Amorphous aluminosilicate catalysts for hydroxyalkylation of aniline and phenol
Transcript of Amorphous aluminosilicate catalysts for hydroxyalkylation of aniline and phenol
Amorphous aluminosilicate catalysts for hydroxyalkylation
of aniline and phenol
C. Perego *, A. de Angelis, A. Carati, C. Flego, R. Millini, C. Rizzo, G. Bellussi
EniTecnologie, via Maritano 26, I-20097 San Donato Milanese, Italy
Available online 18 April 2006
Abstract
Amorphous aluminosilicates with controlled porosity in the region of micro-pores (ERS-8, SA) and meso-pores (MCM-41, HMS and MSA)
have been compared as catalysts in the hydroxyalkylation of phenol with acetone and of aniline with formaldehyde. The comparison has also
included two commercial silica–alumina gels and H-Beta.
In the hydroxylakylation of phenol with acetone to bisphenol A (BPA), the catalysts have been compared by batch test in terms of activity and
selectivity. In the hydroxylakylation of aniline with formaldehyde, also the catalyst life has been investigated. As a general behavior, mesoporous
aluminosilicates have evidenced better catalytic activity, selectivity and longer catalyst life than both microporous ones and commercial silica–
alumina gels. In the hydroxyalkylation of aniline to methylenedianiline (MDA), mesoporous MSA have shown similar performances of H-Beta in
term of MDA yields, but lower catalyst life.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Amorphous; Aluminosilicates; Catalysts
www.elsevier.com/locate/apcata
Applied Catalysis A: General 307 (2006) 128–136
1. Introduction
The condensation of molecules containing carbonyl groups,
such as aldehydes and ketones, with aromatic compounds is
known as hydroxyalkylation. This reaction is widely applied in
the chemical industry to produce important commodities (e.g.
bisphenol A (BPA) and methylenedianiline (MDA)) and several
fine chemicals [1].
Industrial hydroxyalkylation processes are usually catalyzed
by strong mineral acids (HCl, H2SO4 and H3PO4), having
several drawbacks concerning handling, safety, corrosion and
waste disposal. In addition, because the reagents are mixed with
the acid catalyst, the separation of the products from the
catalyst is often a difficult and energy-consuming process. In
order to avoid these problems, many efforts have been devoted
to the search of solid acid catalysts, which should be safer and
environmentally friendly than mineral acids.
Different solid acid catalysts have been proposed as
substitutes of mineral acids in hydroxyalkylation reaction.
Among them, the most important belong to the classes of
* Corresponding author.
E-mail address: [email protected] (C. Perego).
0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2006.03.013
zeolites and zeotypes, heteropolyacids, exchange resins, clays
and amorphous silica–aluminas. In particular for the synthesis
of MDA and BPA, according to the large dimension of the
molecules involved, large pore zeolites (e.g. Beta, mordenite,
ZSM-12) have been preferred as catalysts [1]. In order to reduce
the diffusion limitation in the micropores of zeolitic catalyst for
MDA synthesis, Corma et al. have recently claimed the use of
delaminated zeolites (e.g. ITQ-2, ITQ-6 and ITQ-18) [2].
The application amorphous silica–alumina was described
since 1968: gels of the type used in commercial catalytic
cracking were claimed to be active catalysts for MDA synthesis
[3]. These silica–alumina gels were characterized by a broad
pore size distribution with pores ranging in diameter from few
to a few hundred Angstroms. According to Thomas, these
characteristics of silica–alumina gels ‘‘make them less than
ideal catalysts’’ [4].
Later on new families of aluminosilicates were described.
Their high surface area (larger than 500 m2/g) and ordered pore
distribution open new opportunities for acid catalyzed reactions.
