Aldol condensation of acetic acid and formaldehyde to acrylic acid over a hydrothermally treated silica gel-supported B-P-V-W oxide

Article abstract of DOI:10.1016/j.apcata.2020.117472

Supported on silica B-P-V-W-O_x catalysts of the aldol condensation process, subjected to hydrothermal treatment (HTT), were synthesized and characterized. It was found that HTT of silica has a significant effect on catalyst texture, namely specific surface area, pore size. These properties affect the distribution of the active phase on the support surface. It was suggested that this is the reason of change in acid-base characteristics of the catalysts. It was observed that HTT at 100?150 °C leads to increase in surface, available for the reagents, ability to sorb acetic acid, decrease of acidity, improving catalytic activity and selectivity. It had been demonstrated that HTT allows to increase acrylic acid yield up to 67.6 % which is 10 % higher compared to that for catalyst supported on initial silica. The HTT of silica support before the active phase deposition is suggested as a cheap and efficient way to improve catalytic performance of B-P-V-W-O_x/SiO₂ catalysts.

Full text of DOI:10.1016/j.apcata.2020.117472

Journal Pre-proof
Aldol condensation of acetic acid and formaldehyde to acrylic acid over a
hydrothermally treated silica gel-supported B-P-V-W oxide
R. Nebesnyi, Z. Pikh, V. Sydorchuk, S. Khalameida, I. Kubitska, O.
Khyzhun, A. Pavliuk, T. Voronchak
PII:
S0926-860X(20)30065-X
DOI:
https://doi.org/10.1016/j.apcata.2020.117472
APCATA 117472
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
26 November 2019
11 February 2020
17 February 2020
Please cite this article as: Nebesnyi R, Pikh Z, Sydorchuk V, Khalameida S, Kubitska I,
Khyzhun O, Pavliuk A, Voronchak T, Aldol condensation of acetic acid and formaldehyde to
acrylic acid over a hydrothermally treated silica gel-supported B-P-V-W oxide, Applied
Catalysis A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117472
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Aldol condensation of acetic acid and formaldehyde to acrylic acid over a
hydrothermally treated silica gel-supported B-P-V-W oxide
a
a
b
b*
a
c
a
R. Nebesnyi , Z. Pikh , V. Sydorchuk , S. Khalameida , I. Kubitska , O. Khyzhun , A. Pavliuk ,
d
T. Voronchak
a
Technology of Organic Products Department, Lviv Polytechnic National University, 12
S. Bandera Str., 79013, Lviv, Ukraine
b
Institute for Sorption and Problems of Endoecology, NAS of Ukraine, 13 Naumov St., 03164
Kyiv, Ukraine; *E-mail: svkhal@ukr.net
c
I.M. Frantsevych Institute for Problems of Materials Science, NAS of Ukraine, 3
Krzhyzhanovsky St., 03680, Kyiv, Ukraine
d
Market Central Laboratory, Nestle Ukraine LLC, Malekhiv, 2a Kyivska str., 80383, Lviv
region, Ukraine
Graphical abstract
Higlights


B-P-W-V oxide catalysts based on hydrothermally treated silica support
were developed and studied in aldol condensation of acetic acid with
formaldehyde to acrylic acid reaction.




Hydrothermal treatment of silica support results in formation of catalyst
porous structure more accessible for reagents molecules.
Supported catalysts based on hydrothermally treated silica has lesser content
of acid sites and more ability to acetic acid sorption.
Hydrothermal treatment of silica support at 150°C is optimal for preparation
of catalyst possessing the best catalytic performances.
The best catalyst allows to achieve acrylic acid yield up to 67.6% which is
1
0% higher than for catalyst based on untreated support.
Abstract:
Supported on silica B-P-V-W-O
x
catalysts of the aldol condensation process, subjected to
hydrothermal treatment (HTT), were synthesized and characterized. It was found that HTT of
silica has a significant effect on catalyst texture, namely specific surface area, pore size. These
properties affect the distribution of the active phase on the support surface. It was suggested that
this is the reason of change in acid-base characteristics of the catalysts. It was observed that HTT
at 100-150°C leads to increase in surface, available for the reagents, ability to sorb acetic acid,
decrease of acidity, improving catalytic activity and selectivity. It had been demonstrated that
HTT allows to increase acrylic acid yield up to 67.6% which is 10% higher compared to that for
catalyst supported on initial silica. The HTT of silica support before the active phase deposition
is suggested as a cheap and efficient way to improve catalytic performance of B-P-V-W-O
catalysts.
x
/SiO
2
Keywords – acrylic acid; aldol condensation; solid catalysts; porous structure;
hydrothermal treatment.
1
. Introduction
Acrylic acid (AA) is a large-scale chemical, world production capacity of which exceeds 5
million tons annually. Acrylic acid and its derivatives are used to produce high-quality paints and
varnishes, organic glass, concrete modifiers and as intermediate substances for fine organic
synthesis. Today, main industrial AA production method is two-stage oxidation of propylene via
intermediate stage of acrolein formation [1,2]. This method proved itself to be good in terms of
economic efficiency and equipment design simplicity. However, petroleum-based origin of

