Catalytic Activity of Li1 + xHf2–xInx(PO4)3-Based NASICON-Type Materials for Ethanol Conversion Reactions

Article abstract of DOI:10.1134/S0020168518070117

Li_{1 + x}Hf_2–xIn_x(PO₄)₃ (x = 0, 0.05, 0.1) materials have been prepared by solid-state reactions and characterized by X-ray diffraction, low-temperature nitrogen adsorption measurements, and scanning electron microscopy. The materials consist of NASICON-type lithium hafnium double phosphates with a hexagonal structure. Milling in a planetary mill has been found to increase the specific surface area of the Li_{1 + x}Hf_2–xIn_x(PO₄)₃ materials by almost one order of magnitude (from 1.5 to 13 m²/g in the case of LiHf₂(PO₄)₃). The materials with a larger surface area exhibit catalytic activity for ethanol dehydration reactions and are less active for ethanol dehydrogenation. Ethanol conversion predominantly yields diethyl ether at low temperatures and ethylene at higher temperatures. The diethyl ether selectivity of the catalytic processes reaches 85% at 350°C, with 60% conversion, and the ethylene selectivity reaches 96% at 510°C, with 100% conversion. Indium doping raises the high-temperature acetaldehyde selectivity from 4 to 8% and leads to the formation of C₄ hydrocarbons as reaction products. C₄ selectivity reaches 15 and 17% in the case of the Li_1.05Hf_1.95In_0.05(PO₄)₃ and Li_1.1Hf_1.9In_0.1(PO₄)₃ materials, respectively (420°C, 97 and 92% conversion, respectively).

Full text of DOI:10.1134/S0020168518070117

ISSN 0020-1685, Inorganic Materials, 2018, Vol. 54, No. 7, pp. 676–682. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © S.A. Novikova, A.B. Il’in, N.A. Zhilyaeva, A.B. Yaroslavtsev, 2018, published in Neorganicheskie Materialy, 2018, Vol. 54, No. 7, pp. 713–720.
Catalytic Activity of Li1 + xHf2 – xInx(PO4)3-Based NASICON-Type
Materials for Ethanol Conversion Reactions
S. A. Novikovaa, A. B. Il’inb, N. A. Zhilyaevab, and A. B. Yaroslavtseva, b,
*
aKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
bTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia
*e-mail: yaroslav@igic.ras.ru
Received January 30, 2018
Abstract—Li1 + xHf2 – xInx(PO4)3 (x = 0, 0.05, 0.1) materials have been prepared by solid-state reactions and
characterized by X-ray diffraction, low-temperature nitrogen adsorption measurements, and scanning elec-
tron microscopy. The materials consist of NASICON-type lithium hafnium double phosphates with a hex-
agonal structure. Milling in a planetary mill has been found to increase the specific surface area of the
Li1 + xHf2 – xInx(PO4)3 materials by almost one order of magnitude (from 1.5 to 13 m2/g in the case of
LiHf2(PO4)3). The materials with a larger surface area exhibit catalytic activity for ethanol dehydration reac-
tions and are less active for ethanol dehydrogenation. Ethanol conversion predominantly yields diethyl ether
at low temperatures and ethylene at higher temperatures. The diethyl ether selectivity of the catalytic pro-
cesses reaches 85% at 350°C, with 60% conversion, and the ethylene selectivity reaches 96% at 510°C, with
100% conversion. Indium doping raises the high-temperature acetaldehyde selectivity from 4 to 8% and leads
to the formation of C4 hydrocarbons as reaction products. C4 selectivity reaches 15 and 17% in the case of the
Li1.05Hf1.95In0.05(PO4)3 and Li1.1Hf1.9In0.1(PO4)3 materials, respectively (420°C, 97 and 92% conversion,
respectively).
Keywords: lithium hafnium double phosphate, NASICON, doping, indium, mechanochemical activation,
catalysis, ethanol conversion
DOI: 10.1134/S0020168518070117
INTRODUCTION
supports [39–45]. Another possible example of cata-
lysts is NASICON-type mixed phosphates [18–35].