In 1992, a new mesoporous aluminosilicates designed as MCM-
41 [5] was discovered. MCM-41 has a hexagonal array of
uniform mesopores, which can vary approximately from 15 to
100 A in size. Several amorphous aluminosilicates, prepared as
MCM-41 by surfactant micelle templated syntheses and
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136 129
Table 1
Reagent mixture and product compositions
Samples Gelling agent (R) Reagent mixture molar ratio Product composition (SiO2/Al2O3)
SiO2/Al2O3 R/SiO2 H2O/SiO2 EtOH/SiO2
MCM-41 (CH3)3C16H33NOH 100 0.11 10 8 39
HMS C12H25NH2 100 0.27 10 5 75
MSA (C3H7)4NOH 100 0.09 8 8 100
ERS-8 (C6H13)4NOH 100 0.08 8 8 100
SA HNO3 100 0.01 20 – 100
Grade J639 Grace commercial sample 100
MS 13/110 Grace commercial sample 11.4
Beta Sample prepared according to [17] 22
characterized by a narrow pore size distribution in mesopore
region, have been described by several research groups: FSM-16
[6], HMS [7], SBA [8], MSU [9] and KIT-1 [10]. Other
amorphous aluminosilicate families obtained by cluster tem-
plated sol–gel syntheses in basic medium have been described:
MSA [11] and ERS-8 [12], (micro)/meso- and micro-porous
materials with controlled pore size distribution, respectively.
Mesoporous aluminosilicates, like MCM-41, are good
catalysts even better than large pore zeolites for acid catalyzed
reactions requiring mild acidity and involving bulky reactants
and products (e.g. Friedel–Crafts alkylation 2,4-di-tert-butyl-
phenol with cinnamyl alcohol [13]). Al/MCM-41 aluminosi-
licate showed higher catalytic activity in the hydroxyakylation
of 2-methoxynaphtalene and naphthalene with paraformalde-
hyde than that of some acidic zeolites (e.g. Y, mordenite, ZSM-
5) and amorphous silica–alumina gel [14].
In a recent patent, some mesoporous aluminosilicates,
selected among the previous described families, were patented
by us as active catalysts for MDA synthesis [15].
In order to better understand the role of porosity and acidity,
aluminosilicates with controlled porosity in the micro-region
(ERS-8), meso-region (MCM-41, HMS) and (micro)-meso-
region (MSA)1 were synthesized and evaluated in the
hydroxyalkylation of aniline and phenol. All the catalysts
were prepared by sol–gel synthesis in alkali-free medium via a
polymeric gel route involving alkoxide hydrolysis and
condensation, catalyzed by a proper basic organic gelling
agent. Also a microporous silica–alumina (SA) prepared in
acidic medium without organic gelling agent was used as
catalyst. The comparison to two commercial silica–alumina
gels and to a zeolite Beta completes the catalytic study.
2. Experimental
2.1. Catalyst preparation
All samples were prepared as described in Ref. [16] from
silicon and aluminum alkoxide (Si(OC2H5)4 (Dynasil A,
Nobel), Al(i-OC3H7)3 or Al(sec-OC4H9)3 (Fluka)). Ethyl
alcohol (EtOH) and/or water were used as solvents. The gel
formation was catalyzed by basic aqueous alkali-free gelling
1 MSA is mainly mesoporous with a lower contribution of micropores [16].
agent (selected among: cetyltrimethylammonium hydroxide
(CTMA-OH), tetrapropylammonium hydroxide (TPA-OH),
tetrahexylammonium hydroxide (THA-OH), dodecylamine
(DDA)), or by acid (HNO3). The synthesis parameters are
summarized in Table 1.
All the syntheses gave initially rise to a clear solution. In the
case of MCM-41 and HMS, a progressive flocculation was then
observed. After 15 h ageing at room temperature, the samples
were filtered, washed and dried at 100 8C.
Homogeneous gels without separation of liquid solution
were instead formed for other syntheses: the gel was opalescent
for MSA and transparent for ERS-8 and SA samples. After 15 h
ageing at room temperature, the gels were dried at 100 8C.
All the samples were calcined for 8 h in air at 550 8C.
The synthesized catalysts were compared to two commercial
silica–alumina gels supplied by Grace (J639 and MS 13/110)
and to a zeolite Beta in acidic form, prepared in our laboratories
according to [17].