propylene and volatility of world oil market cause need of development of alternative methods of
AA and its derivatives production. So, the development of alternative methods of acrylic acid
synthesis, in particular from renewable raw materials, became a new trend. Such alternative
method of AA synthesis can be one of the following: 1) AA synthesis based on biomass using
_
*
______________________________
Corresponding author: Svitlana Khalameida. Tel. +38 044 452-93-28; E-mail:svkhal@ukr.net
enzymes; 2) AA synthesis based on glycerol as a by-product of biodiesel production; 3) AA
synthesis by aldol condensation of acetic acid (AcA) with formaldehyde (FA) [2,3]. Production
of AA from renewable raw materials is carried out by fermentation of biomass to 2-
hydroxypropanoic or 3-hydroxypropanoic acid with further dehydration to AA. The principal
weakness of this approach is considerable duration of the process [4-6] and consequently low
productivity of reaction equipment. There are numerous publications dedicated to glycerol
oxy-dehydration into acrolein and acrylic acid [4]. One-pot glycerol oxy-dehydration to AA over
multifunctional catalysts was also reported [5], but the highest AA yield does not exceed 50 –
7
5 % at comparatively low process selectivity.
AA production by aldol condensation of AcA with FA (via the scheme: biomass (natural
gas, coal) – syngas – methanol – acetic acid and formaldehyde – acrylic acid) also can be
considered as a promising approach. Acetic acid is produced mostly via the Monsanto process
(catalytic carbonylation of methanol) and formaldehyde is produced by oxidation of methanol.
Both acetic acid and formaldehyde are relatively cheap and widely available chemicals.
However, industrial-scale AA production by aldol condensation has not been arranged so far,
because of insufficient performance of known catalysts of the process. Besides low efficiency of
known catalysts of aldol condensation of unsaturated carboxylic acids with formaldehyde, their
another disadvantage is short working time because of coke deposition on their surface [2,7].
That is why development of new highly efficient catalysts for AA production by aldol
condensation of AcA with FA without aforementioned disadvantages is still very urgent.
A lot of publications are devoted to AA synthesis on V-P-O catalysts. Early studies
describe the use of vanadium and mixed vanadium-titanium phosphates [8,9]. The used catalysts
possessed insufficient specific surface area, undeveloped porous structure and required
granulation. Therefore, in order to increase their productivity, the supported catalysts
(
predominantly based on silica as a support) were tested in this process [10-12]. The authors
4
+
5+
found that the balance between acid and base sites and the ratio of V and V had significant
influence on catalytic performance and this ratio is dependent on V and P content [11]. The
authors also noticed that calcination temperature effected on catalysts texture, and both textural
properties and ratio of V /V of V-P-O catalyst could be considered to be the main factors
favoring the catalytic performance increase [10]. However, despite the high specific surface area
and developed porous structure, these catalysts did not show good catalytic performance. The
highest yield of AA was only 21.9% with a selectivity respect to AA 97.8%.
4
+
5+
Better results were obtained with V-P-O catalyst deposited on supports with regular
porosity, namely SBA-15 and HZSM-5. The acrylic acid selectivity was 90.8 – 70.2% at the
formaldehyde conversion of 14.3 – 68.7% [13]. It was found that the catalytic performance,
among other factors, depends on acid and base properties of the catalyst, and the acid and base
sites number were significantly affected by the support type and P/V atomic ratio. The SBA-15-
supported V-P-O catalysts with high number of both acid and base sites exhibited high catalytic
activity in the aldol condensation of AcA with formaldehyde to AA. On the other hand, V-P-
O/SBA-15 catalyst possesses the highest specific surface area among the tested catalysts and the
authors noticed that the correlation between catalyst structure and its activity is worth of further
study. The later paper [14] describes the effect of Cs, Ce, and Nd cations on the efficiency of V-
P-O/SiO
2
catalysts. The addition of the metallic cations in the catalysts increased their basicity,
while their acidity increased at a lower ratio of metallic cation to vanadium. It was also noted
5
+
4+
that the presence of the metallic cations also increased the V /V ratio. Maximum AA yield on

Ce-V-P-O/SiO catalysts was 73.9%. It should be noted that in papers [13,14] the acetic
2
acid/formaldehyde mole ratio was 3:1.
The particularity of all above-mentioned papers [10-14] is that the catalysts structure was
changed as a result of change in qualitative and quantitative composition of the catalysts or the
support type and, as a result, a lot of characteristics of the catalysts’ surface were changed
5
+
4+
(
specific surface area, pore size, acid and base properties, V /V ratios). Thus, it is quite
difficult to separate one factor and its impact on catalytic performance from another.
In our previous studies, the catalysts of the following qualitative composition: B–P–V–W–
O
x
/SiO was described. These catalysts are rather efficient in gas-phase aldol condensation of
2
AcA with FA to AA. It was found out that the optimal atomic ratio of the catalyst components
B:P:V:W is 3:1:0.18:0.12 and this type of catalysts is effective even at equimolar ratio of AcA
and FA in the reaction mixture [3, 15-18]. The aim of the present paper is to determine the effect
of the catalyst porous structure on its catalytic performance in aldol condensation of AA with
formaldehyde at constant qualitative and quantitative composition of the catalysts. It is well
known that porous structure of solid catalysts has a significant effect on their catalytic properties.
To modify structure of porous materials such method as hydrothermal treatment is widely used.
Hydrothermal treatment (HTT) is effective procedure for regulation of porous, crystal and
surface structure of adsorbents, catalysts and their supports [19-21]. Particularly, HTT of
different kinds of silica has been extensively studied [19-26]. This method allows to vary the
parameters of silica gel porous structure in a very wide range. The elevation of HTT temperature
results in monotonic and significant reduction in specific surface area and increase of pore size at
constant pore volume [20,23,24]. Besides, increase in mechanical strength of silica gel granules
takes place up to the treatment temperature about 250°C [24, 27-29].
In present work it was important to ascertain the effect of HTT of the silica support for B–
P–V–W–O catalyst on its efficiency in the process of AA production by aldol condensation of
x
AcA with FA. It was also important to find out exactly, what values of the catalyst support
characteristics, achieved by means of HTT, ensure catalytic efficiency of the catalyst in aldol
condensation of AcA with FA to form AA.
2
. Experimental
2
.1. Materials
2
Silica gel with a specific surface area 365 m /g (China) was used as a catalyst support. AR
grade chemicals H BO , (NH HPO , H (P(W )·H O and NH VO (Sigma-Aldrich) were
3
3
4
)
2
4
7
2
O )
7 6
2
4
3
used for the catalyst preparation. AR grade AcA and FA (Sigma-Aldrich) were used for the
catalyst testing in chemical reaction. 37% formalin was used as a source of FA, and the formalin
was obtained from paraformaldehyde (Sigma-Aldrich).
2
.2. Catalyst preparation
To regulate the porous structure of the support, silica gel was subjected to HTT in the
temperature range 100-250°С for 3 h. The treatment was carried out in the vapor-phase [20, 22]
using laboratory steel autoclave of 45 mL volume. The weight of silica (10 g) was placed in a
PTFE vessel and water (5-10 mL depending on temperature of HTT) was poured on the bottom
of the autoclave. After HTT, the support was dried at 100°C for 5 h.
The active phase of the catalyst was deposited on initial and hydrothermally treated
supports by impregnation. Aqueous solutions of H
NH VO taken in amounts to ensure atomic ratio of the catalyst components B:P:V:W to be
:1:0.18:0.12 in finished catalyst, were used for the catalyst preparation. After the impregnation
the catalyst precursor was dried at 150°C for 8 h and then calcined in air at 400°C for 6 h. The
3
BO
3
, (NH
4
)
2
HPO
4
, H
7
(P(W
2
O
7
)
6
) and
4
3
3