For example, it is well documented that CuZr2(PO4)3,
Cu0.5B2(PO4)3 (B = Ti, Zr, Sn), AgB2(PO4)3 (B = Zr,
Hf), Na1 – xCuxZr2(PO4)3, and Na1 + xFexZr2 – x(PO4)3
are active catalysts for the conversion of the butyl alco-
hols and isopropanol [31–34]. Note that ethanol is
one of the most promising alcohols as a biomass
decomposition product [2, 3]. Depending on the cat-
alyst used and process conditions, ethanol conversion
may follow different reaction paths, leading to the for-
mation of ethylene, diethyl ether, acetaldehyde,
hydrogen, water, carbon monoxide, ethyl acetate, or a
mixture of products [29, 35–41].
In the NASICON-type phosphates, iso- or het-
erovalent substitutions on both the cation and anion
sites are possible [1–3]. Such substitutions have a sig-
nificant effect on both the ionic conductivity and
composition of the products of alcohol conversion
catalyzed by NASICON-type phosphates [27, 29, 36].
For example, indium- and molybdenum-doped
LiZr2(PO4)3-based materials offer high catalytic activ-
NASICON-type compounds constitute a large
class of materials with the general formula
AxB2(PO4)3, where A = H, Li, Na, K, H3O, or NH4
and B = In, Sc, Fe, Zr, Ti, Hf, Sn, Ge, etc. [1–4]. The
alternating voids in their framework crystal structure
ensure fast ion transport, which makes these materials
potentially attractive solid electrolytes with consider-
able lithium-ion, sodium-ion, or proton conductivity
[1–3, 5, 15]. The NASICON-type phosphates con-
taining polyvalent cations capable of changing their
oxidation state (Ti, Fe, and V) can be used as electrode
materials [4, 16]. Other advantages of the NASICON-
type materials are their high chemical and thermal sta-
bility, good ion-exchange properties [17], and catalytic
activity for alcohol dehydrogenation and dehydration
processes [18–35].
The catalytic conversion of alcohols is of great
practical interest because it offers the possibility of
obtaining important products, such as ethers, alde-
hydes, ketones, hydrocarbons, and hydrogen, by envi-
ronmentally safe processes [18–42]. Catalysts for such ity for dehydrogenation processes, whereas niobium-
processes typically have the form of metals on oxide doped materials exhibit higher activity for dehydration
676

CATALYTIC ACTIVITY OF Li1 + xHf2 – xInx(PO4)3-BASED
677
processes [29]. The NASICON-type materials with The mechanochemical activation of the samples
the general formula (NH4)1 – xHxHf2(PO4)3 (x = 0–1) was carried out in a Fritsch Pulverisette 7 planetary
mill using agate bowls with an inner volume of 45 mL,
each containing ten agate balls 10 mm in diameter.
The powder load was 750 mg. To each vial, 8 mL of
ethanol was added. The samples were milled for 24 h
at a rotation speed of 400 rpm. Every 3 min, the mill-
ing process was interrupted for 2 min.
The specific surface area of the samples was deter-
mined by BET measurements with a Micromeritics
ASAP 2020 analyzer. The samples were degassed at a
temperature of 350°C for 1 h. Analyses were carried out
at relative pressures (p/p0) in the range from 0.01 to 0.99.
exhibit catalytic activity for ethanol dehydration reac-
tions. Ethanol conversion predominantly yields
diethyl ether at low temperatures and ethylene at
higher temperatures. The composition of ethanol
dehydrogenation and dehydration reaction products
was shown to depend significantly on which singly
charged cation (H+ or
+) enters into the composi-
NH4
tion of the hafnium double phosphate [36].
In this paper, we report the synthesis of Li1 + xHf2 – x
Inx(PO4)3 NASICON-type phosphates, their phase
composition, and their catalytic activity for the etha-
nol conversion process. In addition, we examine the
effect of mechanochemical processing on the particle
size and catalytic activity of the materials.
-
The samples were examined by scanning electron
microscopy (SEM) on a Carl Zeiss NVision 40
equipped with an elemental analysis accessory. The
accelerating voltage was 1 kV.