2.2. Physico-chemical characterization
The textural properties of these catalysts were determined by
nitrogen isotherms at liquid nitrogen temperature by using a
Micromeritics ASAP 2010 apparatus (static volumetric
technique). Before determination of adsorption–desorption
isotherms the samples (�0.2 g) were outgassed for 16 h at
350 8C under vacuum.
The specific surface area was determined from the BET
equation three parameters fit in the range p/p8 0.01–0.20. The
total pore volume (VT) was evaluated by Gurvitsch rule. Mean
pore size (dDFT) was calculated using DFT method (Micro-
meritics’ DFT Plus1 software) for all materials with the
cylindrical pores in oxide surface model.
The acidity was determined by the pyridine adsorption–
desorption method, followed by FT-IR spectroscopy. Self-
supported wafers of 15–20 mg/cm2 thickness were evacuated in
situ (500 8C, 1 h, 10�4 mbar, dynamic vacuum) in a pyrex cell
with KBr windows, contacted with pyridine at 200 8C(13.3 mbar) and stepwise evacuated at increasing temperature
(200–500 8C) in dynamic vacuum, as already described [18].
The spectra were registered at 20 8C with a mod. 2000 (Perkin-
Elmer) FT-IR spectrometer, taken the resolution of 1 cm�1. The
density of the acid sites was evaluated from the peak area of the
IR signals at 1455 (Lewis type) and 1545 cm�1 (Brønsted type),
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136130
taking the extinction coefficient calculated by Take et al. [19].
The error in peak area determination was around 15%. After
desorption at 200 8C, the ‘‘total’’ density of the sites was
measured.
2.3. Catalytic tests: apparatus, procedure and analysis
The catalytic tests were carried either batchwise or by a
continuous flow, fixed bed microreactor.
2.3.1. Hydroxyalkylation of phenol with acetone to BPA
The tests were performed loading 1 g of catalyst, 9 g of
phenol and 1.16 g of acetone in an autoclave (phenol/acetone
molar ratio = 5). The autoclave was closed, stirred and heated at
150 8C for 6 h. Unreacted phenol was evaporated under vacuum
and the sample analysed through GC–MS.
2.3.2. Hydroxyalkylation of aniline with formaldehyde to
MDA—batch tests
Aminal (aminoacetal), product of non-catalyzed condensa-
tion of two molecules of aniline with one molecule of
formaldehyde was produced adding dropwise 22.4 g of aqueous
formaldehyde (37.5%, 0.27 mol) to 100 ml of aniline
(1.096 mol). The organic phase containing the aminal and
unreacted aniline was separated and dried over Na2SO4.
Residual water content was about 1000 ppm.
The tests were performed loading the catalyst (0.25 or 1 g),
4 g of aminal and 10 g of aniline in an autoclave (aniline/
formaldeyde molar ratio, including aminal = 14.7). The
autoclave was closed, stirred and heated at 150 8C for 6 h.
Unreacted aniline was evaporated under vacuum, and the
sample analysed through HPLC [20].
2.3.3. Hydroxyalkylation of aniline with formaldehyde to
MDA—continuous tests
The continuous reactor was a standard laboratory assembly,
built up around a SS tubular microreactor (13 mm i.d., 25 cm
long) equipped with a 1.6 mm o.d. axial thermowell and heated
by an oil bath. The reactor was charged with 3 g of catalyst,
crushed from 5 t pelletized wafers of 3.9 cm diameter and
sieved to 20–40 mesh particles. The catalyst was then flushed in
slowly flowing dry nitrogen while heating up to 180 8C for 3 h.
The stability tests were performed with MSA, SA, Grace
J639 and H-Beta at the following reaction conditions: 3.8 MPa
Table 2
Textural properties of ERS-8, SA, MSA, MCM-41, HMS, Grace J639 and MS 13
Samples Isotherm
MCM-41 Type IV without hysteresis loop
HMS Type IV with a hysteresis loop at
high relative pressure ( p/p8 > 0.8)
MSA Type IV + (I) with a H2 hysteresis loop
ERS-8 Type I
SA Type I
Grace J639 Type IV with a H1 hysteresis loop
MS 13/110 Type IV with a H1 hysteresis loop
a Interparticle void.
total pressure; T = 180 8C; aniline/formaldehyde = 42.5/1;
liquid hourly space velocity (LHSV) = 2–6–10–40 h�1 referred
to the reaction mixture. LHSV was increased every 48 h of time
on stream (tos). In the case of H-Beta, the life test was repeated
with a larger concentration of aminal, corresponding to aniline/
formaldehyde = 7.2/1 and LHSV = 2 h�1.