finished catalysts contain about 20 %w/w of active phase. The prepared supported catalysts
based on initial and treated silica are hereinafter named as Cat-Init and Cat-HTT-t (where t –
temperature of silica HTT), respectively. Bulk catalyst (without support) was prepared under the
same conditions.
2
.3. Catalyst characterization
The porous structure of the prepared supports and catalysts was studied using the
adsorption-structural methods. Nitrogen isotherms of adsorption-desorption were obtained using
an automatic gas adsorption analyzer ASAP 2405N (by Micromeritics Instrument Corp.) after
outgassing the samples at 150°C for 2 h. The specific surface area S, volume of mesopores Vme
and volume of micropores were calculated from these isotherms using the BET, BJH and t-
method, respectively. The total pore volume V was determined by means of impregnation of the
Σ
samples granules after their drying at 150°C with liquid water (so-called incipient wetness
method) [30]. The curves of pore size distribution (PSD) were plotted using the desorption
branches of isotherms. Besides, the isotherms for one of the reagents, namely acetiс acid, were
obtained at 20°C from vapour phase to relative pressure p/p
0
=0.5 using the adsorption setup with
the McBan-Bakr spiral quartz balances [30]. Before measurements, the samples were degassed at
-3
residual pressure 10 Pa and 150°C for 2 h. This data has been used for two purposes: 1) for
calculation of specific surface area accessible for acetic acid SAcA; 2) for estimation of basic
properties for the supported catalysts.
The crystal structure of the catalysts was studied by means of X-ray powder diffraction
(
XRD) using Philips PW 1830 diffractometer with CuKα-radiation. The FTIR spectra in the
-1
range 4000 – 400 cm were registered using the “Spectrum-One” spectrometer (by Perkin-
Elmer). The ratio of sample and KBr powder was 1:20. KBr was dried at 600°C for 2h before the
measurements.
The selected samples have been tested with X-ray photoelectron spectroscopy (XPS) to
measure the binding energies (BEs) of the core-level electrons as well as the energy distribution
of the valence electronic states (XPS valence-band spectra). The XPS spectra were acquired with
the UHV-Analysis-System. The system was designed and assembled by SPECS Surface Nano
Analysis Company (Berlin, Germany).
The curves of DTA-TG for spent catalysts (tested for 8 h) were registered in air using the
Derivatograph-C apparatus (F.Paulik, J.Paulik, L.Erdey system) from 20 to 1000°C at a heating
rate 10 °/min. The initial mass of the sample was 200 mg.
Surface acidity of supported catalysts was determined using temperature-programmed
desorption of ammonia [30]. The sample was placed in a glass reactor and heated to 410°C in a
flow of helium. This temperature of pretreatment is selected since the catalytic tests were carried
о
out at a temperature ≤ 410°C. Then the sample cooled to 20 C and saturated with ammonia.
Next heating of the sample was carried out at a rate 17 °/min. The thermodesorption of ammonia
was carried out in the temperature range 20-650°C. The amount of desorbed ammonia was
determined by titration with hydrochloric acid at the outlet from the reactor.
2
.4. Catalysts performance study
Catalytic performance of the catalysts in aldol condensation of AcA with FA to AA was
studied in a fixed bed flow reactor. The reactor was made of stainless-steel tube (250 mm long,
1
0 mm I.D.) mounted vertically and equipped with electric heating. Amount of catalyst with
granule size of 1.5-3 mm was 3.5 g. Before being used the catalysts were dried at 150°C for 2 h.
A reaction was carried out under atmospheric pressure from 290 to 410°С and space time
8
s. The reactants molar ratio AcA:FA = 1:1. The reaction products composition was determined
by gas chromatography (instrument HP 6890). A capillary chromatographic column Restek with
0 m length and Srabilwax as stationary phase is used. The main products were acrylic acid,
3