Catalytic properties were studied in a flow-type
quartz reactor under flowing helium or argon with
ethanol additions. The reaction products were ana-
lyzed on a Crystallux 4000 M chromatograph
equipped with a thermal conductivity detector and
HayeSep T 60/80 mesh (2 m, 150°C, 30 mL/min,
He), SKT-6 (2 m, 150°C, 30 mL/min, He), and Mole
Sieve 5 A (2 m, 25°C, 30 mL/min, Ar) columns. In our
experiments, a weighed amount (0.3 g) of a catalyst
was mixed with ground quartz (d = 315–400 μm) and
placed in a reactor 6 mm in inner diameter so that the
catalyst layer was 17 cm in thickness. To ensure an
appropriate ethanol vapor concentration, the carrier
gas was passed at a volumetric flow rate of 20 mL/min
through a bubbler containing ethanol and thermo-
stated at 11°C.
EXPERIMENTAL
Li1 + xHf2 – xInx(PO4)3 (x = 0, 0.05, 0.1) materials
were prepared by solid-state reactions according to the
following scheme:
6(NH ) HPO + 4 – 2x HfO
(
)
4 2
4
2
(1)
+ xIn2O3 + (1 + x)Li2CO3
→ 2Li1+xHf2–xInx(PO4)3.
The starting chemicals used were (NH4)2HPO4
(Roth, >97%), HfO2 (Aldrich, 98%), In2O3 (analyti-
cal grade), and Li2CO3 (Merck, >99%). Stoichiomet-
ric ratios of the starting materials were mixed, thor-
oughly ground in a mortar, and fired at a temperature
of 600°C for 6 h in platinum crucibles. Next, the resul-
tant mixture was reground and pressed into pellets to
improve chemical interaction between particles. The
pellets were packed with powder of similar composi-
tion, containing 20% excess lithium carbonate, to pre-
vent the volatilization of lithium compounds from the
main material at high temperatures and annealed at
1000°C for 24 h.
Conversion and selectivity were evaluated using the
following relations:
ϕ0 − ϕ1
(3)
(4)
X =
S =
× 100%,
× 100%,
ϕ0
ϕi
ϕ0 − ϕ1
The phase composition of the samples thus pre-
pared was determined by X-ray diffraction on a Rigaku where ϕ0 and ϕ1 are the initial and final volume fractions
D/MAX 2200 diffractometer (CuKα radiation). For of the alcohol, respectively, and ϕi is the fraction of the
alcohol consumed for the formation of product i.
data processing and qualitative analysis of X-ray dif-
fraction patterns, we used the Rigaku Application
Data Processing and FullProf Suite software pack-
ages. The crystallite size was assessed from X-ray dif-
fraction line broadening using the Scherrer formula:
RESULTS AND DISCUSSION
According to X-ray diffraction data, the synthe-
sized lithium hafnium double phosphate LiHf2(PO4)3
is single-phase and has a hexagonal NASICON-
related structure (PDF-2, card no. 53-0569). The
X-ray diffraction patterns of the Li1 + xHf2 – xInx(PO4)3
kλ
(2)
d =
,
B2 − b2 cos θ
where d is the crystallite size, λ is the X-ray wavelength
used, θ is the diffraction angle, k = 0.9 is the Scherrer (x = 0.05, 0.1) doped materials also contain only
constant, B is the experimentally determined full reflections from a hexagonal phase, which are slightly
width at half maximum (FWHM) of the line, and b is shifted (Fig. 1). In addition, near 2θ ~ 27.3°, 45.3°,
the FWHM of a line of a standard. As a standard for and 53.7° the X-ray diffraction patterns contain
evaluating the instrumental broadening, we used LaB6 reflections from germanium, which was used as a stan-
powder (Standard Reference Material 660a).
dard (card no. 04-0545). The unit-cell parameters of
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678
NOVIKOVA et al.
Ge standard
x = 0.1
x = 0.05
x = 0
2
1
10
20
30
2θ, deg
40
50
14 16 18 20 22 24 26 28 30
2θ, deg
Fig. 1. X-ray diffraction patterns of the Li
Hf
In (PO )
1 + x 2 – x x 4 3
samples with different degrees of indium substitution for haf-
nium.
Fig. 2. Portions of the X-ray diffraction patterns of the
LiHf (PO ) samples (1) before and (2) after milling.
2
4 3
ples had an extremely small surface area: 1–2 m2/g.
Milling increased the specific surface area S to 12–
15 m2/g (Table 2).