The samples were collected and analyzed as above
described.
3. Results and discussion
3.1. Catalyst preparation and characterization
The chemical composition of the catalysts is reported in
Table 1. MCM-41 and HMS samples, whose syntheses
comprise a solid phase separation, show a lower SiO2/Al2O3
molar ratio than the reagent mixture, in agreement with the
higher silica solubility at high pH.
By contrast, the materials obtained by gelation (MSA, ERS-
8, SA) show the same SiO2/Al2O3 as the reagent mixtures,
according to the complete hydrolysis of the alkoxides and to the
absence of phase separation during the preparation.
Therefore, the aluminum content in the catalysts decreases
in the order MCM-41 > HMS > MSA = ERS-8 = SA.
Textural properties are reported in Table 2.
Between the two microporous aluminosilicates, ERS-8,
synthesized in basic medium, shows higher surface area and
pore volume than SA, prepared in acidic medium.
MSA, MCM-41 and HMS aluminosilicates are characterized
by a high surface area and a mean pore size centered in the region
of mesopores. Compared to the commercial silica–alumina gels
(Grace J639 and MS 13/110) the surface areas are larger and the
mean pore size lower. Besides, these aluminosilicates obtained
by surfactant micelle templated syntheses or by cluster templated
sol–gel syntheses are characterized by a narrower pore size
distribution than commercial silica–alumina gels [11,21].
The results of adsorption–desorption of pyridine are
reported in Table 3, together with the acidity of zeolite Beta,
taken as a reference in the catalytic reaction.
The total density of acid sites is almost comparable for all
the amorphous aluminosilicates, except for MCM-41, showing
the largest density of both Brønsted and Lewis sites according
to its larger Al content. For the same reason also the silica–
alumina gel Grace MS 13/110 shows the highest density of
/110
SBET(3p) (m2/g) VT (ml/g) dDFT (A)
1140 0.83 21
948 0.96 19 (936)a
928 0.74 32
1196 0.62 16
509 0.24 13
318 1.44 207
480 1.02 94
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136 131
Table 3
Brønsted and Lewis acid site density (mmol/g)
Samples SiO2/Al2O3 Al content
(mmol/g)
Total acid site
density (mmol/g)
Lewis Brønsted
ERS-8 100 333 108 38
SA 100 333 94 19
MSA 100 333 101 47
MCM-41 39 854 173 120
HMS 75 444 77 37
Grace J639 100 333 59 27
Grace MS 13/110 11.4 2920 258 25
H-Beta 22 1514 340 280
Scheme 1.
Lewis acid sites, but in this case the Brønsted site density is very
low. Zeolite Beta is a very acidic material with respect to both
amorphous aluminosilicates and silica–alumina gels, as already
reported in literature [22–24].
3.2. Hydroxyalkylation reaction
The aim of the present report is to provide a comparison of
the catalytic activity of amorphous aluminosilicates with
controlled porosity and H-Beta in the hydroxyalkylation of
aniline and phenol. Hydroxyalkylation is a variant of the
Friedel–Crafts aromatic alkylation in which the alkylating
agent is a carbonylic compound, rather than an alkene, alcohol
or alkylhalide. This reaction is known to require an acid catalyst
in order to activate the carbonyl group enhancing its ability as
electrophile towards an electron-rich aromatic ring. In the case
of acidic zeolites, the first stage of reaction is usually reported
as the formation of an anionic lattice-associated carbenium ion,
by the attack of a proton on a carbonylic group (>C O). This
carbocation is then attacked by a free or weakly adsorbed
Scheme
aromatic. However, the adsorbed conjugate acid of the carbonyl
reactant has also a strong tendency to react with another
carbonylic compound, giving rise to competitive side reaction
(e.g. aldol condensation). The hydroxyalkylation usually does
not stop at the first step and the alcohol initially produced (1:1
adduct) reacts with another molecule of aromatic compound to
form bisarylalkanes (2:1 adducts), according to Scheme 1. As
reported by Venuto [25], the molecular weight and the steric
requirements of 1:1 adduct may be rather great. Besides, the 1:1
adduct then comprises a bulky alkylating agent for condensa-
tion with a second aromatic molecule, generating a very large
product. Therefore, both porosity and acidity are expected to
play a significant role.