acetone and carbon dioxide. The molar ratios of CO
2
to acetone were close to 1, indicating that
both CO and acetone are formed in the same reaction of acetic acid ketonization. So the
2
catalytic performance was evaluated by acrylic acid selectivity and yield as well as by acetic acid
conversion.
The yield and selectivity were calculated based on the amount of acetic acid fed into the
reactor. The acrylic acid yield (YAA) is defined as 100 moles of acrylic acid / moles of acetic acid
fed in. The acrylic acid selectivity (SAA) is defined as 100 moles of acrylic acid / moles of acetic
acid consumption. The acetic acid conversion (XAcA) is defined as 100 moles of acetic acid
consumption / moles of acetic acid fed in. Carbon balance (>90%) was achieved under reaction
conditions which is acceptable for such reactions [5].
3
3
3
. Results and Discussion
.1. Catalyst characteristics
.1.1. Porous structure
It was very important to find out the effect of HTT on the catalyst performance, so first of
all the porous structure of treated supports and supported catalysts were characterized. The
obtained results for silica gel HTT, based on nitrogen adsorption-desorption isotherms (Fig. S1)
are quite consistent with those described in literature [8, 9, 11, 12]. The PSD curves calculated
from desorption branches of isotherms (insets to Fig. S1) show that the maximum is shifted
towards higher values of pore diameter when temperature of HTT increases.
All samples are mesoporous and the values of V and Vme are almost identical (compare
Σ
columns 4 and 5 of Table 1) and there are no micropores. Thus, we have the uniformly
mesoporous supports, specific surface area of which monotonically decreases while the pore size
monotonically increases, and the pore volume remains almost constant with HTT temperature
o
elevation (Table 1). PSD curves obtained for silica gel, subjected to treatment at 200-250 С, are
diffused (Fig. S1). Consequently, these supports have a less uniform pore structure than those
o
modified at 100-175 С.
Table 1. Porous structure parameters of silica supports
2
3
3
N
Sample
S
N2 (m /g) V
Σ
(cm /g) Vme (cm /g)
d (nm)
1
2
3
4
5
6
1
2
3
4
5
6
7
8
Initial silica
365
1.05
1.05
1.04
1.04
0.99
0.92
0.96
0.98
1.04
1.07
1.04
1.02
1.00
0.91
0.97
0.96
7.9
7.9
8.7
9.7
13.6
17.9
25.0
31.5
o
HTT at 100 C 350
o
HTT at 125 C 316
o
HTT at 150 C 275
o
HTT at 175 C 201
o
HTT at 200 C 141
o
HTT at 225 C 119
o
HTT at 250 C 102
The deposition of active phase onto all supports are accompanied by decrease both of
specific surface area and pore volume V but values of V are almost the same for all supported
Σ
Σ
catalysts (Fig. S2, Table 2). This is associated with partial filling of pores by deposited phase.
Nevertheless, the value of specific surface area of supported catalysts calculated by acetic acid
sorption SAcA is lower than SN2 for the catalysts prepared from silica gel treated at temperature
o
o
<
150 С. Besides, SAcA has maximum value for the catalyst based on silica treated at 150 С (Cat-
HTT-150). This is evidence that part of the surface is inaccessible for larger molecules of acetic
acid – its kinetic diameter is 0.44 nm against 0.36 nm for N . At the same time, both values of
2

specific surface area practically coincide for catalysts prepared using silica treated at 150°C and
higher temperature. In other words, elevation of HTT temperature is accompanied by increase in
pore size both of support and corresponding catalyst and shift of PSD curves towards higher d
values. As a result, accessibility of pores for AcA molecules and therefore SAcA value also
o
increase. After HTT at 150 C, pores diameter from maximum on PSD curves reaches value
(9.7 nm for support and 12.3 nm for catalyst) that ensures complete accessibility of the pore
surface for AcA molecules. At higher HTT temperatures, the pore diameter continues to
increase, and the values of SAcA and SN2 monotonously decrease. This is possible if the supported
catalysts contain bottle-shaped pores, so-called “ink-bottle effect” [31,32]. Such effect can be
eliminated as a result of hydrothermal or mechanochemical treatment by increasing the size of
the entrances to the pores, as was observed for silica gel and zirconium phosphate [19,33].
Therefore, the accessibility of pores for adsorbate molecules, used for specific surface area
measurements, as well as reagents in catalytic processes increases [19,32,33]. The pore size of
the catalysts also increases compared to corresponding supports (compare insets to Fig. S1 and
Fig. S2).
Table 2. Parameters of porous structure of supported catalysts
Fresh catalyst
Spent catalyst
N
Sample
S
N2
S
AcA
V
Σ
d
S
N2
V
Σ
d
2
2
3
2
3
(m /g)
(m /g)
(cm /g) (nm)
(m /g)
(cm /g) (nm)
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
Cat-Init
238
178
180
190
197
128
110
n.d.
n.d.
0.74
0.75
0.74
0.72
0.65
0.73
0.70
0.72
9.7
9.7
238
251
241
199
138
110
79
0.69
0.72
0.75
0.74
0.64
0.74
0.69
0.72
8.0
8.7
9.6
12.3
14.8
17.8
23.4
32.0
Cat-HTT-100 245
Cat-HTT-125 232
Cat-HTT-150 193
Cat-HTT-175 130
Cat-HTT-200 107
Cat-HTT-225 84
Cat-HTT-250 62
11.1
12.6
14.8
17.8
24.4
31.8
55
The prepared supported catalysts have thermostable porous structure in reaction mixture
Table 2): its parameters practically did not change in the temperature range, in which the
(
catalytic tests were performed (subsection 3.2). Only the spent catalyst (after 8 hours of the
reaction) based on initial silica and those ones after HTT at low-temperature (100-125°C) have
less values of total pore volume and size of mesopores compared to those for corresponding
fresh catalysts. This may be due to coking process, which is obviously possible in narrow pores
(up to 10-11 nm) but is less likely in pores of larger size. Indeed, the granules of catalyst,
prepared from initial silica (Cat-Init), become almost black after the catalytic test (Fig. 1). As
higher the HTT temperature for silica (or as wider the pores both of the support and
corresponding catalyst) is, the lighter color of the spent catalyst granules is (Fig. 1). So, the
catalysts, which are based on silica hydrothermally treated at 150°C and above, might have
longer catalyst life.

