%) with a resorcinol conversion of ～70%. The catalyst was recycled thrice with a negligible decrease in the yield for 4-tertiary butylresorcinol. The SZ exhibited the best performance at low temperature (60 °C) among the different types of solid acid catalysts studied so far.

Full text of DOI:10.1039/c9nj01311c

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Sulfated Zirconia an Efficient Catalyst for the Friedel-
Crafts Monoalkylation of Resorcinol with Methyl
tertiary butyl ether to 4-Tertiary butylresorcinol
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Vijaykumar S Marakatti,a Shivanna Marappa,b Eric M Gaigneauxa*
aInstitute of Condensed Matter and Nanosciences (IMCN), Molecular Chemistry,
Materials and Catalysis (MOST), Université catholique de Louvain (UCLouvain),
Place Louis Pasteur 1, L4.01.09, B-1348 Louvain-la-Neuve, Belgium.
bJawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur,
Bangalore, 560064, India.
*corresponding author: eric.gaigneaux@uclouvain.be
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Abstract
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Friedel-Crafts alkylation of resorcinol with methyl tertiary butyl ether was carried out over
sulfated zirconia (SZ) catalysts in the liquid phase. The SZ catalysts were synthesized by
impregnation method with different sulfur amounts and characterized by XRD, FT-IR, Nitrogen
sorption, XPS, SEM, Pyridine-FTIR, and NH3-TPD. The effect of sulfur loading on the total
acidity and catalytic activity was investigated. The influence of the nature of the solvents on the
alkylation reaction was inspected in terms of their acceptor and donor numbers. The sulfur
loading, amount of solvent, temperature, catalyst amount, mole ratio and reusability of catalyst
were examined. The SZ catalyst synthesized by impregnating 1N sulfuric acid was found to be
highly selective for the monoalkylation to 4-tertiary butyl resorcinol (72%) with the resorcinol
conversion of ∼70%. The catalyst was recycled thrice with a negligible decrease in the yield for
4-tertiary butylresorcinol. The SZ exhibited the best performance at low temperature (60 °C)
among the different types of solid acid catalysts studied so far.
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Keywords: sulfated zirconia; resorcinol; 4-tertiary butyl resorcinol, Friedel-Crafts reaction
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1. Introduction
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Alkylation of aromatics is one of the important reactions in organic chemistry due to its
wide application in the petrochemicals, fine chemical, perfume, dye and pharmaceutical
industries.1-2 Among them, Friedel-Crafts butylation of resorcinol to produce mono-tert butylated
products is of the most significant due to its involvement in the synthesis of antioxidants,
polymer stabilizers, therapeutics, and organic synthetic applications.2-4 The alkylation of
resorcinol with methyl tertiary butyl ether (MTBE) gives mixture of alkylated benzenediols and
ethers. Being thermodynamically favoured, the dialkylated 4,6-di-tert-butyl resorcinol (4,6-
DTBR) is usually the major product.5 However, the 4-tert-butylresorcinol(4-TBR) is of particular
interest as antioxidants in food preservative and in treatment of skin diseases. Therefore, selective
synthesis of 4-TBR at high conversion of resorcinol is a difficult and challenging task for the
catalyst designer.6
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Traditionally, homogeneous Lewis (AlCl3, FeCl3) and Brönsted acids (H2SO4, H3PO4)
catalysed the reaction.4 However, the limitation and hazardous nature of these conventional acid
catalysts have been replaced by the use of heterogeneous catalysts. There are few solid acid
catalysts that have been studied for the reaction such as montmorillonite clay, zeolites, ZrO2
supported phosphotungstic acid, dodecatungstophosphoric acid supported on K-10, sulfated
mesoporous Al2O3 and heteropolyacid supported mesoporous materials.1-6 However, the
reactions over these catalysts were carried out at high temperature and/or pressure, with moderate
yields for 4-TBR .
Over the past two decades, application of anion modified metal oxides for catalysis have
received more attention.7-8 ZrO2 has been commonly utilized as a support. However, robust acid
2-
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sites can also be generated on ZrO2 by modifications with WO4 , SO4 and MoO4 ions.8
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Among these, SO4 modified ZrO2 catalyst attracted much attention because of its high activity
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in alkane isomerization at low temperature.9 The H2SO4 treatment of ZrO2 generates Brönsted
and Lewis acid sites on it, and these active sites induce a super acidity in the resulting sulfated
zirconia (SZ). Thanks to their acidic nature, SZ were further exploited in bulk petrochemical
processes like alkylation, acylation, isomerization and cracking. Simple synthesis procedure,
excellent thermal stability, resistance to thermal extrusion, and low cost offer several advantages
over other solid acid catalysts. Thus, continuous development of this catalyst generated interest in
the synthesis of fine and specialty chemicals like nopol, 1.4-dioxane, coumarins, organic esters,
N, N’-diphenylene diamines etc.7, 10-13 The SZ catalyst was utilized as an acid catalyst for the
various Friedel-Crafts alkylation reactions such as alkylation of diphenyl oxide with benzyl
chloride, alkylation of guaiacol and p-Cresol with cyclohexene, alkylation of 4-methoxyphenol
with MTBE and alkylation of benzene with benzyl chloride.7 The alkylation of catechol and
anisole with MTBE was also studied over SZ catalyst but a low catalytic activity was observed.4
However, to the best of our knowledge, SZ catalyst has not been studied so far for alkylation of
resorcinol with MTBE to produce 4-TBR.