3.2.1. Hydroxyalkylation of phenol with acetone to BPA
Bisphenols are formed by the acidic condensation of 1 mol
of ketone (or aldehyde) with 2 mol of phenols. The bisphenol
most largely produced in the chemical industry is bisphenol A
(BPA) ( p,p0-isopropylidenediphenol), which derives from the
acid catalyzed condensation of 2 mol of phenol with 1 mol of
acetone. The industrial production of BPA is performed using
hydrochloric acid or sulphonic resins as catalysts [1]. The
reaction proceeds according to the Scheme 2. The main
2.
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136132
Table 4
Product molar composition (%)
Catalyst Acetone conversion (%) Bisphenol p,p0-Bisphenol (BPA) o,p0-Bisphenol BPX Chromans Mesytil oxide
ERS8 72.47 62.28 36.19 26.09 8.23 1.93 27.56
MSA 64.7 68.22 29.73 38.49 8.40 11.73 11.65
MS 13/110 71.08 58.99 38.74 20.25 8.49 3.23 29.29
H-Beta 100 76.27 48.97 27.3 23.7 0.01< 0.01<
reaction produces BPA together with relevant quantities of the
o,p0-isopropylidenediphenol (o,p0/p,p0 ratio up to 1:2). Both
isomers can undergo to further condensation producing small
amounts of heavier by-products: trisphenols (BPX) and
chromans. Besides acetone by aldol condensation produces
small amount of mesytil oxide.
The tests of the hydroxyalkylation of phenol with acetone
were performed with three different amorphous catalysts: two
ordered microporous (ERS-8) and mesoporous (MSA) alumi-
nosilicates and a commercial silica–alumina gel (Grace MS 13/
110). The results are reported in Table 4 together with that
obtained with H-Beta.
The amorphous aluminosilicates are able to activate the
hydroxyalkylation with comparable catalytic activities. Con-
cerning the selectivity the main differences between ERS-8 and
MSA (i.e. less chromans, higher p,p0/o,p0 and more mesytil
oxide) are probably due to the different pore dimension. In fact,
these two aluminosilicates show similar acidity but different
porosity. The lower pore diameter of ERS-8 with respect to
MSA (16 A versus 32 A) likely hampers the formation of bulky
products like o,p0-BPA and chromans, while favoring the
formation of p,p0-BPA and mesytil oxide having a low steric
hindrance. However, neither ERS-8 nor MSA evidence peculiar
performances with respect to the commercial silica–alumina
gel. Finally, all amorphous aluminosilicates show a lower
hydroxylation activity and selectivity with respect to H-Beta.
The zeolite Beta produces more BPA and practically no mesytil
oxide and chromans. The lower activity of amorphous
aluminosilicates with respect H-Beta can be reasonably
attributed to their lower acidity (particular the Brønsted one)
and lower resistance to the water produced during BPA
synthesis. Being less consumed by hydroxyalkylation, a larger
concentration of acetone is available for aldol condensation to
mesytil oxide, by a second order reaction rate with respect to
acetone. Both acid sites and porosity result play a role in the
reaction, affecting the activity and the selectivity, respectively.