0
Fig. 1. Granules of B–P–V–W–O
x
/SiO catalysts: а –Cat-Init (fresh catalyst), a–Cat-Init (spent
2
0
catalyst), b –Cat-HTT-150 (fresh catalyst) and b–Cat-HTT-150 (spent catalyst)
Besides, the results of DTA-TG analysis confirm the formation of carbonaceous deposits
on the catalysts surface as shown in other studies [34-36]. The mass loss within 300-500°C,
which corresponds to exoeffect at 400-450°C, indicates this. The mass loss and exoeffect are
attributed to combustion of carbon deposits (coke) in the catalyst's pores. The coking degree
decreases with elevation of support HTT temperature, which is associated with increase of pore
size. Minimal mass loss value of 0.7%w/w was registered for spent catalyst based on silica gel
hydrothermally treated at 250°C (Cat-HTT-250) while maximal value, which is about 5%w/w,
was obtained for spent catalyst Cat-Init. For spent catalyst based on treated at 150°C mass loss is
about 1%w/w. The examples of such DTA-TG curves are present on Fig. S3.
3
.1.2. Crystal and surface structure
The crystal structure of bulk and supported catalysts was studied by means of XRD and
FTIR spectroscopy. The obtained diffractograms are shown in Fig. 2. It can be seen, that only
reflexes attributed to boron oxide B
2
O
3
are present in diffractograms for bulk catalysts
(
unsupported B–P–V–W–O ): there are peaks at 2Θ = 14.64; 24.60; 28.06 and several low-
x
o
intensity peaks around 2Θ =39-41 (curves a, b) [37]. The position and intensity of main reflexes
is somewhat different as a result of active phase deposition. Thus, reflexes at 2Θ = 14.64 and
o
2
4.60 is superimposed on the halo (curves c, d), which is typical for amorphous silica. The
diffraction patterns are not changed after catalysis (compare curves a, b and c, d in Fig. 2). The
calculation of crystallite size D for B using Sherrer’s equation allows to estimate the degree
of dispersion of the deposited phase. The most intense reflex 2Θ = 24.60 was used for this
purpose. The comparison of the D values for bulk and supported samples indicates that B
2
O
3
o
2
O
3
crystallites have a smaller size for the latter catalysts. Thus, for the bulk sample D is 37.2 nm, for
the catalyst supported on initial silica gel – 29.6 nm, and for the catalysts supported on silica gel
o
after HTT at 150-250 С – 23-25 nm. The latter indicates the dispersion of the active phase on
the surface of the support.

Fig. 2. X-Ray difractograms for bulk catalyst before (a), after catalysis (b) and catalyst Cat-HTT-
o
1
50 before (c), after catalysis (d) and bulk catalyst calcined at 600 С (e).
Phosphorus oxide, obviously, is a part of amorphous or X-ray amorphous phases. For
example, it can form amorphous boron phosphate BPO
catalysts contain a mixture of crystalline B and BPO
4
with part of boron oxide. Indeed, bulk
after calcinations at 600 С when
o
2
O
3
4
amorphous component is crystallized (Fig. 2, curve e). Particularly, the peaks at 2Θ = 23.21,
o
2
4.52, 40.02, 48.90, 50.20 are attributed to BPO (JCPDS N 00–034–0132) [37-40]. The
4
characterization of bulk catalyst was performed because of small amount of active phase in the
catalyst, so the changes of reflexes for bulk catalyst is easier to notice.
Analysis of FTIR spectra (despite substantial overlapping of absorption bands - a.b. -
attributed to support and active phase) allowed additionally to determine structure of catalysts. In
general, FTIR spectra confirm the XRD data. The most intensive a.b. in the FTIR spectra for
bulk catalysts, which are recorded in the absorbance mode, are also attributed to B
2
O
3
(Fig. S4):
-1
3
9
208, 1459, 1379, 1195, 885, 800, 651, 549 cm [37,41]. Besides, a.b. centered at 1090-1120,
30-950, 610-640 and 550-560 cm are revealed in spectra of bulk catalysts. They are assigned
[37-40].
-1
to BPO
4

Fig. 3. FTIR spectra of the B–P–V–W–O catalyst supported on initial and treated silicas
x
However, these bands cannot be clearly detected for supported catalysts due to the strong -
1
overlapping with bands related to siliceous support (Fig. 3). Intensive a.b. around 3208 cm is
revealed in spectra for bulk and supported catalysts (Fig. 4). This band is attributed to stretching
vibrations of surface OH-groups both for boron oxide and phosphate [39,40,42]. As can be seen
-1
in Fig. 4, the reduction in the intensity of a.b. at 3208 cm is observed as a result of the support
HTT temperature elevation.
-1
Also, a.b. as shoulder at 3650-3700 cm is present in all spectra. It is obviously assigned
to stretching vibrations of surface SiOH-groups of silica support [19,25,26]. Low intensity of this
a.b. compared to that for the pure silica (Fig. 4) is observed. At the same time, its intensity is
reduced with elevation of the support HTT temperature. The latter can be explained by a specific
surface area decreasing (Table 1, column 3), as well as by denser coating of the support surface
with the deposited phase. Indeed, constant content of deposited phase (20 %w/w) is distributed
onto a smaller surface and covers it more densely.

