An adsorption calorimetry study of the adsorption of acetone on raney nickel under conditions of liquid-phase hydrogenation

Article abstract of DOI:10.1134/S0036024406130206

Adsorption calorimetry was used to study the characteristics of the adsorption of acetone on Raney nickel from 2-propanol-water solutions under conditions of liquid-phase hydrogenation. The isotherms of adsorption of acetone on Raney nickel and the dependences of integral and differential heats of adsorption on the adsorption value were obtained. It was established that the adsorption of acetone on Raney nickel occurs by the mechanism of volume filling of micropores in the pore space of the catalyst with adsorbate solution, which consists of partially or completely desolvated acetone molecules irrespective of the concentration of 2-propanol in the solvent. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S0036024406130206

ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, Suppl. 1, pp. S116–S121. © Pleiades Publishing, Inc., 2006.
PROBLEMS OF CHEMICAL KINETICS
AND CATALYSIS
An Adsorption Calorimetry Study of the Adsorption of Acetone
on Raney Nickel under Conditions of Liquid-Phase Hydrogenation
N. Yu. Sharonov, M. V. Ulitin, and M. V. Lukin
Ivanovo State University for Chemistry and Technology, Ivanovo, Russia
E-mail: physchem@isuct.ru
Received February 13, 2006
Abstract—Adsorption calorimetry was used to study the characteristics of the adsorption of acetone on Raney
nickel from 2-propanol–water solutions under conditions of liquid-phase hydrogenation. The isotherms of
adsorption of acetone on Raney nickel and the dependences of integral and differential heats of adsorption on
the adsorption value were obtained. It was established that the adsorption of acetone on Raney nickel occurs by
the mechanism of volume filling of micropores in the pore space of the catalyst with adsorbate solution, which
consists of partially or completely desolvated acetone molecules irrespective of the concentration of 2-propanol
in the solvent.
DOI: 10.1134/S0036024406130206
The main problems in the theoretical interpretation imental methods for studying adsorption [1, 3], since,
of the characteristics of adsorption from solution on the in all cases, the adsorption of the compounds to be
surfaces of solids are known to be associated with the hydrogenated occurs on surfaces the active sites of
displacement or cooperative adsorption interactions of which contain adsorbed hydrogen, with the process
the solutes and solvent in the surface layer [1–3]. being accompanied by parallel chemical interactions
According to the Gills classification [2], there are between the reactants in the surface layer [9]. In a num-
17 experimentally identified types of isotherms of ber of studies, for example [6], it was proposed to study
adsorption from solution, i.e., about four times the the adsorption of organic compounds on spongy blacks
number of types of isotherms for the adsorption of without surface hydrogen (removed by thermal treat-
vapors and gases in the Brunauer classification [1]. The ment), so as to eliminate the effect of side reactions.
adsorption of solutes is commonly described within the This approach is, however, methodologically incorrect,
framework of a thermodynamic approach, which, in since the removal of adsorbed hydrogen [10] is accom-
turn, may be based on the concept of individual phases panied by a rearrangement of the structure of active
of adsorption solutions in the surface layer [1, 4], on sites, a process that may affect the subsequent adsorp-
two-dimensional equations of state [4], or on integral tion of organic compounds.
equations of adsorption isotherms [5]. According to [1–
5], the form of the equations for describing partial
adsorption isotherms is similar to that of the equations
of adsorption isotherms for gases and vapors.
The aim of the present work was to study the char-
acteristics of the adsorption of acetone from 2-pro-
panol–water solvents on Raney nickel under conditions
of liquid-phase hydrogenation.
Note that the principal approaches to the theoretical
description of the characteristics of adsorption from
solutions have been worked out largely for carbon
adsorbents [1, 4], active sites of which exhibit no
marked catalytic or chemical activity. Little is known
about how to describe adsorption on catalytically active
surfaces. In particular, the stage of adsorption of the
reactants in reactions of liquid-phase hydrogenation is
normally described within the framework of the
Temkin and Langmuir models of monolayer adsorption
[1, 3], models that for the most part not completely
reflect the processes in adsorption layers.
The choice of the objects of the study was motivated
by the following reasons. The kinetics of the liquid-
phase hydrogenation of acetone on catalysts of various
types has been rather thoroughly studied [9]. It was
established that, irrespective of the nature of the cata-
lyst, this reaction involves only one stage and occurs
without the formation of noticeable amounts of byprod-
ucts. Raney nickel and aqueous solutions of aliphatic
alcohols are widely used as a catalyst and solvents,
respectively, in various reactions of liquid-phase hydro-
genation of organic compounds [8, 9].
