Synthesis of nanostructured carbon on Ni catalysts supported on mesoporous silica, preparation of carbon-containing adsorbents, and preparation and study of lipase-active biocatalysts

Article abstract of DOI:10.1134/S002315841603006X

This work continues a series of our studies on the synthesis of nanostructured carbon (NSC) by the pyrolysis of H₂ + C₃–C₄ alkane mixtures on nickel and cobalt metal catalysts supported on chemically diverse inorganic materials (aluminosilicates, alumina, carbon) having different textural characteristics (mesoporous and macroporous supports) and shapes (granules, foamed materials, and honeycomb monoliths). Here, we consider Ni catalysts supported on granular mesoporous silica (SiO₂). It has been elucidated how the yield of synthesized carbon depends on the Ni/SiO₂ catalyst preparation method (homogeneous precipitation or impregnation) and on the composition of the impregnating solution, including the molar ratio of its components—nickel nitrate and urea. The morphology of catalytic NSC and Ni distribution in the silica granule have been investigated using a scanning electron microscope with an EDX analyzer. Carbon-containing composite supports (NSC/SiO₂) have been employed as adsorbents for immobilizing microbial lipase. The enzymatic activity and stability of the resulting biocatalysts have been estimated in transesterification reactions of vegetable (sunflower and linseed) oils involving methyl or ethyl acetate, esterification, and synthesis of capric acid–isoamyl alcohol esters in nonaqueous media.

Full text of DOI:10.1134/S002315841603006X

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2016, Vol. 57, No. 3, pp. 394–403. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © G.A. Kovalenko, T.V. Chuenko, L.V. Perminova, N.A. Rudina, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 3, pp. 398–408.
Synthesis of Nanostructured Carbon on Ni Catalysts Supported
on Mesoporous Silica, Preparation of CarbonꢀContaining
Adsorbents, and Preparation and Study
of LipaseꢀActive Biocatalysts
G. A. Kovalenkoa, b, *, T. V. Chuenko , L. V. Perminova , and N. A. Rudina
a
a
a
a
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b
Novosibirsk State University, Novosibirsk, 630090 Russia
*
eꢀmail: galina@catalysis.ru
Received September 15, 2015
Abstract—This work continues a series of our studies on the synthesis of nanostructured carbon (NSC) by
the pyrolysis of H2 C –C alkane mixtures on nickel and cobalt metal catalysts supported on chemically
diverse inorganic materials (aluminosilicates, alumina, carbon) having different textural characteristics
mesoporous and macroporous supports) and shapes (granules, foamed materials, and honeycomb monoꢀ
+
3
4
(
liths). Here, we consider Ni catalysts supported on granular mesoporous silica (SiO2). It has been elucidated
how the yield of synthesized carbon depends on the Ni/SiO2 catalyst preparation method (homogeneous preꢀ
cipitation or impregnation) and on the composition of the impregnating solution, including the molar ratio
of its components—nickel nitrate and urea. The morphology of catalytic NSC and Ni distribution in the silꢀ
ica granule have been investigated using a scanning electron microscope with an EDX analyzer. Carbonꢀconꢀ
taining composite supports (NSC/SiO ) have been employed as adsorbents for immobilizing microbial
2
lipase. The enzymatic activity and stability of the resulting biocatalysts have been estimated in transesterifiꢀ
cation reactions of vegetable (sunflower and linseed) oils involving methyl or ethyl acetate, esterification, and
synthesis of capric acid–isoamyl alcohol esters in nonaqueous media.
Keywords: supported Ni catalysts, nanostructured carbon, lipase adsorption, heterogeneous biocatalysts,
esterification, transesterification
DOI: 10.1134/S002315841603006X
INTRODUCTION
tion of organic (capric, palmitic, lauric, salicylic)
acids with aliphatic (isoamyl, cetyl, ethyl) alcohols in
nonaqueous media in the presence of lipase [2–7].
The synthesis of nanostructured carbon (NSC) on
inorganic matrices differing in their chemical nature
metal oxides, silica, graphite) and in textural properꢀ
Nanostructured carbon is synthesized by hydrocarꢀ
bon pyrolysis on metallic catalysts (Ni, Cu, Fe, and
their alloys) supported on an inorganic material [8, 9].
