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PONOMAREVA et al.
Low-temperature nitrogen adsorption isotherms
Heteropoly acids [3, 4], oxides [5, 6], phosphates
[7, 8], and sulfates [9] were studied as acid catalysts of were recorded on an ASAP2000 automatic porosime-
a single-stage gas-phase process under flow condi- ter (Micromeritics, United States). Micropore volume
tions. The main disadvantages of these systems are was determined by the t-plot method.
their low efficiency in isoprene production, rapid
deactivation of almost all of them, and the impossibil-
ity of regeneration. An exception is provided by nio-
bium- and zirconium-containing phosphates; accord-
ing to [7], these systems exhibit a stable on-stream
behavior; a 30-h reaction does not have a significant
effect on the activity of these catalysts. The authors of
[7] attribute the high deactivation resistance of ZrP
and NbP catalysts to the in situ reduction of Brønsted
acid sites directly during the reaction involving water
present in the reaction zone.
Promising heterogeneous catalysts of this process
are zeolites that exhibit shape-selective properties,
readily undergo regeneration, and readily provide
modification of their acid–base properties by ion
exchange, impregnation, dealumination, and isomor-
phous substitution. Unfortunately, the published data
on isoprene synthesis from isobutylene and formalde-
hyde over zeolites [10–13] were obtained in the batch
mode; it is difficult to compare those data with results
for other heterogeneous catalysts obtained in the flow
mode.
This study is focused on a single-stage gas-phase
synthesis of isoprene from formaldehyde and isobuty-
lene in the flow mode in the presence of Al–BEA,
Zr–BEA, Sn–BEA, and Nb–BEA zeolite catalysts
synthesized by isomorphous substitution methods and
in the presence of zeolites of the different framework
types (MFI, BEA, and FAU(Y)). The effect of the
nature of the active sites and the zeolite structure on
the physicochemical and catalytic properties of the
synthesized samples is studied.
The morphology of the synthesized samples was
studied by scanning electron microscopy (SEM) on a
Hitachi Tabletop Microscope TM3030Plus electron
microscope. The voltage across the accelerating elec-
trode was 15 kV.
The acidic properties of the samples were studied
by temperature-programmed desorption of ammonia
(NH3-TPD) on a USGA-101 universal sorption gas
analyzer. A weighed portion of the sample was placed
in a quartz reactor, heated in a helium stream to a tem-
perature of 500°C, calcined at this temperature in a
helium stream for 1 h, and then cooled to 60°C.
Ammonia saturation was implemented in a stream of
dried NH3/N2 (1 : 9) mixture for 15 min. Physically
adsorbed ammonia was removed at 100°C in a dry
helium stream for 1 h. After that, the temperature in
the reactor was linearly increased to 800°C at a rate of
8 deg/min. Changes in the thermal conductivity of the
flow were recorded using a thermal-conductivity
detector.
Infrared spectra were recorded on a Nicolet Pro-
tege 460 Fourier transform IR spectrometer equipped
with an MCT detector in a range of 4000–400 cm–1 at
a resolution of 4 cm–1. A 20-mg catalyst sample was
compressed into a disc with a diameter of 2 cm. Water
was removed from the samples on a vacuum unit
equipped with absolute pressure sensors at a working
vacuum of 5 × 10–4 Pa. A pellet of the sample was
placed in an IR cell, heated to 450°C for 2 h, and held
at 450°C for 1 h. Carbon monoxide adsorption was
implemented at the liquid nitrogen temperature
(77 K) by dosing the gas until complete saturation.
Pyridine (Py) adsorption was run at 150°C for 30 min;
it was followed by evacuation at 150°C for 1 h. The
recorded IR spectra were processed using the OMNIC
ESP software, version 7.3.
The reaction of isobutylene with formaldehyde was
studied in a catalytic fixed-bed flow system at a tem-
perature of 300°C; a feed space velocity of isobutylene
and formalin of 3.85 and 1.09 g/(g h), respectively;
and an isobutylene: formaldehyde molar ratio of 5 : 1.
Before testing, the catalysts were heated to 350°C in a
helium stream; after that, the temperature was
decreased to the reaction temperature. Formalin con-
taining 37 wt % of formaldehyde, 3 wt % of methanol,
and 60 wt % of water was used without pretreatment.
The chromatographic analysis of liquid and gas-
eous reaction products was conducted on a Chro-
matec Analytic Kristall 2000M gas chromatograph
equipped with a flame ionization detector and a
40 m × 0.32 mm capillary column coated with the SE-
EXPERIMENTAL
The original samples were zeolites BEA with
Si/Al = 12.5 and 150, MFI with Si/Al = 11.5, and FAU
with Si/Al = 15 purchased from Zeolyst. The Sn–
BEA, Zr–BEA, and Nb–BEA samples were synthe-
sized as described in [14], [15], and [16], respectively.
To convert the zeolites from the NH4 form to the
H form, the samples were calcined in a dry air stream
at 550°C for 6 h.
The elemental composition of all the studied sam-
ples was determined by X-ray fluorescent spectrome-
try on a ThermoScientific ARL PERFORM’X
WDXRF instrument equipped with a 2.5-kW operat-
ing rhodium tube.
The phase composition of the samples was deter-
mined on a Bruker D2 PHASER powder diffractome-
ter using CuKα radiation. Diffraction patterns were
processed using the Bruker DIFFRAC.EVA software 30 nonpolar phase. Dioxane and methane were added
package. Phases were identified in accordance with as an internal standard to liquid samples and gaseous
the ICDD PDF2 database.
products, respectively.
PETROLEUM CHEMISTRY
Vol. 59
No. 7
2019