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tive product distribution and superior catalyst stability in the
Sn-Beta-catalyzed reaction of HMF with 2-butanol. Research
into combining such Lewis acid zeolites with a heterogeneous
Brønsted acid in a dual catalyst bed system is needed to ulti-
mately advance a breakthrough in the continuous and scalable
production of HMF ethers.
liquid mixture and controlled to 393 K. The reactors were charged
with the reactant solution (2.5 mL, 1 wt% HMF in alcoholic sol-
vent), an internal standard (1,3,5-tri-tert-butylbenzene), and the de-
sired amount of catalyst (corresponding to a ratio of mol metal/
mol HMF of 3:100). The reactions were allowed to proceed for the
times designated within Tables 1, 3, and 4 and Figure 1. Afterwards,
the sample was prepared for analysis by passing the liquid through
Taken together, our results provide new insights into the
changes of the local environment near the metal centers
within the zeolites and show, for the first time, the partially re-
versible deactivation of these materials in a flow reactor. Efforts
to expand the scope of continuous flow strategies to other
types of Lewis acid-catalyzed cascade reactions are currently
underway in our group.
a 0.2 mm Millipore PTFE syringe filter to remove extraneous particu-
1
lates. Flow reactions were conducted in a = inch tubular stainless
4
steel reactor mounted inside an aluminum block within an insulat-
ed single-zone furnace (850 W/115 V, Applied Test Systems Series
3
210). The catalyst bed consisted of the calcined catalyst loaded
between quartz wool plugs and supported by inert glass beads.
The reaction temperature was monitored inside the reactor using
a K-type thermocouple (Omega). The system was pressurized with
dry compressed air. This pressure was controlled by a backpressure
regulator. Liquid reactants were introduced into the reactor by
using a Waters 515 HPLC pump, and the effluent was collected in
a separator (Gage&Valve Co.) at room temperature. Each experi-
ment was carried out at 393 K and 791 kPa. The effluent liquid was
drained periodically for analysis.
Experimental Section
Catalyst synthesis
The zeolites were synthesized based on the procedure reported by
[20c]
Corma et al.
using the following precursors: hafnium(IV) chlo-
ride, zirconium(IV) oxychloride octahydrate, tin(II) chloride dihy-
drate, titanium(IV) isopropoxide, tantalum(V) ethoxide, and nio-
bium(V) ethoxide. Tin(II), which oxidizes to tin(IV) in water, was
used in place of SnCl ·5H O and resulted in Sn-Beta consistently
Product analysis
4
2
Liquid samples were analyzed using an Agilent Technologies
[28]
free of extra-framework SnO2. Hf-Beta was synthesized as fol-
lows: aqueous tetraethylammonium hydroxide [27.158 g; Sigma–
Aldrich, 35 wt% (TEAOH)] and tetraethylorthosilicate (23.968 g;
Sigma–Aldrich, 99 wt%) were added to a Teflon [polytetrafluoro-
ethylene (PTFE)] dish, which was magnetically stirred at room tem-
perature for 90 min. Additional deionized water (15 mL) was
added, and the dish was cooled in an ice bath. Then, hafnium(IV)
chloride (0.3747 g; Sigma–Aldrich, 98 wt%) dissolved in ethanol
6
890N GC equipped with an Agilent DB-1701 capillary column
(length 30 m; inner diameter 0.25 mm; film thickness 0.25 mm) and
flame ionization detector (split ratio 20:1, column flow rate
2
using an Agilent 7890A GC equipped with a DB-1701 capillary
column (30 mꢂ0.25 mm) and compared with available standard
compounds. HMF ether and acetal compounds were synthesized
and purified according to the methods described by Balakrishnan
À1
.4 mL min ). Product identification was performed by GC–MS
(
2 mL) was added dropwise. The solution was left uncovered on
[5c]
et al.
a stir plate for 10 h to reach a total mass of 33.147 g after evapora-
tion of ethanol and some of the water. Next, aqueous hydrofluoric
acid (2.620 g; Sigma–Aldrich, 48 wt%) was added dropwise, and
the mixture was homogenized using a PTFE spatula, resulting in
a thick gel. Si-Beta (0.364 g), prepared using an identical procedure
without seeding or hafnium(IV) chloride addition, was seeded into
the mixture. The weight of the resulting sol–gel was allowed to be
reduced to 33.956 g over roughly 2 h, which corresponds to a final
molar composition of 1 SiO /0.01 HfCl /0.56 TEAOH/0.56 HF/
Catalyst characterization
ICP-AES were recorded on an Optima 2000 DV spectrometer (Perki-
nElmer Inc.). PXRD patterns were collected using a Bruker D8 dif-
fractometer using CuKa radiation. UV/Vis analysis was performed
using a Varian Cary 5000 UV/Vis NIR spectrometer equipped with
a Praying Mantis diffuse reflectance accessory. The spectra were
2
4
7
.5 H O. The thick paste was transferred to a PTFE-lined stainless
2
collected at 190–450 nm and referenced to BaSO . TGA was per-
4
steel autoclave (45 mL) and heated to 413 K for 20 days under
static conditions. The solids were recovered by filtration, washed
À1
formed by heating the catalyst under a flow of 10 mLmin N and
2
À1
9
0 mLmin air using a Q500 thermal analysis system (TA Instru-
with nanopure H O, and dried at 373 K. The zeolites were calcined
2
ments). After 1 h dehydration at 423 K, approximately 30 mg of the
sample was heated to 1073 K (5 Kmin ). FTIR spectra were ac-
quired using a Bruker Vertex 70 spectrophotometer. Approximately
À1
by heating to 853 K with a 1 Kmin ramp (with 1 h isothermal
À1
steps at 423 and 623 K) and kept at that temperature for 10 h.
After calcination, the overall inorganic oxide yield was 80–90%.
1
0 mg samples were pressed into 13 mm self-supporting pellets
HfO /Si-Beta was prepared by incipient wetness impregnation of
2
and placed into a Harrick high temperature FTIR cell. Each sample
was heated to 623 K (10 Kmin ) and held for 1 h under flowing
He. Once the cell had cooled to room temperature, dynamic
vacuum of a pressure of roughly 0.1 Pa was established. A refer-
ence spectrum was then acquired. Under a static vacuum, the cell
Si-Beta with an aqueous hafnium(IV) chloride solution, followed by
drying at 383 K and calcination in air flow at 533 K. Sn- and Zr-
MCM-41 were synthesized according to previously published pro-
À1
[29]
tocols. All catalysts were synthesized to achieve a silicon/metal
ratio of roughly 100 (see Table 1).
was dosed with excess CD CN vapor. The vapor was evacuated
3
after 10 min, and the cell was dosed once more. Once dynamic
vacuum was re-established, the difference spectrum was acquired.
N2 adsorption–desorption isotherms were measured on a Quan-
tachrome Autosorb iQ apparatus at the liquid-nitrogen tempera-
ture (77 K). All samples were degassed under vacuum prior to use
(623 K, 12 h). Micropore volumes were analyzed by the t-plot
method.
Catalytic reactions
Batch reactions were carried out in a glass reactor (5 mL) under au-
togenous pressure and heated in a temperature-controlled oil bath
with magnetic stirring. The temperature in the reaction vial was
measured by a K-type thermocouple (Omega) placed inside the
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2014, 7, 2255 – 2265 2263