In particular, Ni catalysts are commonly supported on
SiO2 [10–23]. The following methods are used to preꢀ
pare silicaꢀsupported Ni catalysts: (1) impregnation of
a support with a nickel salt solution (Impr) [12, 14,
(
ties and shape (granules, rods, honeycomb monoliths)
provides means to obtain a wide variety of unique
composite supports and materials, including those
usable in heterogeneous biocatalysis. Containing
nanostructured surface carbon, these supports can
efficiently adsorb enzymatically active substances,
such as protein molecules of enzymes. Heterogeneous
biocatalysts prepared by enzyme adsorption on carꢀ
bonꢀcontaining supports can accelerate many practiꢀ
cally important processes, for example, esterification
1
5], (2) intercalation of nickel compounds into a siliꢀ
cate matrix using sol–gel processing [12, 16, 17], and
(
3) homogeneous precipitation (HP) [18–23].
The HP method is based on urea decomposition in
reactions yielding an ester linkage between acid and aqueous solutions of salts at 85 5°С:
±
alcohol residues. These reactions provide a basis for
+
–
CO(NH ) + 3H O
→
2OH + 2NH4 + CO [23].
2
2
2
2
obtaining aroma compounds for the food, fragrance,
and cosmetic industries. By varying the composition The hydroxide ion reacts with Ni2+ to yield colloidal
of the initial substrates (alcohol and acid), it is possible ꢀNi(OH)2 particles (since the solubility product of
to synthesize aroma compounds having various odors: nickel hydroxide is SP < 10 ), and these particles
for example, isoamyl butyrate smells like a pear [1, 2]. precipitate on the surface of an inorganic support [20,
There have been reports on the enzymatic esterificaꢀ 21, 23]. The pyrolysis of alkanes (С , С –С4) or ethylꢀ
α
–15
1
3
394

SYNTHESIS OF NANOSTRUCTURED CARBON ON Ni CATALYSTS
395
Table 1. Textural characteristics of the adsorbents and initial silica
Preparation Supported nickel Carboncontent*,
S
BET
m /g
,
S
,
micro
m /g cm /g cm /g
V
,
V ,
micro
Σ
Sample
Dav, nm
2
2
3
3
method
content, wt %
wt %
Ni/SiO2 HP (0.1 mol/L Ni(NO3)2,
1
3.5
5
145
14.1
1.8
6.1
5.4
0.67
0.74
0.54
0.75
0.008
0.002
0.004
0.004
18.4
mol/L urea)
Impr (0.05 mol/L
Ni(NO ) )
0.8
1.7
0
4
16
0
130
140
150
22.5
15.2
19.7
3
2
Impr (0.5 mol/L
Ni(NO ) )
3
2
KSKꢀG** –
*
According to thermoanalytical data.
** Initial support for the Ni catalysts.
ene (С2) on the Ni/SiO2 catalyst yields carbon as filaꢀ ified in an earlier work [23]. The nickel nitrate conꢀ
mentous structures (nanofibers) [23–25]. Supports, centration in the solution was varied between 0.005
including silicates, that have an NSC layer on their surꢀ and 0.1 mol/L; the urea concentration, between 0.1
face are employed in heterogeneous biocatalysis for and 2 mol/L. The molar ratio of nickel nitrate to urea
adsorptive immobilization of enzymatically active subꢀ was varied between 1 : 2 and 1 : 200.
stances, namely, microbial cells and enzymes [17, 26].
The morphology of the supported catalysts and carꢀ
This work continues a series of our studies on the bon deposits were studied by scanning electron microsꢀ
synthesis of catalytic filamentous carbon by the pyrolꢀ copy (SEM) on a JSM 6460 LV microscope (JEOL,
ysis of С –С alkanes on supported Ni and Co cataꢀ Japan) fitted with an EDX spectrometer (Oxford
3
4
lysts. The support for the nickel catalysts prepared in INCAEnergy).
this study by homogeneous precipitation and impregꢀ
The catalytic pyrolysis of the С –С4 alkanes +
3
nation was mesoporous silica. We studied how the
activity of the catalysts in the pyrolysis of a hydrogen +
hydrogen (8 : 1 v/v) gas mixture was performed in a
fixedꢀbed reactor at 500°C for 1 h, as was described in
earlier works [17, 23]. The amount of synthesized carꢀ
bon was determined by thermal analysis (TA) using an
STAꢀ449 C Jupiter system (Netzsch, Germany) and
was also calculated from the change in the catalyst
С –С alkane mixture and the NSC yield depend on
3
4
the Ni/SiO2 preparation conditions, primarily on the
composition of the impregnating solution. The resultꢀ
ing NSC/Ni/SiO composite carbonꢀcontaining adsorꢀ
2
bents were used to immobilize the enzyme lipase and to
prepare heterogeneous biocatalysts for conversion of
vegetable oil triglycerides. The enzymatic activity and
stability of these biocatalysts was studied in triglyceride
transesterification and fatty acid esterification in nonꢀ
aqueous media.
