Mendeleev Commun., 2021, 31, 385–387
Amorpꢀous Alꢁꢂꢃ
ꢉꢊꢊ ꢋtꢌ
Surface area S ꢅ ꢂꢆꢆ m2 gꢁ1
ꢄbm Alꢂꢅꢂꢆꢇ
ꢃꢊꢍꢎꢏ ꢋtꢌ
S ꢅ ꢂꢂꢇꢁꢂꢈꢈ m2 gꢁ1
ꢈm Alꢂꢅꢂꢆꢇ
ꢎꢐꢍꢑꢊ ꢋtꢌ
S ꢅ 2ꢆ2ꢁ2ꢈꢄ m2 gꢁ1
ꢈm Alꢂꢅꢂꢆꢇ
ꢑꢉꢍꢉꢊꢊ ꢋtꢌ
S ꢅ 12ꢉ m2 gꢁ1
ꢀydrothermal
treatment
110ꢁ1ꢂ0 ꢃC
ꢀydrothermal
treatment
1ꢂ0ꢁ1ꢄ0 ꢃC
ꢀydrothermal
treatment
1ꢄ0 ꢃC
Pore volume V ꢅ 0.ꢇ0 cmꢂ gꢁ1
V ꢅ 0.ꢆ1ꢁ0.ꢄꢊ cmꢂ gꢁ1
V ꢅ 0.ꢄꢂꢁ0.ꢊ1 cmꢂ gꢁ1
V ꢅ 0.ꢄꢄ cmꢂ gꢁ1
ꢀ
ꢀ
ꢀ
ꢋotal amount of
acidic sites Nas ꢅ
ꢄꢄ0 ꢃC
ꢄꢄ0 ꢃC
ꢄꢄ0 ꢃC
ꢂ
ꢂ
ꢂ
ꢅ ꢊꢄ2 ꢌmol Nꢀꢂ gꢁ1
ꢄbm
ꢈm
ꢈm
ꢒAl ꢂ
ꢀꢒAlꢁꢂꢃ
ꢀꢒAlꢁꢂꢃ
ꢀ
ꢁ
ꢃ
S ꢅ 2ꢆꢄꢁ2ꢄꢈ m2 gꢁ1
S ꢅ 1ꢉ2ꢁ210 m2 gꢁ1
V ꢅ 0.ꢊꢆꢁ0.ꢉꢂ cmꢂ gꢁ1
S ꢅ 1ꢄꢈ m2 gꢁ1
V ꢅ 0.ꢆ1ꢁ0.ꢄꢈ cmꢂ gꢁ1
V ꢅ 0.ꢊꢆ cmꢂ gꢁ1
Nas ꢅ 2ꢉꢉꢁꢂꢊꢇ ꢌmol Nꢀꢂ gꢁ1
Scheme 1
Nas ꢅ 2ꢉ2ꢁꢂꢂꢄ ꢌmol Nꢀꢂ gꢁ1
Nas ꢅ 2ꢄꢄ ꢌmol Nꢀꢂ gꢁ1
Maximum fraction of Pbm (63.9 wt%) was found at 130 °C
after 60 min, while after 90 min at constant temperature in the
range of 110–130 °C it crystallized into Bm.
stabilized at 277–369 µmol NH3 g–1 with growth of the
g-Al2O3Pbm content up to 60 wt%, and the amount of weak acid
sites increased from 151 to 195 µmol NH3 g–1. The similar
Bm
The transformation of amorphous alumina into Pbm and Bm
under hydrothermal conditions proceeds through the dissolution–
precipitation mechanism.16,17 Spherical Al2O3 particles of ~11 nm
diameter constitute large aggregates of 3–133 µm size15 and
initially crystallize in the needle- and plate-like Pbm particles
with ~20 nm width and 70–100 nm length. Then these needles/
plates grow into parallelepiped-like Bm particles with 27–45 nm
width and 73–155 nm length composed of rhombic plates
(Figure S3). The influence of these phase and morphologic
transformations on the porous system of the hydrothermal
treatment products was explored in detail (Figures S4–S8). It is
known that starting amorphous alumina contributes to the pore
volume and specific surface area of the products with its pore
volume of 0.90 cm3 g–1 caused by the mesopores with a shoulder
on the differential curve of their volume distribution at ~10 nm
and the micropores formed between the coarse agglomerates of
the spherical particles, as well as with its specific surface area of
344 m2 g–1 due to ~60% of mesopores with 3–5 nm diameter.15
Pbm contributes new thin mesopores with maxima in the
differential curve of their diameter distribution at 2.8 and 3.8 nm.
