Study of Modifier Influence on The Catalytic Conversion of Syngas into High-Molecular-Weight Hydrocarbons

American Journal of Applied Science and Technology

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VOLUME

Vol.05 Issue 07 2025

PAGE NO.

18-30

DOI

10.37547/ajast/Volume05Issue07-04

Study of Modifier Influence on The Catalytic Conversion
of Syngas into High-Molecular-Weight Hydrocarbons

Asliddin Mamatov Sayitmurodovich

Assistant, Department of Inorganic Chemistry and Materials Science, Samarkand State University named after Sharof Rashidov,
Samarkand, 140101, Uzbekistan

Hayitali Ibodullayev Normurotovich

Student, Faculty of Chemistry, Samarkand State University named after Sharof Rashidov, Institute of Biochemistry, Samarkand, 140101,
Uzbekistan

Normurot Fayzullaev Ibodullayevich

Doctor of Technical Sciences, Professor, Department of Polymer Chemistry and Chemical Technology, Samarkand State University
named after Sharof Rashidov, Samarkand, 140101, Uzbekistan

Received:

16 May 2025;

Accepted:

12 June 2025;

Published:

14 July 2025

Abstract:

This study investigates the effect of sodium-based modifiers on the catalytic synthesis of high-molecular-

weight liquid hydrocarbons from syngas (CO + H₂). Catalysts with the composition 20%Co–

20%Fe

–

5%B

–

1.5%Zr

–

(0

–

2)%Na supported on Al₂O₃ and SiO₂ were synthesised using the incipient wetness impregnat

ion method. Various

sodium compounds (NaNO₃, NaCl, Na₂CO₃, and NaOH) were applied as modifiers. The catalysts were characterised

using chromatographic, X-ray diffraction, and technological analysis methods to determine their phase
composition, distribution of active sites, and reaction efficiency.
The study highlights the influence of support material nature and sodium loading on CO conversion, hydrocarbon
productivity, and product selectivity. According to the analysis, Na modification significantly enhanced the

activation of active centres and chain growth probability in Al₂O₃

-based catalysts, while this effect was less

pronounced for catalysts supported on SiO₂. Additionally, both the sodium source and the sequence of metal

deposition on the carrier surface were found to play a critical role in determining overall catalyst performance and
product distribution.
These findings confirm the potential of sodium-modified cobalt

–

iron

–

boron

–

zirconium catalysts in achieving high

productivity and selectivity in Fischer

–

Tropsch-type hydrocarbon synthesis. The results provide practical insights

for the design of advanced catalytic systems for efficient syngas conversion.

Keywords:

Syngas, high-molecular-weight hydrocarbons, catalyst, selectivity, modifier, sodium, cobalt, zirconium,

boron, iron.

Introduction:

Syngas, a mixture primarily composed of carbon

monoxide (CO) and hydrogen (H₂), serves as an

important intermediate for the industrial production
of liquid fuels and various chemicals derived from
coal, natural gas, and other hydrocarbons [1

–

5].

Currently, there are three main technological routes

for large-scale syngas production: steam reforming,
partial oxidation, and autothermal reforming [6

–

8].

Each method is selected based on the desired H₂/CO

ratio and the targeted application.

One of the key strategies for the effective utilisation
of associated petroleum gases (APG)

—

typically

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American Journal of Applied Science and Technology (ISSN: 2771-2745)

released or flared during oil extraction

—

is to convert

them into liquid fuel products. The Gas-to-Liquid
(GTL) technology, particularly the Fischer

–

Tropsch

(FT) synthesis, is widely employed for this purpose. FT
synthesis allows the conversion of gas condensates
and natural gas into sulfur-free, high-quality motor
fuels and chemical feedstocks. This technology not
only improves energy efficiency but also reduces
environmental

impact

by

minimizing

waste

generation and greenhouse gas emissions. Fuel
utilization is one of the most critical contributors to
global warming, with the transportation sector
ranking second in global energy consumption after
industry.

Another promising area involves the use of plasma-

assisted processes to dissociate CO₂ molecules into
CO and O₂ (2CO₂ → 2CO + O₂) [9–

14]. This approach

enables the recycling of excess atmospheric CO₂ to

obtain CO as a key component of syngas. The ability
to produce syngas from renewable sources positions
it as a foundational pillar of future C1 chemical
industry development [15

–

21].