weight measured after the pyrolysis run (500
after annealing (800 C, 4 h). In the calculation of the
°C, 1 h) or
°
carbon content, we took into account the moisture
content of the initial silica (4.8%), which was meaꢀ
sured by drying the material at 110
the weight lost by the initial silica subjected to heat
treatment at 500 and 800 (5.4 and 7.1%, respecꢀ
tively). The weight loss of the Ni catalysts ( ) at
00 in the presence of Н2 and in the absence of
°C for 4 h, and also
°C
–
Δ
EXPERIMENTAL
5
°C
The support for the Ni catalysts was KSKꢀG silica С –С4 alkanes (when there was no pyrolysis) and the
3
gel (Russia) with a granule size of 2—5 mm. The texꢀ weight loss as a result of annealing at 800
°
C
–
were, on
value
tural characteristics of supports and adsorbents were average, 5.7 and 9.6%, respectively. The
Δ
determined by nitrogen porosimetry using AUTOꢀ depended weakly on the supported Ni content in the
PORE 9200 and ASAP 2400 V3.07 devices wide range from 0.5 to 6.0% Ni, indicating that the
(
Micromeritics Instrument Corporation, United metal was possibly carried away at high temperatures.
States). The porosimetry data are presented in Table 1. Taking into account all values enabled us to estiꢀ
The specific surface area (SBET) of the initial silica mate the amount of synthesized carbon and to calcuꢀ
–
Δ
2
was 150 m /g, its total pore volume (
0
1
V
D
) was late the NSC yield (
Y
) in terms of the amount of carꢀ
Σ
3
.75 cm /g, its average pore diameter (
9.7 nm, its micropore volume (Vmicro) was 0.004 cm /g, lica experiments were made, and the scatter of
and the specific micropore surface area (Smicro) was experimental data was the same as in the study of comꢀ
) was bon per unit weight of supported Ni (g/g). Many repꢀ
3
2
5
.4 cm /g.
mercial supports.
Supported Ni catalysts for the pyrolysis of the Н +
The NSC/Ni/SiO supports were used to adsorb
2
2
С –С mixture were prepared by the HP and impregꢀ microbial lipase from Thermomyces lanuginosus
3
4
nation methods. The preparation conditions are specꢀ (SIGMA). For comparison, the same operation was
KINETICS AND CATALYSIS Vol. 57
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396
KOVALENKO et al.
carried out with the initial silica. Adsorption was perꢀ operation, the biocatalyst was washed with the solvent
formed for 2 days at 20–22 while occasionally stirꢀ mixture for 2 days and its initial activity was then meaꢀ
°C
ring a support sample (3 g) and lipase in a buffer soluꢀ sured again.
tion (6 mL, pH 7.0). The initial concentration of the
protein was 12.5 mg/mL. Next, lipase was adsorbed
RESULTS AND DISCUSSION
again under the same conditions but at its initial conꢀ
centration of 8.5 mg/mL. The total amount of lipase
adsorbed was calculated in terms of amount of lipase
Preparation of Supported Ni Catalysts
and CarbonꢀContaining Adsorbents
(
mg) per gram of support.
We carried out a comparative study of the Ni/SiO2
catalysts prepared by the HP and Impr methods and
investigated the effect of the composition of the
impregnating solution on the physicochemical propꢀ
erties of the catalysts themselves and on those of carꢀ
bonꢀcontaining adsorbents.
In the HP method, the molar ratio of the compoꢀ
nents of the impregnating solution— Ni(NO3)2 and
urea—were varied in two ways.
The enzymatic activity of the biocatalysts was
determined in a stirred batch reactor at 20 and 40
°C.
The reactor was placed in a KTꢀ104 temperatureꢀconꢀ
–1
trolled shaker reciprocating at a rate of 90 min . The
duration of one reaction cycle was 5 min to 24 h. The
following reactions were investigated: hydrolysis of
tributyrin (I) under the conditions described in our
earlier study [17], esterification of capric acid with
isoamyl alcohol (II), and transesterification of sunꢀ
flower oil triglycerides involving an acyl group donor
In the first series of experiments, the urea concenꢀ
methyl acetate) or acceptor (dimethyl carbonate), tration was varied between 0.1 and 2 mol/L at a fixed
(
and transesterification of linseed oil triglycerides nickel nitrate concentration of 0.01 or 0.02 mol/L.
involving ethyl acetate (III). The hydrolysis of emulsiꢀ Electron microscopic examinations demonstrated
fied tributyrin (reaction (I)) was carried out in a buffer that, as the urea concentration is increased, the partiꢀ
solution at pH 7.0, and reactions (II) and (III) were cle size of the precipitated nickel hydroxide decreases
performed in organic nonaqueous solvents (hexane or (Fig. 1, images a1, b1, c1). The resulting carbon is in
a hexane–diethyl ether mixture).