This in turn leads to an increase in the specific surface area of the
hydrothermal products to 388 m2 g–1 and the decrease in their
pore volume to 0.41 cm3 g–1. Bm does not actually contain thin
pores, the pore diameter is 21 nm and the corresponding specific
surface area is as low as 127 m2 g–1.
pattern revealed after g-Al2O3 had appeared in the alumina
mixture, with total amount of acidic sites 255 µmol NH3 g–1 for
the samples with more than 80 wt% g-Al2O3Bm, mainly due to
the decrease in the weak acidic sites amount from 208 to
96 µmol NH3 g–1.
The changes in phase composition, texture characteristics and
acidic properties originated from the transformations of
amorphous alumina under hydrothermal conditions are collected
in Scheme 1.
Thus, as a result of hydrothermal treatment of amorphous
alumina at 110–150 °C with pressurised saturated water vapor
for up to 3 h, Pbm, Bm and Bt (up to 8.0 wt%), were formed. The
transformation of 64 wt% of amorphous Al2O3 into Pbm was
accompanied by a slight increase in the specific surface area
from 344 to 388 m2 g–1 and a decrease in the total pore volume
from 0.90 to 0.41 cm3 g–1 due to formation of thin pores with
diameters 2.8–3.8 nm between the primary particles of Pbm.
Further crystallization to ~80 wt% of Bm caused an increase in
the pore volume to 0.53–0.61 cm3 g–1 originated mainly from an
elevation of the volume of pores with diameter more than 5 nm.
After heat treatment at 550 °C for 3 h, the products of
hydrothermal treatment of Pbm and Bm conversed completely
Pbm
into g-Al2O3
and g-Al2O3Bm, respectively, while amorphous
Pbm
Al2O3 was stable up to ~700 °C. g-Al2O3
inherited the fine-
pore system of Pbm, which led to a decrease in the specific
surface area from 344 to 245–258 m2 g–1 and the pore volume
from 0.90 to 0.41–0.58 cm3 g–1. The porous system of aluminum
oxides obtained from Bm is characterized by enlarged pore of
diameter of more than 10 nm, which leads to an increase in the
total pore volume to 0.64 cm3 g–1 and a decrease in surface area to
158 m2 g–1. The total number of acidic sites on the surface of
aluminum oxides correlates well with the specific surface area.
Amorphous Al2O3 is characterized by predominance (47%) of
medium-strength acidic sites with the energy of ammonia
The hydrothermal treatment products were further
transformed into amorphous and/or crystalline Al2O3 after heat
Pbm
treatment at 550 °C. Alumina obtained from Pbm (g-Al2O3
)
inherited thin pores with diameter of 3.8 nm from its precursor,
while the specific surface area and pore volume decreased to
248 m2 g–1 and 0.51 cm3 g–1, respectively, with lowering to
30 wt% the content of amorphous Al2O3 in the hydrothermal
product. Specific surface area value was stabilized (245–
258 m2 g–1) with an increase in the g-Al2O3Pbm part in the product
to ∼60 wt% . Pore volume elevated to 0.41–0.58 cm3 g–1 due to
Pbm
Bm
desorption Ed = 110–142 kJ mol–1. g-Al2O3
and g-Al2O3
the growth of pore diameter to 7.0–14.3 nm. With appearance of
have weak acidic sites with Ed less than 110 kJ mol–1. As a
result of the hydrothermal treatment and the subsequent
maintenance at 550 °C, the total amount of acidic sites reduced
from 652 for amorphous Al2O3 to 255 µmol NH3 g–1 for
g-Al2O3Bm. The results obtained allows one to expect the stable
operation of a catalyst obtained by hydrothermal treatment of
amorphous alumina in, for example, dehydration of
1-phenylethanol into styrene, due to a sufficient number of the
surface acidic sites and a pore size of more than 10 nm.
Bm
g-Al2O3
in the composition of alumina with 40 wt% of
amorphous Al2O3 after the hydrothermal treatment, the pore
volume increased to 0.64 cm3 g–1 while the surface area
diminished to 172 m2 g–1.
Acidic properties of the alumina mixture are in general
determined by superposition of the individual phases acidity.
Starting amorphous Al2O3 is characterized by the highest
acidity of 652 µmol NH3 g–1 due to its maximal surface area,
with medium acidic sites predominating (46%). After
transformation of its 30 wt% into g-Al2O3Pbm, the total amount
of acidic sites decreased to 326 µmol NH3 g–1, i.e., by two
times. The amount of acidic sites in the alumina mixture was
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2021.05.034.
– 386 –
Egorova, Svetlana R.