Information on the adsorption of reactive organic
compounds on metals and catalysts prepared from them
under conditions of liquid-phase hydrogenation is very
EXPERIMENTAL
The Raney nickel catalyst was prepared by treating
scarce [6–8]. In our opinion, this can be explained by a Ni :Al : Fe = 49.5 : 50.2 : 0.3 (wt %) dispersed nickel–
the fact that it is impossible to use the traditional exper- aluminum alloy (4.8-µm-radius grains) with a 7.5 M
S116

AN ADSORPTION CALORIMETRY STUDY OF THE ADSORPTION OF ACETONE
S117
aqueous solution of sodium hydroxide at 273–278 K tone, which is required to calculate the integral heat of
for 1 h and at 370–373 K for 4 h. Every 2 h the worked- adsorption of the hydrogenated compound by using
out solution was replaced by the fresh one. The active Eq. (2), can be measured in individual runs or calcu-
catalyst had a specific surface area of 90 2 m2/g and a lated from adsorption-calorimetry data on the total heat
porosity of 0.5 0.05 cm3/cm3, with the maximum of release in the experiment [8].
the pore radius distribution being situated at 2 nm; i.e.,
The adsorption of acetone under conditions of liq-
the Raney nickel prepared can be classified as an adsor-
uid-phase hydrogenation was studied as follows. A
bent with a developed microporous structure [11, 12].
required amount of the catalyst and an accurately mea-
The hydrogenation of acetone was performed in aque-
sured volume of the solvent were placed into the calo-
ous solutions of 2-propanol with organic component
rimeter, the system was tightened and thermostated at
concentrations of from 0.073 to 1.0 mole fractions both
303 K, after which the catalyst was saturated with
in a liquid-phase-hydrogenation reactor with intense
hydrogen at atmospheric pressure for 15–20 min and
radial stirring [13] and in a special isothermal calorim-
the calorimetric experiment was conducted.
eter intended for studying heterogeneous catalytic reac-
After the stationary regime of heat exchange was
attained, the reaction of hydrogenation was started by
introducing into the system of the known amount of a
solution of acetone in the same solvent (thermostatted
at the temperature of the experiment). Within a certain
period after the beginning of hydrogen absorption, the
reaction mixture in the calorimeter was sampled and
analyzed for the content of acetone by chromatography.
The amount of 2-propanol used was calculated from the
volume of the hydrogen absorbed during the reaction,
which was measured continuously up to the complete
termination of absorption. After the experiment, the
energy equivalent of the calorimeter was determined by
electric current calibration. The heats of dilution of ace-
tone solutions, quantities required in the calculations,
were measured in separate experiments.
tions in the liquid phase [8].
The adsorption of acetone under conditions of liq-
uid-phase hydrogenation was investigated using
adsorption calorimetry, the fundamentals or which are
given in [8, 14]. The method operates as follows.
According to [14], the material and heat balance equa-
tions for an isothermal calorimeter at an instant of time
τ of the measurement period rearranged so as to obtain
equations for calculating the integral heat of adsorption
of the compound being hydrogenated (R) at its concentra-
tion in the solution cR. For the hydrogenation of acetone in
aqueous solutions of 2-propanol, the excess adsorption
and integral heat of adsorption are given by [14]
1
mk
0
-----
ΓR =
(cR – cR – cRH )Vs,
(1)
(2)
2
A chromatographic analysis of the concentration of
acetone in the reaction mixture samples was performed
on a LKhM-80 chromatograph equipped with a flame-
ionization detector, a 1-m-long stainless steel column
packed with a Polychrome containing 10 wt % of poly-
ethyleneglycol sebacate. The carrier gas was helium.
The internal reference was toluene. The temperatures
of the column and vaporizer were 373 and 393 K,
respectively. An analysis of the standard mixture dem-
onstrated that the sensitivity of the analysis to acetone
(with allowance made for dilutions) was 6 × 10–5 M,
while the error in the measured concentration of the
hydrogenated compound in the reaction mixture was
within 2.0–2.5%.