the form of nanofibers (Fig. 1, image b2). As the
Ni(NO3)2 : urea molar ratio is changed from 1 : 10 to
To carry out reactions (II) and (III), a biocatalyst
sample (0.1–0.5 g) predried in air at 22°C for 3 days
was placed in the reactor, and the reaction medium
1
2
0
0
: 100, the supported Ni content decreases by 25–
9%: for example, at a Ni(NO3)2 concentration of
.02 mol/L, the nickel content decreases from 0.7 to
.5%. As a consequence, the amount of the resulting
(
2–5 mL) was poured thereto. In reaction (II), the
reaction medium consisted of 0.1 mol/L capric acid,
.4 mol/L isoamyl alcohol, and a hexane–diethyl
carbon decreases by one order of magnitude, namely,
from 2–3 to less than 0.1%.
0
ether (1 : 2) mixture as the solvent. In reaction (III),
the reaction medium consisted of 0.1 mol/L vegetable
In the other series of experiments, the nickel nitrate
concentration was varied at a fixed urea concentration
of 1 mol/L. In this case, as the Ni(NO3)2 concentraꢀ
tion is decreased, the particles size of precipitated
nickel hydroxide increases (Fig. 2, images a1–e1).
According to earlier data [23], the urea conversion at
(
sunflower or linseed) oil, 1.85 mol/L methyl acetate
or dimethyl carbonate or 2.3 mol/L ethyl acetate, and
a solvent (hexane). Transesterification products were
identified by thinꢀlayer chromatography (TLC), as
was described in our earlier publication [27]. The
tributyrin hydrolysis and ester synthesis rates were
determined by acid–base titration from the increase
and decrease in the concentration of butyric acid and
capric acid, respectively. The activity of biocatalysts
was estimated in international Units of enzyme activꢀ
ity (U), that is, in terms of substrate conversion rate in
85°C in 3 h (as in this study) is 10% and, at a compoꢀ
–
nent molar ratio of 1 : 10, there are two OH ions per
2
+
Ni ion; that is, the process yields nickel hydroxide
Ni(OН)2. When the component ratio is below 1 : 10,
for example, at a nickel nitrate concentration higher
than 0.1 mol/L and a fixed urea concentration of
1
mol/L, supported Ni forms both from precipitated
µ
mol/min per gram of biocatalyst. The specific activꢀ
nickel hydroxide and from nickel(II) nitrate. At reacꢀ
tant ratios of 1 : 2 and 1 : 5, small Niꢀcontaining partiꢀ
ity of adsorbed lipase was expressed as U per milligram
of protein. Capric acid conversion was calculated via
cles on the silica surface form aggregates 0.1–0.5 m in
µ
C0 − Ct
the formula x =
,
where С0 is the initial conꢀ diameter (Fig. 2, images a1, b1). The carbon resulting
C
0
from catalytic pyrolysis has a wellꢀdefined filamentous
structure (Fig. 2, images a2–e2). It can clearly be seen
that, when the ratio between the components of the
centration of capric acid and Сt is its concentration at
the point in time (in mol/L).
t
The stability of biocatalysts in reactions (I)–(III) impregnating solution is 1 : 10 to 1 : 50, the Ni/SiO2
was estimated under the aboveꢀspecified conditions. catalyst and NSC/Ni/SiO adsorbents have a “crust”
2
At the beginning of each reaction cycle, we measured structure. Chemical and gravimetric analyses demonꢀ
the initial reaction rate or substrate (capric acid) conꢀ strated that, as the initial nickel nitrate concentration
version for 5–60 min; next, the biocatalyst was left in is raised under the conditions examined, the supꢀ
the reaction medium for 1–3 days. After 500ꢀhꢀlong ported nickel content increases nonlinearly to 5.6%
KINETICS AND CATALYSIS Vol. 57
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SYNTHESIS OF NANOSTRUCTURED CARBON ON Ni CATALYSTS
397
1 µm
1 µm
(а1)
(b1)
1 µm
1 µm
(c1)
(b2)
Fig. 1. SEM images of supported Ni catalysts and an NSCꢀcontaining adsorbent synthesized at an initial nickel nitrate concenꢀ
tration of 0.02 mol/L in the impregnating solution and nickel nitrateꢀtoꢀurea molar ratio of (a1) 1 : 10 (0.72% Ni), (b1) 1 : 50
(0.52% Ni), and (c1) 1 : 100 (0.50% Ni). (b2) Nanostructured carbon on catalyst b1 (1% C).