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Recently, we reported heteropolyacid supported silica as an active catalyst for the
alkylation of resorcinol.6 In continuation of our investigation of simple, efficient and active
catalyst for this reaction, herein we report the application of SZ as a solid acid catalyst for the
conversion of resorcinol to 4-TBR. In this study, SZ catalyst with different sulfur loadings was
synthesized and studied for the alkylation reaction. The effect of sulfation, the nature of the
solvent used and the recyclability of the catalysts were investigated. The physicochemical
properties of the catalysts were correlated with the activity of the alkylation reaction.
2. Materials and Methods
2.1. Catalyst Preparation
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SZ catalysts were prepared by the method as reported in the literature.12 Zr(OH)4, from the
MEL Chemicals, Manchester, UK, was treated with different concentrated solutions of H2SO4
and calcined at high temperature. In a typical procedure, 10 g of Zr(OH)4 was stirred into the 100
ml of a H2SO4 solution of the desired concentration (0.5N, 1N, 2N, 3N, 4N). After stirring for 60
min, the catalyst was filtered, washed with water, dried and finally calcined at different
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temperatures (500 and 650 C) for 4 h. Obtained catalysts are indicated by the term SZ-X-T,
where X stands for the concentration of H2SO4 used and T stands for the calcination temperature.
SZ-T indicates the whole series of catalysts treated with different concentrations of H2SO4 but all
calcined at the same temperature T. For comparison purpose, 10% Ga-BEA zeolite and 20%
DTP/K10 catalysts were synthesized by the literature method.2-4
2.3. Catalyst characterization
The phase purity and crystalline properties of all SZ catalysts were determined by the X-ray
diffraction on a Siemens D5000 diffractometer equipped with a Cu Kα source. The samples were
analysed in the 2 theta range of 5 and 80° with a scan rate of 0.02°/s. The sulfur present on the
catalyst was determined by the CHNS analysis using Thermo Finnigan FLASH EA 1112 CHNS
analyzer.14
Specific surface area, pore volume, and pore size distributions were obtained through N2
sorption experiments performed using a Micrometrics Tristar 3000. Before the measurements,
catalysts were degassed overnight under vacuum (6ꢀPa) at 150ꢀ°C. The measurements were
performed at −196ꢀ°C and with relative pressures in the range of 0.01–1.00 (p/p0). The specific
surface area was calculated from the adsorption isotherm in the p/p0 relative pressure range of
0.05–0.30 using the Brunauer–Emmett–Teller method (BET). The pore size distribution, mean
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pore diameter, and pore volume were calculated by the Barrett–Joyner–Halenda method (BJH)
using the data from the isotherm desorption branch.6
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Surface acidity measurements of the SZ catalyst were done by pyridine adsorption
technique as reported earlier.6 The wafers of SZ catalyst were prepared, weighed and then placed
in a sample holder inside a Pyrex cell specially designed for the controlled heating of the sample
under vacuum and equipped with an optical NaCl window. In a typical measurement, the catalyst
was pre-treated at 300ꢀ°C under vacuum (between 10−4 and 10−5ꢀPa) for 2ꢀh in order to remove
impurities from the surface. After cooling under vacuum, 1000ꢀPa of pyridine was sent at room
temperature in the cell and adsorption was allowed for 30ꢀmin. The sample was then outgassed at
10−5ꢀPa at 100ꢀ°C. FT-IR spectra were taken in transmission mode before and after pyridine
adsorption, using a spectrometer IFS55 Equinox (Bruker) equipped with a DTGS detector. The
spectra were recorded with 100 scans between 400 and 4000ꢀcm−1 with a resolution of 4ꢀcm−1.