Over the past decade, significant progress has been
made in the synthesis of high-selectivity products
from syngas, such as diesel fuels, intermediate
chemicals, and various organic compounds [22

–

28].

In FT synthesis, product distribution and efficiency are

strongly influenced by the H₂/CO ratio, reaction

conditions, catalyst type, and reactor configuration.
Catalysts based on iron (Fe) and cobalt (Co) are
commonly employed in FT reactions. However,
stabilisers

used

during

catalyst

suspension

preparation often act as catalytic poisons and reduce
activity. Consequently, the use of polymer additives
for catalyst stabilization has attracted increasing
attention [29

–

35].

This study focuses on FT synthesis under gas

–

liquid

–

catalyst (three-phase) conditions, using ultrafine iron-
based catalysts in a hydrocarbon medium. The
influence of various polymer additives on catalytic
performance was investigated. It was found that
polymer modification improved the dispersion
stability, preserved the active sites, and enhanced the
reaction selectivity. In addition, the kinetic

characteristics of the Fischer

–

Tropsch reaction were

analyzed in the presence of polymer-modified
nanoscale catalytic dispersions [36

–

40].

The results reveal new opportunities for optimising
product selectivity and yield in FT synthesis,
contributing to improved syngas utilisation and the
production of high-value motor fuels and speciality
organic compounds.

EXPERIMENTAL SECTION

Catalyst Preparation

Catalysts with high catalytic activity, selectivity, and
productivity were synthesized using the incipient
wetness impregnation method. The formulation

included aqueous solutions of Co(NO₃)₂∙6H₂O,
Fe(NO₃)₃∙9H₂O,

and

ZrO(NO₃)₂,

which

were

impregnated onto powdered supports in a single
step.

After impregnation, the resulting catalysts were dried
in a water bath, pressed into tablets, crushed to 1

–

3

mm particles, and calcined in an air stream a

t 400 °C

for 1 hour. In all catalyst formulations, the cobalt
content was maintained at 10 wt%.

Supports used were Al₂O₃ (in extrudate form) and
SiO₂ (in spherical granule form). For some samples,

the impregnation sequence was varied to evaluate its
effect on catalytic behaviour. In one approach, Co and
Na nitrates were co-impregnated. In another, alkali

metal salts (KNO₃, KOH, K₂CO₃, KCl, LiNO₃, NaNO₃,
RbNO₃, CsNO₃) were introduced in concentrations of

0.3

–

5 wt% (Li, Na, K, Rb, or Cs) in the first step,

followed by the addition of 20 wt% cobalt via

Co(NO₃)₂∙6H₂O in the second step.

After each impregnation, the samples were dried in a

water bath and calcined in air at 400 °C for 1 hour.

Chromatographic Analysis

The analysis of gaseous reaction products was
performed using an LHM-80 gas chromatograph with
a thermal conductivity detector (TCD). Helium was
used as the carrier gas, and the separation was
carried out on an activated carbon column (1 m × 3
mm) using the gas-adsorption chromatography (GAC)
method.

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Figure 1. Typical chromatogram of gaseous products obtained from synthesis over the selected high-

performance catalyst.

1

–

CO, 2

–

CH₄, 3 –

CO₂, 4 –

C₂H₆, 5 –

C₃H₈, 6 –

C₄H₁₀.

The composition of liquid paraffins was determined
using a GC method on a Biokhrom-1 gas
chromatograph equipped with a flame ionisation
detector (FID). Nitrogen was used as the carrier gas.
The separation was conducted on a capillary column

(50 m × 0.25 mm) with a stationary phase of OV

-101

under a programmed temperature regime ranging

from 40 °C to 220 °C.

X-ray Diffraction Analysis (XRD)

X-ray diffraction (XRD) patterns of the catalysts were
recorded using a DRON-

4 diffractometer with CuKα

radiation (Ni filter). XRD was used to identify the
phase composition of the catalysts and supports.
Qualitative phase analysis was performed by
matching the position and intensity of the diffraction
peaks with standard reference patterns.

RESULTS AND DISCUSSION

Catalyst activity and performance under the
influence of the nature of the carrier and the Na
modifier

. In the synthesis of liquid hydrocarbons from

syngas (a mixture of CO and H₂), the effectiveness of

catalysts is determined not only by the composition
of the active components but also by the nature of
the support material and the effect of modifiers. In
this context, the catalytic properties of the systems
20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃

and

20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/SiO₂

were

studied under reaction conditions at Tmax and 200 °C.