1
Qa(R) = ----- – (Qτ – Vs∆hG(R)cRH ),
2
mk
where c0R is the initial concentration of acetone, cRH is
2
the concentration of 2-propanol at an instant of time τ,
Qτ is the heat released by the reaction of hydrogenation
by an instant of time τ, ∆hH(R) is the heat of hydroge-
nation of acetone to 2-propanol, mk is the catalyst mass,
and Vs is the solution volume.
The amount of 2-propanol formed by the reaction
can be calculated from the volume of absorbed hydro-
gen, as measured by the volumetric method. Using the
excess adsorption ΓR, the adsorbate equilibrium con-
Processing the chromatographic analysis data
centration, and the volume of the micropores in the within the framework of Eq. (1) yielded the excess
adsorbent, it is possible to determine the absolute adsorption of acetone, which, when combined with the
adsorption of acetone aR [13]. Determining aR values at equilibrium concentration of the adsorbate in the solu-
various cR, one can plot adsorption isotherms under the tion and the adsorption volume of the catalyst, made it
conditions of the reaction. The corresponding integral possible to calculate the absolute adsorption aR. The
heats of adsorption can be used to calculate the differ- adsorption volume of the Raney nickel sample was set
ential heat of adsorption as a function of the absolute equal to the volume of the micropores in it. The calori-
absorption aR. Thus, to determine the adsorption values metric data in conjunction with the method described in
and heats of adsorption under conditions of liquid- [8] made it possible to calculate the change of the calo-
phase hydrogenation, it is necessary to measure the rimeter temperature at a current instant of time τ and at
numbers of moles of acetone subjected to hydrogena- the end of the reaction with consideration given to cor-
tion and 2-propanol formed, as well as the heat released rections for heat exchange, after which the quantity Qτ
by a given instant of time of the calorimetric measure- and the total heat release in the experiment Q∞ were
ment. The heat of liquid-phase hydrogenation of ace- determined. Next, the integral heat of adsorption of the
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 Suppl. 1 2006

S118
SHARONOV et al.
Table 1. Effect of the relative pressure of hydrogen on the
values of adsorption aR (mol/(g Ni)) of acetone on Raney
nickel in 2-propanol (x = 1.0) under conditions of liquid-
phase hydrogenation (cR is the equilibrium concentration of
acetone)
It is known [9, 13] that liquid-phase hydrogenation
is accompanied by the displacement of hydrogen
adsorbed on active sites of the catalyst’s surface. The
heat effects of the interaction of hydrogenated com-
pounds with adsorbed hydrogen are expected to affect
adsorption-calorimetry measurements [14]. To estab-
lish whether this side reaction occurs, we studied how
the partial hydrogen pressure changes the value of
adsorption of acetone under conditions of liquid-phase
hydrogenation. The results obtained are summarized in
Table 1. The adsorption values at the atmospheric pres-
sure of hydrogen were measured by adsorption calo-
rimetry, as described above. At hydrogen pressures
below atmospheric, these quantities were obtained by
processing kinetic curves for the reaction of hydrogena-
tion in a closed system with the use of the method
described in [14].
prel
cR × 102, M
aR × 104, mol/(g Ni)
0.93 0.01
0.13 0.01
59.3 0.6
60.0 0.7
59.0 0.6
19.6 1.0
21.5 2.0
18.6 1.5
(1.2 0.1) × 10–4
Table 2. Heats of hydrogenation ∆hH°(R), solution ∆solH°(R),
and hydration ∆rH°(R) of acetone in 2-propanol–water
binary solutions (kJ/mol) at 303 K
x
–∆hH°(R)
∆solH°(R)
–∆rH°(R)
The results obtained show that, over a wide partial
hydrogen pressure range, which corresponds to a cov-
erage of the Raney nickel surface with hydrogen of
from 0.25–0.30 to 0.95 [16], the adsorption of acetone
is independent of the value of preliminary adsorption of
hydrogen. The data of Table 1 suggest that the adsorp-
tion of hydrogen and acetone are noncompetitive pro-
cess, as can be seen from the absence of a marked
removal of hydrogen adsorbed on active sites at the cat-
alyst surface during acetone hydrogenation. Thus, the
contribution from the heat effects of the interaction of
acetone with adsorbed hydrogen to the value of Qτ is
expected to be comparable with the measurement error,
and, therefore, this side effect can be disregarded in
processing the experimental data. Note that the data
obtained indirectly support the notion that the adsorp-
tion of hydrogen and hydrogenated compounds occur
independently, a key concept of the theory of hydroge-
nation reactions, which underlies many kinetic models
[9, 17].