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NH3 adsorption and subsequent temperature-programmed desorption (NH3-TPD) were
performed in a Hiden CATLABPCS combined micro reactor and mass spectrometer (MS) system
as reported earlier.15 Experiments were performed according to the following three steps: (1)
stabilization of a flow of pure He (30 mL/min) at 50 °C for 25 min in order to check the
sensitivity factor of He and therewith to determine that of NH3 during the later TPD (through the
previously calibrated He/NH3 sensitivity factors ratio) and subsequent heating to 150 °C (still
under 30 mL/ min of pure He); (2) adsorption of NH3 at 150 °C for 1.5 h from a 95:5 He/NH3
flow (25 mL/min) mixed with a flow of pure He (5 mL/min); (3) flush at 100 °C under pure He
(30 mL/min) for 2.5 h in order to eliminate physisorbed NH3 and subsequent TPD from 100 to
650 °C (10 °C/min, still under 30 mL/min of pure He) in order to desorb chemisorbed NH3. XPS
measurements were carried out with a SSX 100/206 spectrometer from Surface Science
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Instruments as reported earlier.15 The pressure within the analysis chamber was about 10−6 Pa.
The zone analysed was about 1.4 mm2, and the pass energy was set at 150 eV for the general
spectrum and at 50 eV for the elementary spectra. An electron gun set at 8 eV and a nickel grid
placed 3 mm above the surface of the samples were used to stabilize the charge. The following
sequence of spectra was recorded: general spectrum, C 1s, O 1s, S 2p and Zr 3d. The C-(C, H)
component of the C 1s peak of adventitious carbon was fixed at 284.8 eV to set the binding
energy scale. When required, spectra were decomposed with the Casa XPS program (Casa
Software Ltd., UK) with a Gaussian/Lorentzian product function after subtraction of a nonlinear
baseline. For the quantification of the elements, the sensitivity factors and acquisition parameters
provided by the manufacturer were used.
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2.4. Catalyst activity studies
The alkylation of resorcinol with MTBE was carried out in the liquid batch reactor. In a typical
reaction, 2.2 g of resorcinol was dissolved in 4.8 ml of MTBE, 0.9 ml of o-xylene (internal
standard) and 2 ml of solvent in a 100 mL two neck round bottom flask fitted with the condenser.
The reactor was placed in an oil bath under magnetic stirring (1200 rpm) and heated at 60 °C
under atmospheric pressure. Then, 0.22 g of catalyst was added. The reaction was monitored by
GC–FID analysis. For the sampling, 10 µl of the reaction mixture was mixed with 1.8 mL of
methanol and was analysed by gas chromatography (Shimadzu-2041, FID detector) equipped
with RTX-5 column (0.25 mm I.D and 30 m length). The oven temperature programme was 60
°C (1 min), 60–180 °C (10 °C/min), 180 °C (5 min). All the products were identified by injecting
the standard sample followed by the NMR analysis as done in our previous work.6 The
conversion and selectivity were directly calculated from GC peak areas in mol% using a multi-
point calibration curve through the internal standard method. The mass balance was also
calculated after the reaction and was found to be in the range of 95-98 %.
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3. Results and discussion
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3.1. Characterization of catalyst
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The SZ catalyst calcined at 500°C shows no crystalline phases, whereas the catalyst
calcined at 650 °C exhibits some crystallinity in the XRD analysis. (Figure S1 and Figure 1)
The low crystallinity of sample calcined at 500°C is due to the presence of amorphous sulfate that
has dispersed in the matrix.16 The XRD pattern of ZrO2 and SZ catalysts calcined at 650 °C are
shown in Figure 1. The ZrO2 displayed peaks at 2θ = 24, 28.2, 31.5 and 30.2, 34.5, 35.4, 50.2,
60.2, 62.9 due to the presence of monoclinic and tetragonal phases, respectively. However, the
SZ catalysts exhibited predominant peaks only due to the tetragonal phase, indicating the strong
influence of sulfation on the modification of thermodynamically stable monoclinic to tetragonal
phase, as observed in the literature.7, 10
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Figure 1. XRD patterns of ZrO2 and sulfated zirconia catalysts calcined at 650 °C.
The amount of sulfur in all the SZ catalysts was determined by the CHNS analysis and are
tabulated in Table 1. There is a linear correlation between the concentration of H2SO4 used and
the amount of sulfur present on ZrO2 calcined at 650 °C, however no correlation existed for the
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catalyst calcined at 500 °C. This could be because SZ catalyst calcined at a lower temperature of
500 °C
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Table 1. Physio-chemical properties and catalytic activity of SZ catalysts in alkylation of resorcinol.
Catalyst
S
(wt.