For the catalysts prepared in series, the addition of
1% sodium significantly affected the activity of the
alumina-based catalyst. In particular, the CO
conversion decreased from 94% to 67% (Figure 2),
which may be explained by the blocking of active sites
or structural transformations.

Figure 2. Effect of 20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃ and 20%Co–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/SiO₂

catalyst

s on CO conversion (XCO, %) and C₅⁺ product yield (YC₅⁺, g/m³) in hydrocarbon synthesis from CO and H₂

at T = 200 °C.

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At the same time, the productivity of liquid
hydrocarbon products at elevated synthesis
temperatures increased significantly

—

from 80 to

127 g/m³. This occurred alongside an increase in the

α parameter (which reflects the probability of carbon
chain growth in the C₅⁺ fraction) from 0.68

to 0.91.

These results demonstrate how the nature of the

support and the modifier influence the catalyst’s

phase composition, dispersion level, and the
properties of active centers. In general, the physical

and chemical properties of Al₂O₃ and SiO₂ suppor

ts

—

especially their beta-structure, surface area, porosity,
and surface activity

—

play a significant role in

determining reaction efficiency. Notably, in the
alumina-based system, the effect of the sodium
modifier resulted in high selectivity and productivity

toward C₅⁺ products, although limited by a lower CO

conversion. This confirms that the selection of the
support and modifier is one of the key factors in
enhancing the activity and efficiency of catalysts for
producing long-chain liquid hydrocarbons from
syngas.

Effect of Sodium Modifier on the Selectivity of
20%Co

–

20%Fe

–

5%B

–1.5%Zr/SiO₂

Catalyst

.

The

20%Co

–

20%Fe

–

5%B

–1.5%Zr/SiO₂ catalyst stood out

in the process of synthesising liquid hydrocarbons

from a CO and H₂ mixture, due to its high catalytic

activity, high selectivity, and good productivity. When
sodium was added in the range of 0

–

2%, the

selectivity indicators

—

namely, selectivity toward

the C₅⁺ fraction and toward CH₄ —

remained virtually

unchanged.

According to the obtained data, the selectivity toward

C₅⁺ hydrocarbons remained in the range of 87–

88%,

while CH₄ selectivity remained at about 5%. This

stability is likely due to the passivity of active sites on

the SiO₂ surface toward sodium, or due to the lack of

sufficient acidity and surface polarity. As a result, the
sodium modifier had little effect on the catalyst's
selectivity profile.

Based on this, it can be concluded that the use of SiO₂

as a support does not benefit from the addition of a
sodium modifier in terms of optimising the cata

lyst’s

selectivity. This implies that the selection of modifiers
must consider the specific nature of the support
material, and that comprehensive analysis of the

catalyst’s structure and surface chemical properties is

necessary.

Figure 3. Effect of the support nature in 20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃(SiO₂) catalysts on

selectivity toward C₅⁺ and CH₄ during hydrocarbon synthesis from CO and H₂ at T = 200 °C.

According to the described experimental results, the
nature of the support material significantly influences

both the selectivity toward C₅⁺ fractions and the
formation of methane (CH₄) in the synthesis of
hydrocarbons from CO and H₂ at 200 °C. As shown in
the graph, for the Al₂O₃

-based support, the addition

of a sodium promoter increased selectivity toward

the C₅⁺ fraction from 41% to 92%, which is attributed

to the redistribution or enhancement of active

centres on the catalyst surface. Furthermore, CH₄

selectivity decreased sharply from 29% to 4%,
indicating a notable suppression of methanation
reactions.

For the SiO₂

-based support, the addition of sodium

had almost no effect: C₅⁺ selectivity decreased slightly
from 88% to 87%, while CH₄ selectivity remained

stable at around 5%. This behaviour reflects the
differences in interaction strength between the

support’s structural and surface properties and the

active metal components.

Overall, these findings confirm that Al₂O₃

-based

supports offer a better environment for organizing
active sites and are more responsive to promoter
effects. This is a key factor in developing highly
selective and productive catalysts.

