phase separation under nonequilibrium conditions.5 This
phenomena prompted us todevelop enzyme-inspired poly-
meric acid catalysts that realize phase-separated non-
equilibrium reaction conditions. We designed a fairly
hydrophilic meso/macroporous acid catalyst for this pur-
pose. Fairly hydrophilic meso/macropores in the catalyst
should readily capture a fairly hydrophilic substrates and
reactants, alcohols and carboxylic acids, to convert to the
corresponding more hydrophobic esters that could be
kicked out from the hydrophilic nano/macropores not to
be captured and mediated by the catalyst. This is our
working hypothesis of in-water dehydrative reaction with
phase separation under nonequilibrium conditions to af-
ford the desired esters in high yield. Here, we report the
development of a macroporous phenolsulfonic acidÀ
formaldehyde resin catalyst as a novel heterogeneous
polymeric acid catalyst. By using the catalyst, the direct
dehydrative esterification proceeded smoothly in the pre-
sence of less than1 mol % ofthe macroporousacid catalyst
without removal of water to afford the corresponding
esters with high yield. Moreover, biodiesel fuel was pro-
duced under the flow reaction conditions by using a
column-packed macroporous acid catalyst.
material with a pore size of 1À5 μm wide (Figure 1a,b).
Energy-dispersive X-ray spectroscopy analysis on the SEM
(EDX/SEM) of 1 showed sulfur atoms uniformly dispersed
on the polymeric matrix (Figure 1c,d). The surface area
of 1 was 11.5 m2/g from Kr absorptionÀBET analysis.
Phenolsulfonic acidÀformaldehyde resins have been
used as cation exchangers or electron-conductive composi-
tions.6 However, to the best of our knowledge, there are no
reports on their catalytic use as heterogeneous catalysts.
The phenolsulfonic acidÀformaldehyde resin 1 was pre-
pared by the condensative polymerization of p-phenolsul-
fonic acid and formaldehyde (5 mol equiv) in H2O at
120 °C for 6 h (Scheme 1), after which the reaction mixture
was gradually cooled to 25 °C over 12 h to give a pale
brownish gel. After being dried under reduced pressure, the
gel became a reddish brown, hardly soluble solid. Elemental
analysis and the IR spectrum (773 and 708 cmÀ1) (Figure S1,
Supporting Information) of 1 showed 80% degradation of
the sulfonic acid group via thermal decomposition.
Figure 1. (a, b) SEM images (bar scale: (a) 1 μm, (b) 30 μm), (c)
EDX/SEM image of 1, and (d) EDX/SEM mapping of sulfur
atom in 1.
With a phenolsulfonic acidÀformaldehyde resin 1 in
hand, we investigated the catalytic activity in the direct
esterification of 2a and acetic acid with a variety of homo-
geneous and heterogeneous acid catalysts at 50 °C for 12 h
without removal of water (Figure 2). When the esterifica-
tion of benzyl alcohol (2a) and acetic acid (1.2 mol equiv)
was performed with 1 (0.7 mol % SO3H) at 50 °C for
12 h, we were pleased to find that the reaction with less
than 1 mol % catalyst proceeded smoothly to give benzyl
acetate (3a) with 96% conversion. While a homogeneous
counterpart p-phenolsulfonic acid and a common homo-
geneous catalyst p-TsOH led to the formation of 3a with
72% and 77% within 6 h, a prolonged reaction time did
not afford the generation of 3a owing to the equilibrium
(12 h; 79% and 82% conversion, respectively). The cata-
lytic activity of common heterogeneous acid catalysts
zeolite (MS3A),7 Amberlyst,8 and DOWEX9 was lower
than that of 1 (12 h, 7%, 31%, and 51% conversion,
respectively) under similar conditions.
Scheme 1. Preparation of p-Phenolsulfonic AcidÀFormaldehyde
Resins
Using the best heterogeneous catalyst 1, direct esterifica-
tion of various alcohols and acetic acid (1.2 mol equiv) was
performed with 1 (0.7 mol %) at 50 °C for 12 h under
similar conditions (Table 1). The esterification of benzyl
alcohol (2a), 2-phenylethanol (2b), and 3-phenyl-1-propanol
(2c) with acetic acid proceeded smoothly to afford the corre-
sponding acetates (3aÀc) with 93À94% yield (entries 1À3).
When the high-resolution SEM of 1 was observed, we
were surprised to find that it was an aggregated macroporous
(5) (a) Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.;
Walter, P. Molecular Biolology of the Cell, 5th ed.; Garland Science:
New York, 2008.
(6) (a) Klausener, P.; Woermann, D. J. Membr. Sci. 2000, 168, 17. (b)
Mayer, K.; Woermann, D. J. Membrane Sci. 1997, 127, 35. (c) van
Keulen, H.; Hollander, J. G.; Smit, J. A. M. J. Colloid Interface Sci.
1997, 185, 119. (d) Hahn, O.; Woermann, D. J. Membr. Sci. 1996, 117,
197. (e) Roettger, H.; Woermann, D. Langmuir 1993, 9, 1370. (f)
Weisshaar, D. E.; Lamp, B.; Merrick, P.; Lichty, S. Anal. Chem. 1991,
63, 2380. (g) Wehn, R.; Woermann, D. Polymer 1987, 28, 1729.
(7) CAS: 1318-02-1, aluminosilicate (MS3A) (Merck) was used. For a
use of MS3A for the esterification, see: Harrison, H. R.; Haynes, W. M.;
Arthur, P.; Eisenbraun, E. J. Chem. Ind. 1968, 45, 1568.
(8) CAS: 125004-35-5, Amberlyst 16 (Fluka) was used.
(9) CAS: 69011-20-7, DOWEX 50WX2-200 (Wako) was used.
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