%)
S/Zr
(At
%)
Specific
Surface
area
Average Average
Acidity Conversion Selectivity
Pore size
(Å)
Pore
volume
cm3/g)
(μ
of
mol/g)
resorcinol
(mol %)
4,6-
DTBR
TON
Others (10-3)
(m2/g)
4-TBR
R-
MTBE
SZ-0.5-500 4.79
0.25
0.25
0.26
0.26
0.29
0.11
0.14
0.13
0.13
0.12
0.0
191
184
154
155
34
109
119
113
103
68
52
52
46
61
158
51
52
53
61
67
0.162
0.107
0.119
0.144
0.098
0.141
0.154
0.146
0.165
0.105
0.095
0.168
406
404
358
340
148
182
176
165
152
115
0
61
64
53
44
43
57
58
57
55
51
0
79
77
76
77
75
71
72
75
75
79
0
16
20
8
2
2
10
9
13
1
1
2
2
6
0
2
2
6
5
6
1
1
1
2
5
0
0
9
8
SZ-1-500
SZ-2-500
SZ-3-500
SZ-4-500
4.73
5.12
5.82
4.40
12
15
35
21
22
24
27
38
0
9
6.
27
25
22
21
10
0
SZ-0.5-650 1.06
SZ-1-650
SZ-2-650
SZ-3-650
SZ-4-650
ZrO2-650
Zr(OH)4
1.15
1.34
1.39
1.53
0.0
30
306
120
49
0.0
0.0
0
0
0
0
0
0
Reaction Conditions: Catalyst amount = 0.22-gram, Temperature = 60 °C; Resorcinol= 2.2-gram, MTBE = 7.2 ml, time= 10 h, * TON is
calculated by mmol of resorcinol converted per acid site determined byTPD-NH3.
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contained both physically and chemically adsorbed sulfur in large quantity (4-5 wt %), whereas
SZ calcined at a high temperature of 650 °C exhibited lower sulfur content (1-2 wt %), with the
sulfur in majority likely present in a chemically adsorbed state. The surface S/Zr ratio determined
by the XPS analysis further supports the fact that SZ-500 catalysts possess double amount of
sulfur in comparison to SZ-650 catalysts. (Table 1) Moreover, for SZ-650 series of catalysts (SZ-
0.5N to SZ-4N) the S/Zr ratio (determined by XPS) - almost remained the same, but CHNS
analysis reveals an increase of the total sulfur content from SZ-0.5N to SZ-4N, indicating the
presence of extra sulfur in the bulk phase.12
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The N2 adsorption-desorption isotherm of the SZ catalysts calcined at 650 °C showed the
type-IV isotherm with H2 type hysteresis loop confirming the presence of mesopores. (Figure 2)
z
Figure 2. Nitrogen adsorption-desorption isotherms and pore size distribution of SZ catalysts
calcined at 650 °C.
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The SZ calcined at 500 °C did not exhibit any hysteresis indicating the absence of mesopores.
(Figure S2) These above results indicate that high temperature is required for the generation of
mesoporosity in the ZrO2. The surface area of ZrO2 increased considerably after sulfation due to
the change in its textural properties. However, SZ catalyst showed lower surface area compared
to Zr(OH)4, due to the sulfate loading.(Table1) The specific surface area of SZ-650 catalysts
decreased with the increase in the sulfur amount, except for the SZ-0.5N. This decrease in surface
area and change in porosity could be due to a sulfate migration into the bulk phase of ZrO2.17 The
average pore diameter increased with the increase in the sulfate loading as reported by other
researchers. 18-19
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FT-IR analysis of all the SZ catalysts, revealed the presence of sulfate ions and bonding
of sulfate ions with ZrO2 as shown in Figure 3. (Figure S3) The FT-IR analysis in the fingerprint
region showed the presence of 997 cm−1 peak due to the symmetric (ʋ1) vibrations of S-O
whereas
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Figure 3. FT-IR spectra of sulfated zirconia catalysts calcined at 650 °C.
1034, 1070, 1144 and 1273 cm−1 peak were observed due to the antisymmetric (ʋ3) vibrations of
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O- S-O bonds. The results indicate the presence of sulfate ions on the surface of ZrO2 in a
bidentate fashion.12
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XPS analyses were performed for all SZ catalysts to verify the presence of sulfate species,
to estimate the S/Zr ratio at the surface and the oxidation state of S. The values are presented in
Table 1. No impurities were found, except the contamination carbon inherent to the analysis. All
the SZ catalysts show the peak associated to the S2p3/2 (Binding energy = 169.3 eV) and S2p1/2
(Binding energy =170.4 eV), confirming the presence of sulfur with an oxidation state +6 as
observed for sulfate moieties. (Figure S4) The binding energies of Zr peaks 3d3/2 and 3d5/2 in
ZrO2
Experimental
Decomposed
Decomposed
Fitted
Zr-3d5/2
Zr-3d3/2
185.3 eV
182.9 eV
ZrO2
SZ-1-650
SZ-2-650
SZ-3-650
SZ-4-650
178
180
182
184
186
188
Binding Energy (eV)
Figure 4. Zr 3d5/2 and 3d3/2 XPS spectra of the SZ-650 catalysts.