(
Fig. 3a, curve
1). The amount of synthesized carbon average, 1.3 g/g (at most 1.9 g/g) and 6.2 g/g (at most
also increases nonlinearly, the maximum amount of 8.7 g/g), respectively. A comparison of the highest carꢀ
NSC being 8% (Fig. 3b, curve
1).
bon yields obtained under fixed pyrolysis conditions
on all HP catalysts hitherto studied by the authors
In the case of the Impr method, the supported
nickel content increases almost linearly to 2.5% with
an increasing initial nickel nitrate concentration in the
impregnating solution (Fig. 3a, curve 2). As the silicaꢀ
supported nickel content increases, the amount of
synthesized carbon increases linearly, the maximum
demonstrates that the
work, is about 30 times smaller than the same value for
Ni/foam glass catalysts ( 29 g/g) [23]. Obviously,
Y
≈
1 g/g value, observed in this
Y
≈
this is due to the effect of the porous structure of the
support on the distribution of catalytically active
nickelꢀcontaining particles. For example, foam glass
NSC content being 27% (Fig. 3b, curve 2).
2
(
SBET = 0.4 m /g) is a macroporous support, whereas
The maximum supported nickel content of the catꢀ
alysts prepared by the HP method is approximately
2
silica (SBET = 150 m /g) is a mesoporous one (Table 1).
Owing to the macroporous structure of foam glass,
nickel hydroxide particles are uniformly distributed
inside the glass granule [23], while they are practically
absent inside the mesoporous silica granule, and this
leads to the formation of a crustꢀtype carbonꢀcontainꢀ
ing adsorbent.
3
times higher than that of the samples prepared by the
Impr method (Fig. 3a). At the same time, the amount
of carbon forming on the catalysts prepared by
impregnation is approximately 5 times larger than the
amount of carbon forming on the catalysts prepared by
the HP method (Fig. 3b). Calculations demonstrated
that the carbon yield
Y
on the Ni/SiO catalysts
In cleaved granules of the carbonꢀcontaining
2
obtained by the HP and Impr methods is practically adsorbents obtained on HP catalysts, the naked eye
independent of the amount of supported Ni and is, on can see a black carbon crust and a light inner part of
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398
KOVALENKO et al.
0.5
µ
m
2 µm
(а1)
(а2)
1
µ
m
2 µm
(b1)
(b2)
1
µ
m
2
1
1
µ
m
(
c1)
(c2)
(d2)
(e2)
1
µ
m
µ
m
(d1)
1
µ
m
µ
m
(e1)
Fig. 2. SEM images of (a1–e1) supported Ni catalysts prepared at a fixed initial urea concentration of 1 mol/L in the impregnating
solution and nickel nitrateꢀtoꢀurea molar ratios of (a1) 1 : 2 (5.68% Ni), (b1) 1 : 5 (3.93% Ni), (c1) 1 : 10 (2.76% Ni), (d1) 1 : 50
(
(
0.52% Ni), and (e1) 1 : 200 (0.15% Ni). (a2–e2) NSC formed on catalysts a1–e1; the carbon content of the adsorbents is (a2) 4,
b2) 4, (c2) 2, (d2) 1, and (e2) <0.1%.
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SYNTHESIS OF NANOSTRUCTURED CARBON ON Ni CATALYSTS
399
(b)
NSC content, wt %
26
(а)
24
22
20
18
16
14
12
10
8
6
4
2
Ni content
of the catalysts, wt %
2
6
.0
.0
.0
.0
.0
.0
1
2
5
4
3
2
1
1
0
.1 0.2 0.3 0.4 0.5
[Ni(NO ) ] , mol/L
1
2
3
4
5
6
Supported Ni content, wt %
3
2 0
Fig. 3. (a) Supported nickel content on silica as a function of the initial Ni(NO ) concentration in the impregnating solution in
3
2
the (
prepared by the (
1) HP and (2) Impr methods. The urea concentration is 1 mol/L. (b) Amount of synthesized carbon on Ni/SiO catalysts
2
1) HP and (2) Impr methods.
silica; that is, carbon synthesis does not take place
inside the granules. Electron microscopic examinaꢀ
tions with EDX analyses confirmed this observation.
Indeed, as is clear from Fig. 4, a wellꢀdefined crustꢀ
Preparation of LipaseꢀActive Biocatalysts
For preparing biocatalysts by the adsorptive immoꢀ
bilization of lipase, we chose NSC/Ni/SiO adsorꢀ
2
bents, whose characteristics are presented in Table 1
and in Fig. 5. The initial silica was also used for comꢀ
parison. The adsorbents that were selected for lipase
adsorption differed in their carbon content, carbon
deposit morphology, and carbon distribution inside
type distribution of Ni is typical of the Ni/SiO cataꢀ
2
lysts prepared by the HP method (Fig. 4a), while
nickel in the catalysts prepared by impregnation is
more uniformly distributed in the granule (Fig. 4b).