Materials with a larger surface area are more active
catalysts for ethanol conversion [21]. In view of this,
we used ground materials in our catalytic ethanol con-
version experiments. Those materials exhibited cata-
lytic activity for ethanol dehydration and dehydroge-
nation reactions (Fig. 4). The main observed transfor-
mations can be represented by the following reaction
schemes:
the phosphates increase with an increase in the degree
of indium substitution for hafnium (Table 1), which
points to the formation of solid solutions. The
observed increase in unit-cell volume can be
accounted for by the larger ionic radius of In3+ in com-
parison with Hf4+ (0.80 and 0.71 Å, respectively [46]).
The mechanochemical processing of the samples
in the planetary mill led to X-ray diffraction line
broadening (Fig. 2). The crystallite sizes evaluated
from linewidths are presented in Table 2. For example,
the crystallite size of the as-prepared LiHf2(PO4)3,
Li1.05Hf1.95In0.05(PO4)3, and Li1.1Hf1.9In0.1(PO4)3 mate-
rials was 62, 41, and 42 nm, respectively. Mechanical
processing reduced the crystallite size by a factor of
1.3–1.5, to 41, 32, and 33 nm, respectively (Table 2).
X-ray diffraction line broadening can be due to both a
small particle size and increased defect density in the
structure [47, 48]. Electron microscopy data confirm
that milling reduced the average particle size (Fig. 3).
The as-prepared samples had the form of polydisperse
crystalline materials ranging in particle size from 1 to
10 μm (Figs. 3a, 3b). After mechanical processing, the
particles ranged in size from 30 nm to 1 μm (Figs. 3c–3f).
These values considerably exceed the crystallite size,
suggesting that the samples consist of agglomerates of
individual crystallites.
2C2H5OH → C2H5OC2H5 + H2O,
(5)
°
ΔH298 = –88.48 kJ mol;
C2H5OH → CH3CHO + H2,
(6)
(7)
°
ΔH298 = +17.36 kJ mol;
C2H5OH → C2H4 + H2O,
°
ΔH298 = +28.99 kJ mol.
The major reaction products are ethylene, diethyl
ether, C4 hydrocarbons, acetaldehyde, and hydrogen.
We also observed the formation of a small amount
(<0.3%) of C3 hydrocarbons.
According to low-temperature nitrogen adsorption
experiments, the unmilled Li1 + xHf2 – xInx(PO4)3 sam-
The onset temperature for noticeable ethanol con-
version (≥5%) and the composition of the product
Table 1. Unit-cell parameters of the samples with the general formula Li1 + xHf2 – xInx(PO4)3
Vcell, Å3
Sample
a, Å
c, Å
LiHf2(PO4)3
8.825 (2)
8.827(2)
8.829(1)
22.007 (3)
22.014(1)
22.016(1)
1484.4(9)
1485.4(7)
1486.6(4)
Li1.05Hf1.95In0.05(PO4)3
Li1.1Hf1.9In0.1(PO4)3
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CATALYTIC ACTIVITY OF Li1 + xHf2 – xInx(PO4)3-BASED
679
Table 2. Particle size and specific surface area of the Li1 + xHf2 – xInx(PO4)3 samples before and after milling
S, m2/g
Sample
Crystallite size, nm
Particle size (SEM), μm
LiHf2(PO4)3 (as-prepared)
LiHf2(PO4)3
62
41
32
33
1–10
0.05–1
0.03–1
0.03–1
1.5
13
Li1.05Hf1.95In0.05(PO4)3
Li1.1Hf1.9In0.1(PO4)3
12
15
1 μm
10 μm
(b)
(a)
200 nm
1 μm
(c)
(d)
1 μm
200 nm
(e)
(f)
Fig. 3. SEM images of the as-prepared LiHf (PO ) (a, b) and the milled samples: LiHf (PO ) (c, d) and Li Hf In (PO ) (e, f).
2
4 3
2
4 3
1.1 1.9 0.1
4 3
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680
NOVIKOVA et al.
On heating, the Gibbs energy of reactions (6) and (7)
(a)
becomes negative and the reactions become feasible.
With increasing temperature, dehydration to ethylene
according to scheme (7) becomes the main ethanol con-
version path on all catalysts, in spite of the poorer ther-
modynamics of this process. Ethylene selectivity
reaches 96, 88, and 85% for the LiHf2(PO4)3,
Li1.05Hf1.95In0.05(PO4)3, and Li1.1Hf1.9In0.1(PO4)3 sam-
ples at 510, 450, and 450°C, respectively (at 100% con-
version).