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are typically of 181.8 eV and 184.2 eV, respectively. Whereas Zr doublet in SZ catalysts
appeared at binding energies of 182.9 eV (3d3/2) and 185.3 eV (3d5/2).(Figure 4) The shift of
around 1.1 eV toward higher binding energy observed for SZ catalysts is due to inductively
withdrawal of electron density from ZrO2 by sulfate groups.20
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The use of IR spectroscopy to detect adsorbed pyridine allows distinguishing the Brönsted
and Lewis acidity present on the SZ catalysts.21 The intensity of Brönsted (B) and Lewis (L) acid
sites obtained from the absorbance at 1541 and 1446 cm−1 respectively and corresponding B/L
ratios were measured. FTIR pyridine adsorption spectra of SZ catalysts calcined at 650 °C are as
shown in Figure 5.
Figure 5. Acidity measurement of SZ catalysts calcined at 650 °C by pyridine FT-IR analysis (B-
Brӧnsted acid site, L-Lewis acid site).
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The SZ showed a peak at 1541 cm−1 and 1446 cm−1 indicating the presence of both Brönsted and
Lewis acidity. It is well known that sulfate groups inductively withdraw the electron density from
ZrO2, as a result Lewis acid sites are created on ZrO2. The Brönsted acid sites are due to protons
generated by sulfate groups. The B/L ratio of the SZ-650 catalysts increased from SZ-0.5N to SZ-
2N and later decreased as seen on Figure 5. (Table S1) The decrease of B/L ratio for SZ-3N and
SZ-4N could be due to the formation of polysulfate groups by the excessive sulfur on the ZrO2.12
To find the amount and strength of acid sites, a temperature programmed desorption NH3
profile of all the SZ samples was measured as shown in Figure 6. The marked initial desorption
in the temperature range 150-300 °C is due to the NH3 with medium acid sites. The desorption
peak of NH3 in the range 300-500 °C is due to the strong acid sites. The total amount of acid
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Figure 6. TPD-NH3 profiles of sulfated zirconia catalysts calcined at 650 °C.
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sites measured were tabulated in Table 1. The SZ-500 catalysts exhibited higher acidity
compared to the SZ-650 catalysts due to their higher surface area. (Figure S5) The number of
total acid sites decreased from SZ-0.5N to SZ-4N. (Table 1) For SZ catalysts with low sulfur
content (SZ-0.5), the sulfation takes place on the O-Zr-O sites where both Brönsted and Lewis
acidities are generated by sulfate and -Zr- sites respectively resulting in high acidity.22 As the
sulfur content increased (SZ-4N), greater number of sulfate groups masks the Lewis acidic sites
(O-Zr-O); the migration of sulfate to the bulk of the catalyst also contributes decreasing the total
acidity as measured by pyridine adsorption. Excess sulfate groups on the surface of zirconia in
the form of isolated and poly-sulfates do not contribute to acidity.22 To check the morphology of
the synthesized SZ-1-650 catalyst SEM images were taken as shown in Figure S6. The images of
SZ-1N-650 showed the presence of large bunches of particles with average size of 1-2 µm; high
resolution of these particles exhibits agglomerated small crystallites of 80-100 nm in size.
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3.2. Catalytic activity studies
3.2.1. Catalytic activity over different SZ catalysts.
Alkylation of resorcinol to 4-TBR is a two-step reaction. The first step is the formation of
R-mTBE; in the second step R-mTBE isomerizes to the 4-TBR. (Scheme1)
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Scheme 1. Schematic presentation of resorcinol alkylation with methyl tertiary butyl ether with
different products.
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In this reaction pathway, R-MTBE acts as an intermediate. The formed 4-TBR is more
reactive than the resorcinol and hence further undergoes alkylation to 4,6-DTBR. 2-TBR is also
formed in small amount due to the steric hindrance at the C2 position of resorcinol making the
carbocation attack difficult. (Scheme1)
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The resorcinol alkylation was investigated for SZ catalysts with different sulfur loadings
and compared with its precursor. Zr(OH)4 and ZrO2 showed no activity due to the absence of any
strong acid sites. The catalytic activity of SZ catalysts with different amounts of sulfur and
calcination temperatures are shown in Table 1. The SZ-650 catalysts with low sulfur content, low
surface area and low acidity exhibited almost similar activity compared to the SZ-500 catalysts
having high surface area, acid sites and sulfur content. (Table 1) This could be due to the more
marked presence of crystalline phase in SZ-650 catalysts than that of amorphous phase in SZ-500
ones. This confirms that conversion of resorcinol is influenced by the structural and surface
properties of the catalyst. The presence of crystalline tetragonal ZrO2 phase in the SZ-650 catalyst
has a major influence on the activity. The SZ-500 catalysts with amorphous ZrO2 are not Lewis
acidic and possess only Brӧnsted acidity due to the sulfates; whereas SZ-650 with their
crystalline ZrO2 possess both Lewis and Brӧnsted acidity. Hence, SZ-500 even with high surface
area, sulfur content and total acidity possess almost similar catalytic activity as that of SZ-650.