A TA study of the synthesized carbonꢀcontaining
adsorbents demonstrated that carbon is burned away
above 450°C. The shape of the thermogravimetric
profiles of the adsorbents depends on the Ni catalyst
preparation method (Fig. 5). Carbon on the catalysts
prepared by the HP method is burned away in a narꢀ
rower temperature range (Fig. 5a) than carbon on the
catalysts prepared by impregnation (Figs. 5b, 5c). This
fact is further evidence of crustꢀtype carbon distribuꢀ
tion, in which carbon is relatively highly accessible to
oxygen and, as a consequence, is readily oxidized
the granule. The crustꢀtype (5% NSC)/Ni/SiO (HP)
adsorbents are hereafter designated I; the adsorbents
that are characterized by a uniform carbon distribuꢀ
tion and have no wellꢀdefined filamentous structure,
2
(
4% NSC)/Ni/SiO (Impr), are designated II; the
2
adsorbents with a uniform distribution of nanofibers,
16% NSC)/Ni/SiO (Impr), are designated III
(
.
2
It was discovered that lipase adsorption depends
on the NSC/Ni/SiO adsorbent type and structure.
2
The amount of lipase adsorbed on the crustꢀtype
adsorbent I was 3–5 times as small as the amount of
(
burned away). It follows from Fig. 5 that NSC with a
lipase adsorbed on adsorbents II or III or on the initial
silica (Table 2). This finding cannot be explained in
terms of textural distinctions between the adsorbents,
because the porous structures of all of the samples
examined here have similar parameters (Table 1). The
adsorbents differ slightly in micropores specific surface
wellꢀdefined filamentous structure forms on the
nickelꢀrich catalysts. The carbon deposits on the cataꢀ
lysts that contain <1% Ni and were prepared by
impregnation do not have a distinct filamentous strucꢀ
ture (Fig. 5b).
Thus, in this study we prepare supported Ni catalysts area and micropore volume. For the crustꢀtype adsorꢀ
by using different preparation methods and obtained bent, these parameters are approximately 2 times larger
different types of NSC/Ni/SiO composite adsorbents than for the others (Table 1). Micropores are inaccesꢀ
2
by the pyrolysis of H + C –C alkanes. The carbonꢀ sible to the enzyme being adsorbed, because the size of
2
3
4
containing adsorbents on the Ni catalysts synthesized the lipase molecule is ~10 nm. Since the micropores
by the HP method have a crustꢀtype structure. The carꢀ account for <1% of the total pore volume (Table 1),
bon deposited on the Ni catalysts obtained by impregꢀ the multifold decrease in the amount of lipase
nation is relatively uniformly distributed inside the SiO2 adsorbed on the crustꢀtype adsorbent
granules. attributed to this specific feature of the porous strucꢀ
I cannot be
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400
KOVALENKO et al.
Nickel content, wt %
(а)
1
2
0
8
6
4
2
1
1 µm
0.5
1.0
(
b)
5
4
3
2
1
1 µm
0.5
1.0
Distance from the granule center, mm
Fig. 4. EDX curves illustrating the distribution of nickel in the cut section of a granule (left column) and electron micrographs of
supported Ni catalysts cracks (right column): (a) catalyst prepared by the HP method (Ni content of 3.52%, initial nickel nitrate
concentration of 0.1 mol/L, initial urea concentration of 1 mol/L); (b) catalyst prepared by the Impr method (Ni content of
1.72%, initial nickel nitrate concentration of 0.5 mol/L).
ture. It is possible that the carbon crust hampers the were also examined in an esterification reaction,
transport of protein molecules into the granule bulk. whose product is water. Under the isoamyl caprinate
The activity of the biocatalysts was studied in the synthesis conditions, the water molecules forming in
hydrolysis of emulsified tributyrine in an aqueous the organic solvent medium can accumulate in the silꢀ
medium at pH 7.0. The biocatalysts rapidly lose their ica matrix in the immediate vicinity of adsorbed lipase.
enzymatic activity, irrespective of the NSC/Ni/SiO2 Calculations showed that, under the given conditions,
adsorbent type (Fig. 6a). Earlier [27, 28], we demonꢀ the total conversion of capric acid yields 9 L of water,
µ
strated that one of the causes of the inactivation of bioꢀ which can be strongly bound by KSKꢀG silica gel,
catalysts in reactions involving “oil” substrates is the since the pore volume of this support is one order of
dehydration of adsorbed lipase as a result of triglycerꢀ magnitude larger than the volume of the resulting
ides displacing “essential” water molecules from the water.