Fig. 4. Band of stretching vibrations of surface OH-groups in FTIR spectra for initial silica and
B–P–V–W–O catalyst supported on initial silica and silica treated by HTT at various
x
temperatures
Surface state of atoms was evaluated using XPS. Thus, survey XPS spectra of the fresh
catalysts 1 and 4 from Table 2 as well as the bulk catalyst are presented in Fig. 5. XPS O1s,
Si2P, P2p, B1s core-level spectra for these catalysts are presented on Figs. S5-S8. As can be
seen, the positions and shape of peaks change a little as a result of deposition on silica. Their
binding energies are as follows: 532.9, 103.4, 134.6-134.9 and 193.2 eV. First value is close to
those for oxygen in B
correspond to silicon in SiO
respectively [44]. It should be noted that binding energies of B1s for B
2
O
3
and SiO
2
– 533.5 and 532.3 eV [42, 43]. Other values obviously
(and/or BPO
and BPO differ only
2
, phosphorus in meta-phosphates, boron in B
2
O
3
4
),
2
O
3
4
by 0.3-0.5 eV [37]. XPS spectra associated with W is rather small by its intensity (its content is
about 1.0 %w/w in the supported catalysts), but it is possible to determine binding energy of the
most informative W 4f7/2 core-level spectra of this chemical element (36.6 eV). Comparison with
literature data [44,45] indicates that tungsten is in the effective charge states +6 in studied
catalysts. The presence of vanadium is very difficult to determine in the analysed layers even for
the bulk sample and especially for the both supported samples under consideration due to its low
content (about 1.5 %w/w in supported catalysts) and perhaps also due to the fact that it is mainly
located in the subsurface layer (deeper than 3 nm). Nevertheless, a low-intensity diffuse peak
with a maximum, which can be divided into two components сentered about 516.4 and 517.4 eV,
4
+
5+
is visible (Fig. S5, inset). They can be attributed to V 2p 3/2 and V 2p 3/2, respectively [11].
4
+
5+
However, it is not possible to establish the V /V ratio, as was done in [11]. Indeed, the
catalysts are greenish, indicating the presence of V (Fig. 3).
4
+

Fig. 5. Survey XPS spectra of the bulk, Cat-Init and Cat-HTT-150 catalysts.
Table 3 shows surface content of analyzed elements for bulk catalyst as well as for Cat-Init
and Cat-HTT-150 catalysts, namely supported on initial and hydrothermally treated at 150°С
silica gel. Two differences between these catalysts should be noted: /i/ decrease of phosphorus
content in a row: bulk catalyst - Cat-Init – Cat-HTT-150; this may be the reason for the
minimum concentration of acid sites for Cat-HTT-150 (see below, Table 4, column 5); /ii/
simultaneous decrease in silicon concentration and increase in boron concentration (two
elements with the highest content) or increase in surface B/Si ratio from 0.73 to 1,28. The latter
may indicate more complete covering the silica surface by deposited phase for Cat-HTT-150.
The presence of nitrogen in bulk catalyst (Fig. 5, Table 2) can be associated with incomplete
thermodestruction of ammonium salts that are used as reagents for the preparation of catalysts.
Indeed, maximum positioned at 401.9 eV corresponds to nitrogen in oxidation state -3 [44]. The
absence of this peak in the spectra obtained for supported catalysts can be explained by complete
destruction of ammonium salts on silica surface.
Table 3. Elemental content (at. %) in the analysis surface layers of the bulk and supported
catalysts.
Sample
O
Si
P
B
W
C
V
0.4
-
N
3.1
-
Bulk catalyst
Cat-Init
45.9
58.1
58.7
-
7.3
1.3
1.1
27.1
15.6
21.3
0.9
0.3
0.2
15.3
3.2
2.0
21.5
16.7
Cat-HTT-150
-
-
Therefore, based on the XRD and FTIR results, it can be assumed that the deposited layer
consists of phases of boron oxide and phosphate in amorphous and crystalline states. The state of
vanadium and tungsten in the catalyst structure cannot be determined using indicated techniques
due to their low concentration (about 1.5 %w/w in supported catalysts). Perhaps they play the
role of dopants like other metals for supported V-P-O catalysts of aldol condensation of AcA
with FA, which regulate their acid-base properties [14].

The examples of TEM micrographs of catalyst Cat-HTT-150 are presented in Fig. 6.
Particularly, image 6a demonstrates sufficiently uniform distribution of active phase on the
structure of the support. At the same time, the histograms based on images 6b and 6c with higher
resolution have maxima for particle size about 75-80 nm. Obviously, these are aggregates of
crystallites with a size of about 23-25 nm, as was shown above by the Scherrer method.
Fig. 6. TEM micrographs of supported catalyst Cat-HTT-150. The magnification was 56000 (a),
7
2000 (b) and 100000 (c)
3
.1.3. Active acid sites concentration by NH -TPD
3
The curves of ammonia TPD presented in Fig. 7 contain one broad peak of ammonia
desorption with maximum within temperature range 207-233°С (Table 4). Only for the catalyst

Fig. 7. NH
3
-TPD curve for B–P–V–W–O catalyst supported on initial silica and silica
x
hydrothermally treated at different temperature
This indicates the presence of predominantly weak and medium acid sites on the surface.
This is consistent with results of other work devoted to investigation of boron oxide acidity [42].
However, asymmetry of peaks toward higher temperatures also evidences existence of more
o
acidic groups on the surface. Thus, the second peak at about 544 С is detected in the TPD curve
for catalyst Cat-HTT-250 (Fig. 7). These sites can be attributed to phosphate-groups of BPO
acidity of which were studied using ammonia TPD [46,47]. So, it can be assumed that the
surface groups B-OH and P-OH are weak and medium or strong acid sites, respectively.
4
,
The results calculated from TPD curves are presented in Table 4. The catalyst based on
initial silica is an exception since maximum on ammonia TPD curve for this catalyst is detected
at 172°C. This may be associated with the contribution of silanol groups, which were registered
in FTIR spectra (Fig. 4) and are weakly acidic [24,46,48]. These groups can be accessible for
ammonia molecules because the coverage of initial (untreated) silica with the active phase is still
not continuous as mentioned above. The degree of surface coating with the deposited phase for
other catalysts is higher, and therefore, the peak from SiOH-groups decreases.
The total acidity (in mmol/g) monotonically decreases with the support HTT temperature
elevation (column 4). This is obviously associated with the same reduction in specific surface
area (Table 1). The intensity of a.b. corresponding to surface hydroxyl groups in FTIR-spectra of
supported catalysts decreases in the same sequence (Fig. 4).
Table 4. The acid sites concentration calculated from ammonia TPD curves
Temperature
of peak (°C)
3
Total concentration of acid sites
N
Sample
2
(mmol/g)
(μmol/m )
1
2
4
5
1
2
3
4
5
6
7
8
Cat-Init
172
207
215
216
233
230
217
220
0.44
0.42
0.24
0.16
0.12
0.13
0.10
0.08
1.85
1.72
1.05
0.82
0.92
1.21
1.24
1.29
Cat-HTT-100
Cat-HTT-125
Cat-HTT-150
Cat-HTT-175
Cat-HTT-200
Cat-HTT-225
Cat-HTT-250