All the SZ-650 catalysts almost showed similar activity after the 10 h of reaction time. Therefore,
comparing their initial catalytic activity was more practical and is tabulated in Table S1. The
resorcinol conversion determined after 2 h of reaction time decreases with the increase of sulfur
content and decreases with total acidity from SZ-0.5N to SZ-1N.This indicates that the
conversion of resorcinol mainly depends upon the total number of acid sites. The TON were also
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higher for SZ-650 catalysts compared to the SZ-500 catalysts.(Table 1) The decrease in
resorcinol conversion with the decrease of total acidity was observed from SZ-0.5 to SZ-4 as seen
from Figure 7. Furthermore, the proportion of Brӧnsted and Lewis acidities of SZ-650 catalysts
did not correlate well with the activity or selectivity in the alkylation reaction. (Table S2) This
indicates that both Brӧnsted and Lewis acid sites catalyse the reaction. The SZ-1-650 catalyst
showed 58 % resorcinol conversion with 72 % of 4-TBR selectivity and 25% selectivity for 4,6-
DTBR. Hence, further, optimization studies were done using SZ-1-650 catalyst.
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SZ-1
SZ-2
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SZ-4
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SZ-650 Catalysts
Figure 7. Correlation of acidity of SZ-650 catalysts with conversion of resorcinol in alkylation
reaction.
3.2.3. Study on nature of solvents
The effect of different solvents in the alkylation of resorcinol over SZ catalysts was
examined. The reaction with SZ-1N catalyst without solvent gave a conversion of 58%. The use
of solvent was necessary in order to dissolve the solid resorcinol during the reaction. Hence,
reaction was carried out in different solvents. Results are shown in Table 2.
In order to correlate the solvent properties with the catalytic activity, the conversion of
resorcinol with the dialectic constant was plotted as seen in the Figure S7. Dielectric constant of
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solvent was varied from 2.0 (cyclohexane) to 36.7 (N, N-dimethyl formamide) but did not
correlate well with the catalytic activity. The solvent N,N-dimethyl formamide (DC=36.7) and
nitrobenzene (DC=34.8) with almost similar DC showed different conversion for resorcinol as
shown in Table 2.
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Table 2. Effect of solvent on resorcinol alkylation reaction.
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Solvent
Dielectri Acceptor Donor Conversion of
Selectivity (%)
c
constant
(DC)
--
Number numbe
Resorcinol
(%)
RmTB 2-TBR
4-
4,6-
Others
(AN)
r
(DN)
--
E
TBR DTBR
No solvent
--
--
57
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52
62
70
63
47
25
0
0.5
0.6
0.7
0.6
0.5
0.5
1.1
4.3
0
0.3
0.5
0.5
0.4
0.3
0.4
3.2
10
72
72
77
76
72
72
85
80
0
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21
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27
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4
0.6
0.6
0.5
0.4
0.4
0.4
0.3
0.8
0
MTBE
--
--
Cyclohexane
Benzene
1,2-Dicholoroethane
Nitrobenzene
1,4-dioxane
1,2-dimethoxyethane
N,N-Dimethyl
formamide
2.0
0
0
2.2
8.2
16.7
14.8
10.3
10.2
16
0
0
4.4
14.3
20.0
26.6
10.3
34.8
2.2
7.2
36.7
0
0
Reaction Conditions: catalyst =SZ- 1-650, catalyst amount = 0.22 g, temperature = 60 °C; resorcinol= 2.2-gram, MTBE=4.8 ml =, solvent = 2 ml,
time =10h.
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A further study on the effect of the solvent properties via their AN and DN led to a better
understanding of the role of the solvent in the reaction. AN and DN of different solvents were
reported by Gutman23. They are a measure of the acidic or basic nature of solvents. Subsequently,
these values have been accepted and used by several researchers.12-13, 24-26 Table 2 reveals that an
increase in the AN of the solvent (at constant DN = 0) from cyclohexane to 1,2-dichloroethane,
increases the resorcinol conversion from 52 to 70 %. At the opposite, an increase in DN (keeping
AN in its best range = 10-20) of the solvent from nitrobenzene to N,N-dimethylformamide,
decreased the conversion from 63 % to 0.
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From the above results, it is clear that solvents with high AN and low DN give a high
resorcinol conversion. A high AN for solvent represents a marked presence of Lewis acidic
properties, which improve the catalytic activity without affecting the acid sites of the catalyst.