0.0727
0.190
0.679
1.0
70.25 0.70 –0.08 0.01 70.3 0.7
72.1 0.8
72.4 0.8
76.9 0.6
1.30 0.04 70.8 0.8
3.02 0.03 69.4 0.7
6.68 0.08 70.2 0.7
Note: x is the mole fraction of 2-propanol in the solvent; the exper-
imentally measured values of ∆ H°(R) are given; the tabu-
r
lated value of this quantity is 70.59 kJ/mol [19].
hydrogenated compound was calculated by Eq. (2).
The measurement errors in the adsorption values and
heats of adsorption were estimated at 3 and 5–7%,
respectively; they were largely determined by uncer-
tainties in measuring the acetone concentration.
RESULTS AND DISCUSSION
Thus, the preliminary experiments demonstrated
that, acetone hydrogenation proceeds in one stage, pro-
ducing no byproducts, that it takes less than 10 s to
attain the state of equilibrium at the stage of adsorption
of the hydrogenated compound, that the sorption of
2-propanol produces no effect on the calculated heat of
adsorption on Raney nickel during the reaction, and
that adsorbed hydrogen is not displaced from the cata-
lyst surface during the reaction. The results of the pre-
liminary experiments are in close agreement with the
data reported in [8, 15].
Preliminary experiments, the aim of which was to
substantiate the applicability of Eqs. (1) and (2) to cal-
culating the values and integral heat of adsorption of
acetone from adsorption calorimetry measurements,
demonstrated that, for all the solvents used, the hydro-
genation of 1 mole of acetone consumed 22400
200 cm3 of hydrogen, a proportion that coincides with
the stoichiometry of the reaction with an accuracy of
0.9%. As long as the reaction mixture was sampled at
extents of conversion of acetone below 5%, the values
and integral heats of adsorption of acetone changed
proportionally to the mass of the catalyst, being inde-
pendent of the moments of sampling and the initial con-
centration of the reactant.
Table 2 lists the experimentally measured heats of
liquid-phase hydrogenation of acetone on Raney nickel
in aqueous solutions of 2-propanol ∆hH°(R) and the
heat of solution of acetone ∆solH°(R) [18], quantities
required to calculate the heat of hydrogenation of liquid
acetone to liquid 2-propanol.
A specific feature of solutions of ketones is keto-
enol equilibrium [9, 15]. According to [15], the concen-
tration of the enol form does not exceed 2.5 × 10–4%.
Table 2 shows that the measured heat effects of
Therefore, keto-enol equilibrium is not expected to interaction of liquid acetone with hydrogen with the
influence the results of adsorption calorimetry mea- formation of liquid 2-propanol coincide with the ther-
surements.
modynamically calculated values within 0.6%, a result
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AN ADSORPTION CALORIMETRY STUDY OF THE ADSORPTION OF ACETONE
S119
aR × 103, mol/(g Ni)
Qa(R), J/g
4
2.0
1.5
1.0
0.5
40
20
3
1
2
3
4
2
1
0
0.5
1.0
1.5
2.0
0
0.2
0.4
0.6
0.8
ÒR, mol/l
1.0
aR × 103, mol/(g Ni)
Fig. 1. Isotherms of adsorption of acetone on Raney nickel
from 2-propanol–water binary solvents of various compo-
sitions: x = (1) 0.073, (2) 0.190, (3) 0.679, and (4) 1.0 mole
fractions of alcohol; T = 303 K.
Fig. 2. Dependence of the integral heats of adsorption of
acetone Q (R) on Raney nickel from 2-propanol–water
a
binary solutions on the adsorption value a at 303 K; for
R
1−4, see Fig. 1.
indicative of a high reliability of determination of heat centration of the adsorbate increases still further, the
effects of liquid phase hydrogenation in the calorimeter heat of adsorption decreases to values close to the
used. At the same time, the heat of liquid-phase hydro- heat of solution of acetone in the solvent but with
genation of acetone depends on the solvent composi- opposite sign. According to [7, 15], the extremal
tion due to a change in the heat of solvation of the character of the dependence of the heat of adsorp-
hydrogenated compound [18].
tions of organic compounds on the adsorbate concen-
tration can be explained by rearrangement of the sur-
face layer with the formation of saturated adsorption
solutions, the composition of which is determined by
the chemical nature of the adsorbate and its affinity
to the solvent.