3.2.2. Hydroxyalkylation of aniline with formaldehyde to
MDA
Methylenedianiline (MDA), product of condensation of
2 mol of aniline with 1 mol of formaldehyde, is used as
precursor, through phosgenation, of methylenediisocianate
(MDI). This important chemical commodity is nowadays
largely produced by adding formaldehyde to stoichiometric
amount of hydrochloric acid and aniline at 60–80 8C in agitated
reactor. The reaction mixture is then heated at 100–160 8C for
about 1 h to complete the condensation. The reaction mixture is
then neutralized with an excess of NaOH, producing almost
stoichiometric amount of sodium chloride [1].
The reaction can also be performed in two steps according to
Scheme 3, by the non-catalytic condensation of aniline and
formaldehyde to produce aminal (anilinoacetal), followed by
the rearrangement of aminal to MDA catalyzed by acids. In this
procedure, the water produced in the first step is removed,
taking advantage in the catalytic activity for the rearrangement
[26]. The acid catalyzed rearrangement of aminal first produces
benzylamines, by the eletrophilic substitution on an aromatic
ring carrying a strong activating group, e.g. –NH2, which is
para–ortho orientating. This reaction is relatively rapid and the
formation of p-bonded benzylamines is favored according to
the lower steric hindrance of p position versus o position. Than
the dissociation of benzylamines follows, giving rise to the
formation of a benzylic carbenium ion, which reacts with
aniline forming MDA (see Scheme 3). This reaction is
considered to be the rate-determining step [27]. Beside aniline,
the carbenium ion can also react with MDA, forming
oligomeric amines (triamines and tetraamines). 2,20- and
2,40-MDA are more reactive than 4,40-MDA because of the
availability of p positions for further reactions. An undesirable
side reaction produces small amounts of N-methyl MDA.
The variables affecting the product distribution are aniline
concentration, catalyst concentration (e.g. HCl) and tempera-
ture. The higher the excess of aniline the higher is the
concentration of diamines (MDA isomers) in the reaction
mixture. According to the steric hindrance p isomer (4,40-MDA) is favored at low temperature. However, 4,40-MDA can
isomerize to 2,40- and 2,20-isomers at high temperatures via an
acid catalyzed protodealkylation [28]. At 190 8C in HCl, the
isomer equilibrium composition is 4,40:2,40:2,20 = 49:49:2 [27].
Finally, triamines and tetraamines can react with the
excess of aniline giving back MDA enriched in the 2,40-isomer
[28]. This transalkylation is supposed to involve an initial
C-protonation, followed by loss of the benzyl cations [28]
(Scheme 4).
The results of the aminal rearrangement catalyzed by
amorphous aluminosilicates are reported in Tables 5 and 6.
These catalytic tests were performed batchwise, at the same
reaction conditions (aniline/formaldeyde = 14.7, T = 150 8C,
reaction time = 6 h), but different catalyst amount (i.e. 1 and
0.25 g, respectively). Data reported in Table 5 show that the
aminal completely rearranged with all catalysts. Also
benzylamine was always completed rearranged, but for SA.
Accordingly the overall amines production (MDA and
oligomeric amines) is always close to 100%, with the exception
of SA. In order to better evidence differences in catalytic
activity, the tests were repeated for some catalysts lowering the
catalyst amount (i.e. 0.25 g) at the same conditions (Table 6).
Again, all catalysts are able to completely rearrange
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136 133
Scheme 3.
Scheme 4.
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136134
Table 5
Condensation of aniline with formaldehyde in a batch reactor at 150 8C, 1 g of catalyst, after 6 h. reaction time
Catalyst Amines
(%)
Benzylamines
(%)
N-Methylated
(%)
Diamines
(MDA) (%)
Oligomeric amines
(triamines and tetraamines) (%)
MDA isomer ratio 4,40-MDA/(2,40- + 2,20-MDA)
MSA 98.84 0 1.16 73.21 25.63 5.99
MCM 41 99.14 0 0.86 74.06 25.08 4.98
HMS 99.07 0 0.93 76.05 21.14 5.58
SA 94.6 4.49 0.91 60.23 34.37 6.38
ERS 8 99.18 0 0.82 80.82 18.36 5.41
J 639 99.12 0 0.88 76.05 23.07 5.55
Rearrangement of the aminal obtained in the neutral condensation of aniline with formaldehyde.