In contrast to total acid sites concentration per mass unit, the dependence of total acid
2
sites concentration expressed in μmol/m on HTT temperature of the support has a sharp
minimum at 150°С (Table 4). Perhaps it is associated with a different thickness and structure of
the layer of the deposited active phase in pores of different sizes (Table 2), different active phase
distribution on surface of the support (Table 3) and, as a result, different acid-base properties (the
ratio of acid and base sites) of the catalysts supported on silica hydrothermally treated at 100-
2
50°C.
3
.1.4. Active base sites concentration by acetic acid adsorption
2
The absolute isotherms of AcA adsorption (in μmol/m ) obtained for initial silica and
some supported catalysts are presented in Fig. 8. It should be noted that the initial parts of
isotherms, corresponding to the formation of a monolayer and obeying the BET equation (p/p
0
value up to 0.25-0.28), are informative for characterizing the surface. It is well known that acetic
acid, as well as other carboxylic acids, is adsorbed strongly, but physically, on silica -OH groups
[
24, 49 and references to these papers]. As can be seen in Fig. 8, the isotherm obtained for initial
silica support is located below the others. Four isotherms obtained for catalysts, supported on
initial silica and silica treated at different temperature, are located close to each other and
partially superimposed. Finally, the isotherm for catalyst based on silica, hydrothermally treated
at 150°C, is significantly higher than all the others. The excess of AcA adsorption value for the
supported catalysts over that for the initial silica can be considered as a measure for basicity of
deposited phase. Therefore, an interesting experimental fact is observed: the catalyst Cat-HTT-
1
50 has the maximum ability of acetic acid sorption. On the other hand, this catalyst is also
2
characterized by the minimum of acid sites concentration expressed in μmol/m (Table 4). This
may indicate bi-functional nature of studied supported catalysts. As a result, HTT of the support
can be considered as a way for acid-base ratio regulation.
Fig. 8. Acetic acid sorption on initial silica and B–P–V–W–O catalyst supported on initial silica
x
and silica treated by HTT at different temperature

3
3
.2. Catalytic performance
.2.1. Effect of support HTT temperature
Preliminary HTT of silica support has significant effect on catalytic properties of B–P–
V–W–O
x
/SiO catalyst, namely reagents conversion and products selectivity. Elevation of
2
support HTT temperature from 100 to 150°C results in increase of AcA conversion (X, %), while
further elevation of HTT temperature leads to considerable decrease in conversion (Fig. 9a). Rise
of reaction temperature naturally results in AcA conversion increase. The highest achieved value
of AcA conversion is 81.2%. Such kind of dependence is connected with the porous structure of
both supports and catalysts based on them. Firstly, although the specific surface area SN2 of
catalysts based on supports treated at 100-150°С decreases, its accessibility for the molecules of
reagents as well as the ability of the reaction products to leave the catalyst’s surface increase
(Fig. 10, Table 2). The latter is confirmed by some increase in SAcA and decrease in excess of SN2
over SAcA in indicated range of HTT. Besides, increase of predominant pore size from 9.7 to 12.6
nm takes place (inset to Fig. S2, Table 2). Therefore, possible diffusion resistance is eliminated.
Fig. 9. The effect of the support HTT temperature on acetic acid
conversion (a), acrylic acid selectivity (b) and acrylic acid yield (c) at different reaction
temperatures (space time 8 s, the reactants molar ratio AcA:FA = 1:1)
Secondly, for catalysts based on supports treated at higher temperature (>150°С), the
value of specific surface area has decisive importance: its reduction is accompanied by a
decrease in conversion. As can be seen from Table 2, SN2 and SAcA values are almost the same in
this case. Obviously, the pore size of silica support about 10 – 13 nm is optimal for preparation
of active supported catalyst.