microenvironment of the enzyme. For verifying this
Table 2 lists data characterizing the activity of the
hypothetical inactivation mechanism, the biocatalysts biocatalysts and the specific activity of adsorbed lipase
Table 2. Properties of the prepared biocatalysts in esterification
Specific activity
of adsorbed lipase,
EA/mg
Adsorbent for lipase
immobilization
Amount of lipase
adsorbed, mg/g
Initial activity*,
EA/g
Time to 85–90%
conversion, min
I
II
5.4
32.6
20.1
17.3
6.1
45.4
36.4
45.4
1.1
1.4
1.8
2.6
120
20
III
30
KSKꢀG
30
*
The activity of the biocatalysts was determined at 20
alcohol, and 10 wt % biocatalyst.
°C in a reaction medium containing 0.1 mol/L capric acid, 0.4 mol/L isoamyl
KINETICS AND CATALYSIS Vol. 57
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SYNTHESIS OF NANOSTRUCTURED CARBON ON Ni CATALYSTS
401
TGA, %
00
DTA,
µ
V/mg
(а)
1
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
80°C
99
98
97
96
95
94
93
92
91
–
3.6%
1 °C
–
47
0.5%
4 °C
53
–
3.9%
0.5
1
1
1
µ
µ
µ
m
m
m
0
1
00 200 300 400 500 600 700
TGA, %
00
63°C
DTA, µV/mg
(
b)
4
.0
1
–
5.1%
3.5
9
9
9
9
8
6
4
2
3
2
2
.0
.5
.0
1 °C
29
–
1.0%
4 °C
55
–1.7%
1.5
.0
5 °C
14
1
–
1.4%
5 °C
70
–
0.4%
0.5
0
100 200 300 400 500 600 700
TGA, %
DTA, µV/mg
(
c)
568°C
67°C
100
–15.3%
–4.1%
8
7
6
5
4
3
2
1
0
9
9
8
8
5
0
5
0
4 °C
15
–0.9%
1
00 200 300 400 500 600 700 800 900
Temperature,
°
С
Fig. 5. Thermoanalytical curves (left column) and SEM images (right column) of NSC/Ni/SiO adsorbents: (a) adsorbent conꢀ
2
taining 5% NSC, based on the 3.52% Ni catalyst prepared by the HP method; (b) adsorbent containing 4% NSC, based on the
0.8% Ni catalyst prepared by the Impr method; (c) adsorbent containing 16% NSC, based on the 1.72% Ni catalyst prepared by
the Impr method.
in the esterification reaction. The specific activity of taining biocatalysts show an increased activity in the
lipase adsorbed on the NSCꢀcontaining supports is second or third reaction cycles (after 150ꢀhꢀlong operꢀ
1
.4–2 times lower than that of lipase adsorbed on the ation), as is clear from Fig. 6b, and this is possibly eviꢀ
initial silica. The lowest specific activity of lipase was dence of the formation of a favorable, aqueous
observed with the crustꢀtype adsorbent. The adverse microenvironment around adsorbed lipase. A comꢀ
effect of the carbon surface on the activity of adsorbed parison between inactivation curves 2 and 4 in Fig. 6b
lipase was also reported in our previous works [17, 29]. demonstrates that the higher the NSC content of the
It was hypothesized in those works that, in lipase adsorbent the lower the stability of the biocatalyst.
adsorption on hydrophobic carbon, there can be a Note that the onꢀstream stability of the biocatalysts in
“
wrong” orientation of lipase on the surface. In other the isoamyl caprinate synthesis medium is fairly high.
words, the active site of the enzyme can be “blocked” On being operated for over 500 h, the biocatalysts
screened) by the carbon surface and its accessibility to retain more than 70% of their initial activity (Fig. 6b).
(
the substrate can thus be diminished. On the hydroꢀ The activity of the biocatalysts does not change as they
philic surface of the initial (carbonꢀfree) silica, lipase are stored in the hexane–diethyl ether mixture. The
is “rightly” oriented toward the substrate, maximizing abrupt changes in the activity of the biocatalysts
the activity of the biocatalyst. All of the carbonꢀconꢀ observed on passing from one reaction cycle to
KINETICS AND CATALYSIS Vol. 57
No. 3 2016

402
KOVALENKO et al.
who carried out the enzymatic transesterification of an
(
а)
Activity, U/g
inedible oil from Pistacia cinensis in dimethyl or
diethyl carbonate, which was used as the solvent and
acyl donor. In that study, the yield of fatty acid esters
was as high as 97.6%.