The dehydrogenation process (6) on these catalysts is
less active and has a higher rate in the case of the indium-
doped materials. Acetaldehyde selectivity reaches 4% for
LiHf2(PO4)3 (510°C, 100% conversion) and 8% for
Li1.05Hf1.95In0.05(PO4)3 and Li1.1Hf1.9In0.1(PO4)3 (420°C;
97 and 92% conversion, respectively).
100
80
60
40
20
0
Ethylene
Acetaldehyde
Diethyl ether
Conversion
250
300
350
400
450
500
Temperature, °C
(b)
In
addition,
in
the
case
of
the
100
Li1.05Hf1.95In0.05(PO4)3 and Li1.1Hf1.9In0.1(PO4)3 sam-
ples we observed the release of C4 hydrocarbons. The
formation of C4 hydrocarbons was found to pass
through a maximum at 420°C. C4 selectivity reaches
15 and 17% in the case of the Li1.05Hf1.95In0.05(PO4)3 and
Li1.1Hf1.9In0.1(PO4)3 samples, respectively (420°C).
Thus, the addition of a small amount of indium makes
it possible to obtain C4 hydrocarbons, which were not
detected in the case of LiHf2(PO4)3 (Fig. 4).
80
60
40
20
0
Ethylene
C4
Acetaldehyde
Diethyl ether
Conversion
Moreover, indium doping reduces the catalytic
transformation temperature. Ethanol conversion
reaches 100% at 510°C for LiHf2(PO4)3 and at 450°C
for Li1 + xHf2 – xInx(PO4)3. Comparison of the specific
surface areas of the catalysts (Table 2), which differ
very little, with catalysis results leads us to conclude
that, in the indium-doped materials, the number and
activity of centers responsible for the formation of C4
hydrocarbons and acetaldehyde are higher. This
changes the reaction path and the activity of the cata-
lysts at high temperatures. The materials studied here
were shown to be capable of stable operation for 24 h.
250
300
350
400
450
500
Temperature, °C
(c)
100
80
60
40
20
0
Ethylene
C4
Acetaldehyde
Diethyl ether
Conversion
CONCLUSIONS
We have synthesized Li1 + xHf2 – xInx(PO4)3 (x = 0,
0.05, 0.1) NASICON-type materials, which crystal-
lize in hexagonal symmetry. Milling in a planetary mill
has been found to reduce the size of primary particles
of Li1 + xHf2 – xInx(PO4)3 by a factor of 1.3–1.5 and
increase the specific surface area of the materials by
almost one order of magnitude (from 1.5 to 13 m2/g in
the case of LiHf2(PO4)3). Our results on the catalytic
properties of the synthesized materials in ethanol con-
version reactions demonstrate that they exhibit cata-
lytic activity for ethanol dehydration reactions and are
less active for ethanol dehydrogenation. Ethanol con-
version predominantly yields diethyl ether at low tem-
peratures (85% at 60% conversion at 350°C on
LiHf2(PO4)3) and ethylene at higher temperatures
(96% at 100% conversion at 510°C on LiHf2(PO4)3).
250 300 350 400 450 500
Temperature, °C
Fig. 4. Temperature dependences of selectivity for the for-
mation of ethylene, acetaldehyde, diethyl ether, and C
4
hydrocarbons in the presence of the (a) LiHf (PO ) , (b)
2
4 3
Li Hf In (PO ) , and (c) Li Hf In (PO )
4 3
1.05 1.95 0.05
4 3
1.1 1.9 0.1
materials after milling and ethanol conversion as a func-
tion of temperature.
mixture at low temperatures (270–350°C) are similar
for all of the materials studied (Fig. 4). At low tem-
peratures, the most thermodynamically favorable pro-
cess if dehydration to diethyl ether according to
scheme (5). The selectivity for this reaction product is
80–85%, with 60% conversion at 350°C (Fig. 4).
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ACKNOWLEDGMENTS
This work was supported by the Presidium of the
Russian Academy of Sciences (program no. 37P, proj-
ect no. I.37.3) and was carried out using equipment of
the Joint Research Center of IGIC RAS.
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