The solvents with high DN are Lewis basic in nature, making that these solvents tend to interact
with the acidic sites of the SZ catalyst.27 As a consequence, solvents with high DN like N,N-
dimethylformamide and 1,2-dimethoxyethane preferentially adsorb on the acid sites of SZ
resulting in blockage of active sites. The other reason for the lower activity obtained in high
donor number (Lewis basic) solvents could be due to the stabilization of tertiary butyl
carbocation during the reaction. Stabilization of these carbocation will essentially lead to decline
the rate of reaction. Among all the solvents, 1,2-dichloroethane with high AN and low DN thus
showed excellent performance.
Furthermore, the effect of varying the amount of solvent was also carried out with the 1,2-
dicholoroethane. As the amount of solvent increased from 0 to 8 ml the conversion of resorcinol
dropped from 58 to 49 % likely due to the dilution effect but the selectivity for the 4-TBR
increased from 72 to 84 %. (Figure 8)
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conversion
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2-TBR
others
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Amount of solvent (ml)
Figure 8. Effect of amount of solvent (1,2-dichloroethane) on resorcinol alkylation.
3.2.2. Influence of reaction conditions
The effect of molar ratio resorcinol to MTBE during the catalytic reaction was studied at
60 °C using SZ-1N catalyst. The ratio resorcinol to MTBE was varied from 1:4 to 1:1 as shown
in Figure 9a. The conversion of resorcinol remained almost constant whatever the MTBE
concentration. However, as the Resorcinol: MTBE ratio decreased from 1:4 to 1:1, the selectivity
for the 4-TBR decreased at the same time selectivity for 4,6-DTBR increased. This indicates the
further alkylation of 4-TBR occurs more markedly at lower MTBE concentration. Based on the
above results, 1:3 mole ratio was selected for this reaction to get high conversion of resorcinol
(58 %) with good (72 %) selectivity for 4-TBR.
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Conversion
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mRTB
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2TBR
others
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others
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Amount of Catalyst ( wt %)
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conversion
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others
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Reaction Temperature( C)
Figure 9. Influence of reaction conditions. a) Effect of resorcinol: MTBE mole ratio: conditions: catalyst = SZ-
1N, catalyst amount = 10 wt%, temperature = 60 °C, time =10 h.(b) Effect of catalyst amount: conditions: catalyst = SZ-1N, resorcinol
= 2.2 g, MTBE =4.8 ml, temperature = 60 ◦C, time = 10 h. (c) Effect of reaction temperature: conditions: catalyst = SZ-1N, catalyst
amount = 10 wt%, resorcinol = 2.2 g, MTBE =4.8 ml, temperature = 60 ◦C, time = 10 h.
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The study of the effect of the catalyst amount on alkylation was conducted by taking different
weight % of SZ-1N catalyst with respect to resorcinol weight. As the catalyst amount was
increased from 5 to 20 wt%, the conversion of resorcinol increased from 45 to 66 % and
selectivity for 4-TBR dropped from 81 to 72 % respectively. (Figure 9b) The use of more
catalyst increased the secondary alkylation reaction. The 10 wt. % of catalyst was good enough to
produce selectively 4-TBR with good conversion of resorcinol.
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The effect of reaction temperature was also studied using SZ-1N catalyst with resorcinol
to MTBE mole ratio of 1:3 using 10 wt% of catalyst. The conversion of resorcinol increased with
the increase of reaction temperature as shown in Figure 9c. The selectivity for 4,6-DTBR
increases at higher temperature above 70 °C due to the predominant secondary alkylation
reaction.
An attempt to use different butylating agents such as tertiary butanol (TBA) and di
tertiary butyl dicarbonate (DTBDC) was finally made and are tabulated in the Table S3. The
TBA produces water as by product, which resulted in a lower conversion likely as due to the
blockage of the catalyst acidity. Whereas MTBE and DTBDC produce methanol and CO2 as by-
products, respectively, without influencing the catalyst, hence relatively good conversion of
resorcinol was observed over these alkylating agents.
3.2.6. Leaching and recycling test
Leaching test was carried out for alkylation reaction to investigate the stability of the
active sites at the surface of the catalysts into the reaction conditions. The reaction was carried
out using SZ-1N catalyst at reaction temperature of 60 °C with resorcinol to MTBE mole ratio of
1:3. Reaction was stopped after 2 h (35 % conversion) and the hot reaction mixture was filtered.
The reaction was continued with the filtrate; the conversion remained unchanged along the next
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10 hours, indicating that no leaching of active sites into the reaction medium had occurred during
the first part of the experiment. (Figure S8)
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As a recycling test, the catalyst SZ-1 was filtered after the reaction, washed with acetone
followed by drying in static air for 1 h at 120 °C and finally calcined in static air at 500 °C for 2
h. It was then used in the alkylation reaction with a fresh reaction mixture. The catalyst showed
good recyclability with a minimal decrease in activity after 3 cycles. (Figure 10).