We focused on linseed oil transesterification with
ethyl acetate. The mixture of the ethyl esters of essential
500
400
300
200
100
0
ω
ꢀunsaturated acids from linseed oil (linoleic (18 : 2)
and linolenic (18 : 3) acids), called vitamin F, is used in
cosmetics, pharmaceuticals, and diets. In the transesꢀ
terification reaction, as in esterification, the highest
activity (~2 U/g) is shown by the biocatalyst based on
adsorbent II; the lowest activity (~0.5 U/g), by the bioꢀ
catalyst based on the crustꢀtype adsorbent I. The full
conversion of linseed oil triglycerides is achieved in 24 h
and does not change throughout ten reaction cycles
2
4
6
Onꢀstream time, h
8
(
biocatalyst onꢀstream time of 330 h). The biocatalysts
were tested in linseed oil transesterification with ethyl
acetate for 1000 h. Their halfꢀinactivation time was
determined to ~500 h at 40°C.
(
b)
Activity, U/g
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
CONCLUSIONS
Different types of NSC/Ni/SiO carbonꢀcontainꢀ
2
ing adsorbents were obtained by the pyrolysis of H +
С –С4 alkane mixtures on supported Ni catalysts preꢀ
2
4
3
pared by homogeneous precipitation or by impregnaꢀ
tion of mesoporous silica. On the catalysts prepared by
homogeneous precipitation, NSC appeared as carbon
nanofibers localized near the granule surface, and
these adsorbents had a crustꢀtype structure. In the case
of the catalysts prepared by impregnation, NSC synꢀ
3
2
1
thesis took place inside SiO granules, carbon was relꢀ
2
5
0 100 150 200 250 300 350 400 450 500 550
Onꢀstream time, h
atively uniformly distributed in the granules, and, at a
supported Ni content of <1%, the carbon deposits did
not have a wellꢀdefined filamentous structure.
Biocatalysts possessing a lipase activity were preꢀ
pared by lipase adsorption on the resulting NSC/Ni/
SiO2 composites. The study of lipase adsorption and of
the activity and stability of the adsorbed enzyme demꢀ
onstrated that the optimal supports are the adsorbents
Fig. 6. Activity and stability of biocatalysts containing
immobilized lipase: (a) tributyrin hydrolysis over the
(᭿
) initial SiO2 and (
bent; (b) esterification reaction (isoamyl caprinate syntheꢀ
sis) over the ( ) (5%NSC)/Ni/SiO2 (HP) adsorbent,
) (16%NSC)/Ni/SiO2 (Impr) adsorbent, ( ) initial
SiO , and ( ) (4%NSC)/Ni/SiO (Impr) adsorbent.
᭺
) (3%NSC)/Ni/SiO (HP) adsorꢀ
2
1
(2
3
4
2
2
obtained using Ni/SiO catalysts prepared by impregꢀ
2
nation. The amount of lipase adsorbed on these adsorꢀ
another might be due to uncontrolled changes in reacꢀ bents, was 20–30 mg/g. These biocatalysts showed the
tion conditions inside prepared biocatalysts. We are highest activity and stability in hydrolysis, transesterifiꢀ
planning to optimize the ester synthesis conditions, cation, and esterification reactions. With crustꢀtype
supports, both the amount of lipase adsorbed and the
activity of the biocatalysts were many times lower. The
stability of the biocatalysts prepared was estimated by
performing batch esterification and transesterification
processes for 500–1000 h. The halfꢀinactivation time of
the biocatalysts in linseed oil transesterification with
including by improving the reactor design.
The activity of the biocatalysts was also studied in
the transesterification of vegetable oils with methyl or
ethyl acetate or dimethyl carbonate. It turned out that
the methyl or ethyl esters of fatty acids form only in the
presence of methyl or ethyl acetate. The reaction rates ethyl acetate was determined to be ~500 h at 40°C.
in the presence of these acyl group donors are the
same. The formation of fatty acid esters involving dimꢀ
ethyl carbonate proceeds very slowly. Noticeable
ACKNOWLEDGMENTS
amounts of the product were detected by TLC as late
as 144 h after the beginning of catalyst operation. This of Sciences and Federal Agency for Scientific Orgaꢀ
This study was supported by the Russian Academy
result is at variance with data of Erzheng Su et al. [30], nizations.
KINETICS AND CATALYSIS Vol. 57
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SYNTHESIS OF NANOSTRUCTURED CARBON ON Ni CATALYSTS
403
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Translated by D. Zvukov
KINETICS AND CATALYSIS Vol. 57
No. 3 2016