Table 6
Condensation of aniline with formaldehyde in a batch reactor at 150 8C, 0.25 g of catalyst, after 6 h reaction time
Catalyst Amines
(%)
Benzylamines
(%)
N-Methylated
(%)
Diamines
(MDA) (%)
Oligomeric amines
(triamines and tetraamines) (%)
MDA isomer ratio 4,40-MDA/(2,40- +2,20-MDA)
MSA 99.18 0 0.82 73.49 25.68 5.99
HMS 99.3 0 0.7 56.74 40.17 6.71
SA 52.73 46.8 0.47 7.43 45.3 6.51
ERS 8 99.27 0 0.73 75.29 23.98 5.50
J 639 99.61 0 0.39 73.51 26.1 6.53
MS13-110 99.31 0 0.69 78.44 20.86 6.37
H-Beta 99.57 0 0.43 83.98 15.53 2.55
Rearrangement of the aminal obtained in the neutral condensation of aniline with formaldehyde.
Fig. 1. Aminal to MDA in fixed bed reactor (3.8 MPa; 180 8C; aniline/
formaldehyde = 42.5/1).
benzylamine, with the exception of SA for which the
concentration of unconverted benzylamine sensibly increases
from 4.49 to 46.8%. With the other aluminosilicates, the amines
production is practically the same in both the tests, while some
differences can be evidenced in the distribution between
diamines and oligomeric amines.
Lowering the amounts of catalysts results in an increase of
oligomeric amines with respect to diamines (i.e. the ratio
oligomeric amines/diamines increases). This increase is largest
for SA and reduces in the order SA > HMS > ERS-
8 > J639 > MSA. Hence, the larger the increase the lower is
the transalkylation activity. MSA behavior is practically not
affected by the catalyst loading; a higher stability of MSA with
respect tootheramorphous aluminosilicatescan behypothesized.
Concerning the MDA isomers, 4,40-MDA prevails on 2,20-and 2,40-MDA for all amorphous aluminosilicates and the
isomer distribution is practically the same for all of them and
independent on catalyst amount. Lowering the catalyst amount
also results in a reduction of N-methylated, this trend being
confirmed for all aluminosilicates.
Comparing the results of amorphous aluminosilicates with
that of H-Beta, two main behaviors are evidenced. Zeolite Beta
produces the lowest amount of oligomeric amines and the
lowest MDA isomer ratio 4,40/(2,40 + 2,20) = 2.55. Hence, H-
Beta shows the largest isomerization activity with respect to
amorphous aluminosilicates, though the ratio is still far from
the equilibrium one (�1).
The data collected with these kinds of tests do not allow to
evidence significant differences among amorphous aluminosi-
licates and commercial silica–alumina gels. Only SA evidenced
a limited catalytic activity that seems to be due to both its
porosity and its acidity. In fact, among amorphous alumino-
silicates SA has the smaller pore diameter and pore volume and
the smaller Brønsted acid site density. The role of porosity and
acidity was further investigated by continuous test. Micro-
porous (SA), mesoporous (MSA) and silica–alumina gel (Grace
J639) having the same SiO2/Al2O3 molar ratio have been
selected for these tests. The tests were performed so to collect
information also on catalyst life. The results are reported in
Fig. 1, where conversion includes the rearrangement of aminal
to all products (diamines, oligomeric amines, N-methylated),
but benzylamine. In fact, for all catalysts the rearrangement of
aminal is always complete and the deactivation is revealed by
the appearance of benzylamine.
The difference of catalysts life among aluminosilicates is
relevant. In fact, while SA quickly deactivates, probably
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136 135
Fig. 2. Product distribution for the continuous rearrangement of aminal in fixed
bed reactor (3.8 MPa; 180 8C; aniline/formaldehyde = 42.5/1) with MSA.
Fig. 4. Product distribution for the continuous rearrangement of aminal in fixed
bed reactor (3.8 MPa; 180 8C; aniline/formaldehyde = 7.2 /1) with H-Beta.
because of its small pore diameter, pore volume and Brønsted
acid site density. MSA has a relevant life and Grace J639
behaves in between. With respect to MSA, Grace J639 is
characterized by a broader pore size distribution [16] and a
smaller surface area. These can be the reason of a lower
stability, together with the smaller acid site density.