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Resrcinol
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3
No of recycle
Figure 10. Recycle study of SZ catalyst in resorcinol alkylation reaction. Reaction Conditions:
catalyst amount = 0.22-gram, temperature = 60 °C; resorcinol= 2.2 g, MTBE = 7.2 ml, time= 10 h.
XRD of the catalyst after the third cycle revealed the still intense unchanged pattern of
tetragonal phase indicating the structural integrity of the catalyst (Figure S9). Furthermore, the
XPS spectra of fresh SZ-1N catalyst and after the third recycle showed a similar surface S/Zr
ratio (0.15), surface area (113 m2/g) and sulfur content (1.10 wt %), indicating that no leaching of
the sulfur from the surface and textural changes has occurred during none of the steps of the
experiment. (Figure S10 and Figure S11)
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Table 3 shows the comparison of catalytic activity of SZ-1N with other reported solid
acid catalysts from the literature.1-6 The 10% Ga-BEA zeolite and 20% DTP/K10 are the only
catalysts to exhibit 100 % selectivity for 4-TBR. However, for 20% DTP/K10 and Ga-Beta
catalysts, the
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8
Table 3 Comparison of catalytic activity of the SZ catalyst with other reported solid acid catalysts.
9
Conversion of
Selectivity (%)
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
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Catalysts
Resorcinol
References
RmTBE 2-TBR 4-TBR
4,6-
Other
(%)
70
4
DTBR
SZ-1-650
0.5
17
4
0.3
35
9
72
54
25
0.4
0
Present work
Present work
10%Ga-BEA
0
2
Acidified Montmorillonite K10
ZrO2-PTA
17
67
59
100
97
45
35
85
0
Present work
5
13
--
--
--
--
-
55
26
--
6
2
1
4
3
6
10%Ga-BEA
100
25
--
--
--
--
15
Acidified Montmorillonite K10
20% DTP/K10
--
75
--
--
100
78
ITQ-6 zeolite
--
--
4
--
HPW/SBA-15
3
52
26
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New Journal of Chemistry
Page 28 of 31
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reaction was carried out at high temperature of 150 °C and 80 °C, respectiveVlyiew.2A-r4ticle Online
DOI: 10.1039/C9NJ01311C
5
6
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8
Furthermore, the use of less abundant thus more costly metal like Ga is less attractive
process. The 10% Ga-BEA zeolite and 20% DTP/K10 catalysts exhibited lower resorcinol
conversion compared to SZ-1N-650 in the present reaction conditions. ITQ-6 zeolite showed a 45
% resorcinol conversion with a 78% selectivity to 4-TBR at 100 °C.3 The other catalysts such
as ZrO2 supported phosphotungstic acid (PTA), acidified montmorillonite K-10 and
heteropolytungstate (HPW) supported SBA-15 exhibited lower activity and selectivity in the
alkylation reaction compared to SZ-1N.1,5-6 SZ in the present work exhibited a good
resorcinol conversion of 70% with 72.5% of selectivity to 4-TBR at lower temperature (60
°C) than most of the previously reported catalysts tabulated in Table 3. The SZ catalyst
designed in the current study being simple, efficient and easily scalable, it makes it an
attractive catalyst for alkylation reaction.
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10
11
12
13
14
15
16
17
18
19
20
21
22
23
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4. Conclusions
Application of sulfated zirconia (SZ) as highly selective and reusable catalyst for the
synthesis of 4-tertiary butyl resorcinol from resorcinol and methyl tertiary butyl was reported.
The SZ showed the best catalytic performance at low temperature among different types of
acid catalysts. SZ calcined at 650 °C showed higher catalytic activity compared to the SZ-500
°C. The total acidity of SZ optimized by different concentrations of H2SO4 treatment
influences the acidity and the conversion of resorcinol. Treating ZrO2 with high concentration
of H2SO4 decreased the total number of acid sites in SZ. The presence of solvent with high
acceptor and low donor numbers is necessary for SZ catalyst to show high activity and
selectivity in alkylation reaction. The SZ synthesized by treating 1N H2SO4 was found to be
highly active (∼70 %), selective (72.5%) and reusable catalyst for the alkylation reaction.
Acknowledgement
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New Journal of Chemistry
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View Article Online
DOI: 10.1039/C9NJ01311C
The authors thank the FRS-FNRS for the Projet de recherche n° T.0064.16, and the
Postdoctoral position allocated to VSM from it. The authors also thank Francois Devred for
the technical support.
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References
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