Fig. 10. The effect of the support HTT temperature on specific surface area SN2 and SAcA of
supported B–P–V–W–O /SiO catalysts
x
2
Besides, increase of the reagents conversion using the Cat-HTT-150 additionally might
be connected with increase of its basicity and ability to sorb acetic acid (Fig. 8) that is consistent
with publication [13].
Selectivity of AA formation (S, %) also increases with elevation of HTT temperature of
support up to 150°C and decreases with further elevation of HTT temperature. Maximal value of
AA selectivity (93.1%) is achieved at reaction temperature equal to 350°С. However, some
catalysts (after HTT of the support at 125, 150 and 175°С) allow to obtain almost the same result
at 320°С (Fig. 10). So, the highest acrylic acid formation selectivity is achieved at reaction
temperature 320–380°C on the catalysts based on silica gel treated hydrothermally at 125-175°C.
It is known that usually selectivity for acid catalytic processes depends on the number and
strength of surface acid sites [10, 11, 46, 47, 49]. Surprisingly, the maximum in dependence
selectivity – temperature of support HTT (Fig. 9b) corresponds to minimum in the same
2
dependence of total acidity expressed in μmol/m (Table 4, column 5). Such behaviour is
associated with better ability of acetic acid to sorption on surface of the catalysts, which are
modified hydrothermally at about 150°С (Fig. 8), as well as with bi-functional nature of the
catalysts as was mentioned above. It can also be noted that the presence of acid sites of low and
medium strength (as was mentioned above) promotes the condensation processes that is
consistent with the literature data [10-12, 49, 50]. Decrease of AA selectivity over the catalysts
based on silica gel treated at higher temperatures (>150°С) is also associated with presence of
strong acid sites on the catalysts surface. The latter is confirmed by slight shift of TPD-NH
3
curves toward higher temperatures when increasing the temperature of HTT, and by appearance
of the second peak on the curves at high temperature – at 544°С (Fig. 7).
As a result, acrylic acid yield (Y, %) dependence on support HTT temperature also has a
maximum at 150°С (Fig. 9c). Maximal yield (67.6 %) is achieved on this catalyst at reaction
temperature 380°С.
3
.2.2. Catalyst supported on hydrothermally treated silica vs catalyst supported on non-treated
silica
To evaluate the expediency of catalyst support HTT in case of aldol condensation of AcA
with FA, let us compare the efficiency of the catalyst supported on silica undergone HTT at the
temperature found to be optimal (150°С) and the catalyst supported on initial silica gel.

The catalyst supported on silica after HTT allows to achieve higher AcA conversion
compared to the catalyst supported on non-treated silica at reaction temperatures above 300°С
(Fig. 11a).
Fig. 11. Comparison of the catalysts Cat-Init and Cat-HTT-150 in the terms of acetic acid
conversion (a), acrylic acid selectivity (b) and acrylic acid yield (c) at different reaction
temperatures (space time 8 s, the reactants molar ratio AcA:FA = 1:1)
Hydrothermal treatment of the catalyst support has no substantial positive effect on acrylic
acid selectivity (Fig. 11b). Slight increase of selectivity takes place at the reaction temperatures
above 350°С and taking into account the fact that suitable acrylic acid yield is also achieved
within this temperature range (350 – 410°С), generally HTT has positive effect on catalysts
selectivity.
Acrylic acid yield increases with increasing the reaction temperature and reaches
maximum at 380°С, afterward it starts decreasing (Fig. 11c). Maximum value of the yield
achieved using the catalyst Cat-Init is 57.0 %. On the other hand, the yield achieved under the
same conditions using the catalyst Cat-HTT-150 is 67.6%. So, the increase in acrylic acid yield
due to the hydrothermal treatment of the catalyst support before the active phase deposition is
1
0.6 %, and this is achieved mainly because of increase in acetic acid conversion and slightly
because of increase in acrylic acid selectivity.
The acetone formation as main side product at the reaction conditions was observed.
Acetone selectivity fits within the range 3 - 18 % (in the presence of HTT 150 °C catalyst) and
increases with elevation the reaction temperature. At optimum reaction conditions acetone
selectivity was 6.5 %.
It must be admitted that support has crucial impact on the performance of B-P-V-W-O
catalysts. Thus, unsupported catalyst of the same composition shows quite low catalytic
x
performance and AA yield does not exceed 3.5 % based on acetic acid feed in. The difference is
caused mainly due to the low specific surface area and lower surface concentration of active site
as well as their accessibility for molecules of reagents. This emphasize the huge impact of
porous structure of the catalysts for aldol condensation reactions, and the porous structure can be
easily adjusted in wide limits via support modification.
4
. Conclusions
B–P–V–W oxide catalyst supported on mesoporous silica gel is efficient in gas-phase aldol
condensation of acetic acid with formaldehyde to acrylic acid. Hydrothermal treatment of the
silica support before deposition of the active phase even at constant qualitative and quantitative
composition of the catalyst changes its physical and chemical properties in wide range resulting
in its catalytic performance improvement. Preliminary hydrothermal treatment of the silica
support within the temperature range 100 – 150 °C increases specific surface area (available for
the reagents) of the catalyst, pore diameter and decreases the number of acid sites per mass unit

of the catalyst, change of strong/weak acid sites ratio, as well as ability to acetic acid sorption
and basicity of the catalysts.
The optimal temperature of the hydrothermal treatment of the support in case of its
duration 3 h is 150°С. The optimal temperature to carry out the condensation reaction at contact
time 8 s is 380°C. Acrylic acid yield achieved under these conditions was 67.6% at 90.5%
selectivity. The catalyst supported on the silica treated under optimal temperature 150°С allows
to achieve acrylic acid yield about 10% higher compared to that achieved with the same catalyst
supported on non-treated initial silica.
Thus, hydrothermal treatment of the catalyst support at certain temperature has positive
effect on the catalyst performance and increases both acetic acid conversion and acrylic acid
selectivity. These catalysts possess thermostability in the temperature range of catalysts
operation (up to 410 °С). The hydrothermal treatment of support prevents coking of the catalyst
which is promising in terms of catalyst life. The hydrothermal treatment of support can be
suggested as a cheap and efficient way to modify the supported catalysts structure and their
catalytic performance.
Based on high catalytic performance, it can be concluded that the developed catalysts are
quite promising for industrial application. However, the conversion of initial reagents can be
further improved, the data, shown in the work, were achieved at equimolar acetic
acid/formaldehyde ratio (as opposed to most known results where an excess of acetic acid (3 – 5)
is used). So the use of the B–P–V–W–O catalyst will potentially reduce energy consumption for
x
recycling procedure.
Competing interest.
The authors declare no competing interest.

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