According to space velocity and yield, the catalyst life of
MSA corresponds to a productivity of around 120 g MDA/g
catalyst. Figs. 2 and 3 report the selectivity for MSA and H-
Beta during the life tests. At the beginning of the stability test,
the distribution between diamines and oligomeric amines is
constant and comparable for both MSA and H-Beta. On the
contrary the MDA isomer ratio, after an initial decrease,
constantly increases. However, the MDA isomer ratio is always
lower for H-Beta than MSA. Besides, the smallest value for H-
Beta (�1.4) approaches the equilibrium one, indicating that the
fresh H-Beta has a good isomerization activity, larger than that
of amorphous aluminosilicates. A similar behavior was already
reported for the isomerization of o-, p- and m-cymene and
attributed to the larger strong acid site density of the Brønsted
type of H-Beta with respect to MCM-41 and MSA [29].
In the stability test with MSA just before the deactivation,
which was evidenced by the appearance of benzylamine at
around 120 h, diamines progressively reduce in favor of
oligomeric amines. Simultaneously the MDA isomer ratio
Fig. 3. Product distribution for the continuous rearrangement of aminal in fixed
bed reactor (3.8 MPa; 180 8C; aniline/formaldehyde = 42.5/1) with H-Beta.
increases more rapidly. The increase of oligomeric amines and
of 4,40-MDA isomer evidences a reduction of transalkylation
and isomerisation activity of MSA along with the time,
probably related to the progressive deactivation of the Brønsted
acid sites. In fact, the MDA isomerization and oligomeric
amine transalkylation, both proceeding via protodealkylation
[28], are more acid demanding than aminal and benzylamines
rearrangements. In case of H-Beta, even over 200 h,
benzylamine is never detected. Repeating the stability test
with a larger aminal concentration (58% corresponding to
aniline/formaldehyde = 7.23, with respect to the 42.5 of the
previous tests). Benzylamine appears after around 130 h t.o.s.,
with a global productivity of around 300 g MDA/g catalyst. The
selectivity is summarized in Fig. 4. As for MSA, to the
deactivation corresponds a progressively decrease of diamines
in favor of oligomeric amines and the increase of MDA isomer
ratio.
The longer stability of H-Beta with respect to the
mesoporous aluminosilicate MSA may be due to the larger
acid side density.
4. Conclusion
Amorphous aluminosilicates with narrow pore size dis-
tribution, prepared by surfactant micelle templated syntheses
(e.g. MCM-41, HMS) or by cluster templated sol–gel syntheses
(e.g. MSA, ERS-8) proved to be suitable catalysts for the
hydroxyalkylation of phenol and aniline, with performances
depending on porosity and acidity.
In the hydroxylakylation of phenol with acetone to
bisphenol, the catalytic activities of microporous (ERS-8)
and mesoporous (MSA) are similar and comparable with that of
a commercial silica–alumina gel (Grace MS 13/110). MSA is a
little more selective to bisphenol. However, H-Beta shows a
better activity and selectivity with respect to amorphous
aluminosilicates, probably related to its higher acidity and
higher resistance to the water produced during condensation.
In the case of rearrangement of the aminal, obtained by not
catalyzed condensation of aniline and formaldehyde, only the
microporous silica–alumina SA shows insufficient catalytic
activity and stability. The mesoporous MSA evidences a larger
C. Perego et al. / Applied Catalysis A: General 307 (2006) 128–136136
catalytic stability than other amorphous silica–aluminas. This
has been confirmed by stability tests with MSA and a
commercial silica–alumina gel with the same SiO2/Al2O3
ratio. This may be due to its larger surface area and narrower
pore size distribution. The selectivity to diamines (MDA) and
oligomeric amines is comparable with that of H-Beta. However,
H-Beta is more stable than MSA, with an overall productivity
more than double (300 g versus 120 g MDA/g catalyst).
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