The experimental results obtained were processed in
the linear coordinates of the equations of the classical
adsorption isotherms. We found that none of the known
adsorption isotherm equations described the adsorption
equilibrium characteristics over the entire range of ace-
tone concentrations. What is more, the limiting adsorp-
tion values, even those calculated from the initial seg-
ments of the isotherms plotted in the linear coordinates
Figure 1 shows the experimentally measured iso-
therms of adsorption of acetone on Raney nickel from
aqueous solutions of 2-propanol. As can be seen, the
shape of the isotherm does not depend on the composi-
tion of the solvent. According to the Gills classification,
all the isotherms belong to types 3L and 4L, which are
characteristic of adsorption with parallel orientation of
adsorbate molecules and weak interactions in the sur-
face layer. According to [2], inflections and second pla-
teaus in adsorption isotherms are associated with the
reorientation of the molecules on the surface and with
the polymolecular character of adsorption. That the
shapes of the isotherms of adsorption of acetone in all
the solvents are identical to each other can be explained
by the unlimited solubility of the adsorbate in aqueous
solutions of 2-propanol.
Figure 2 displays the integral heats of adsorption
Qa(R) of acetone on Raney nickel in 2-propanol aque-
ous solutions at various absolute adsorptions aR. As
can be seen, all the curves monotonically ascend, a
behavior characteristic of exothermic adsorption from
solutions. Performing numerical differentiation of the
spline functions obtained by approximating the data
presented in Fig. 2 by a smoothing-interpolating
spline, we calculated the differential heats of adsorp-
tion of acetone ∆aH(R); the results are displayed in
Fig. 3.
–∆aH(R), kJ/mol
60
4
40
2
3
20
1
0
0.5
1.0
1.5
2.0
Figure 3 shows that, irrespective of the solvent com-
position, the differential heats of adsorption of acetone
pass through a maximum at adsorption values of (0.3–
1.0) × 10–3 mol/(g Ni). The maximum values of the
heat of adsorption of acetone increase with the con-
centration of 2-propanol in the solution. As the con-
aR × 103, mol/(g Ni)
Fig. 3. Dependence of the differential heats of adsorption
of acetone ∆ H(R) on Raney nickel from 2-propanol–
a
water binary solutions on the adsorption value a at 303 K;
R
for 1–4, see Fig. 1.
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S120
SHARONOV et al.
ues and heats of adsorption [8]. At adsorption values
–lnaR
above (0.5–0.8) × 10–3 mol/(g Ni), the effect of the
adsorption potential of the pore walls causes changes in
the composition of the adsorption solution, probably
due to an increase in the intensity of adsorbate–adsor-
bate and adsorbate–metal interactions, a factor that
manifests itself through the growth of the heat of
adsorption to a maximum value, individual for each
solvent. At high concentrations, the character of the
adsorption process is governed by the condensation of
acetone solutions in transport pores and on the external
surface of catalyst grains, a process responsible for the
decrease of the heat of adsorption to a value close to the
heat of solution of the adsorbate in the solvent but with
opposite sign.
10
9
8
7
1
2
3
4
6
0
20
40
60
[ln(cs/c)]2
Fig. 4. Isotherms of adsorption of acetone on Raney nickel
from 2-propanol–water binary solvents plotted in the linear
coordinates of the main TVFM equation at 303 K; for 1–4,
see Fig. 1.
In our opinion, this explanation holds for all systems
involving the adsorption of organic compounds from
solutions on solid adsorbents with branched porous
structures. A general discussion of the adsorption
mechanism can be conducted based on the thermody-
namic characteristics of adsorption equilibria, mainly the
entropy of adsorption, and on an analysis of the state of
the organic compounds in the adsorption solutions filling
the pore space of the Raney nickel catalyst.
of the Langmuir and Aranovich equations proved to
be three to five times higher than the monolayer sorp-
tion capacity of Raney nickel. The results obtained
show that the adsorption of acetone on hydrogena-
tion catalysts can be described only formally within
the framework of the existing models of monomolec-
ular and polymolecular adsorption. In our opinion,
the data obtained can be interpreted only by invoking
models capable of describing the specific nature of
adsorption on adsorbents with branched porous
structure.
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