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A comparative analysis of the catalyst series
synthesised at optimal temperatures showed that the
introduction of 1% sodium ions into the 20%Co

–

20%Fe

–

5%B

–

1.5%Zr system led to significant

modification and redistribution of active centres. This
was clearly reflected in the enhanced process
efficiency indicators

—

increased CO conversion, C₅⁺

hydrocarbon productivity, and selectivity.

In particular, for the Al₂O₃

-based catalyst, the

addition of sodium as a promoter increased CO
conversion from 72% to 82%. This improvement is
explained by the enhanced transformation of active
metals (Co and Fe) into highly dispersed active
centres and their more effective interaction with the
activated gas molecules.

In parallel, the C₅⁺ hydrocarbon yield increased from
112 g/m³ to 138 g/m³, indicating that the surface of

the modified catalyst favoured reaction pathways
leading to carbon chain growth. Selectivity toward
target products also increased from 75% to 81%,
confirming a higher yield of desired long-chain
hydrocarbons.

Furthermore, the presence of sodium resulted in
improvements to the textural and electronic
properties of the catalyst, including increased surface
acidity, redox capability, and adsorption capacity for
reactive gases. These changes enhanced overall
reaction performance and confirmed the critical role
of promoter elements in forming effective active
sites. These findings form a strong theoretical and
practical basis for designing future highly selective
catalysts.

The SiO₂

-based 20%Co

–

20%Fe

–

5%B

–

1.5%Zr catalyst

was also evaluated as a system with high catalytic
activity, selectivity, and productivity. However, the
introduction of sodium as a promoter had a more
limited effect compared to the A

l₂O₃

-based system.

Notably,

CO

conversion

remained

virtually

unchanged at ~86

–

87%, even with the sodium

additive. This suggests that sodium interacts less

effectively with Co and Fe active sites on SiO₂ surfaces
than on Al₂O₃.

However, the C₅⁺ liquid hydro

carbon yield increased

modestly from 140 g/m³ to 147 g/m³, and selectivity

rose from 79% to 82%. These changes likely result
from the sodium ions modifying the environment
around active sites, slightly enhancing the pathway
toward higher molecular weight liquid products.

Importantly, the yield of gaseous hydrocarbons (C₁–
C₄) decreased from 35 g/m³ to 27 g/m³, and their

selectivity fell from 18% to 13%. This suggests that the
promoter inhibits methane and low-molecular-

weight hydrocarbon formation and encourages
reactions producing heavier hydrocarbons.

A general trend observed across all tested catalysts
was that the presence of Group I alkali metals

(sodium) led to an increase in CO₂ yield. For example,
in the Al₂O₃

-

based system, CO₂ production increased

fr

om 17 g/m³ to 24 g/m³, and in the SiO₂

-based

system, from 21 g/m³ to 24 g/m³. This could be due

to enhanced oxygen adsorption and oxidation activity
on the catalyst surface, stimulated by sodium.

Additionally, the introduction of sodium into the
20%Co

–

20%Fe

–

5%B

–

1.5%Zr system increased the

optimal synthesis temperature by 10

–20 °C. This

indicates

a

higher

thermodynamic

energy

requirement for catalyst activation, possibly due to
sodium ions either passivating surface active sites or
enhancing thermal stability.

In summary, the presence of sodium significantly
influenced the nature of the active sites, the
adsorption of gas molecules, and the reaction
mechanism on the catalyst surface

—

leading to

observable changes in overall activity and product
selectivity.

Specifically:

•

For Al₂O₃

-based catalysts, sodium addition

substantially

improved

CO

conversion,

C₅⁺

hydrocarbon productivity, and selectivity. These
favourable effects are attributed to the high surface

activity of Al₂O₃ and strong interactions between

sodium ions and the active centres.

•

For SiO₂

-based catalysts, the effects were

milder

—

no significant improvement in CO

conversion was observed, but a slight increase in C₅⁺

selectivity and a reduction in gaseous by-products
were recorded.

•

In all systems, sodium addition increased CO₂

production, indicating intensified oxidation processes
and a more active participation of oxygen on the
catalyst surface.

These findings, supported by detailed analysis and
tabular data, demonstrate that reaction activity and
selectivity in modified catalysts can be significantly
enhanced, and that the influence of sodium
promoters varies depending on the nature of the
support material.

Analysis of the Fractional Composition of Liquid

Hydrocarbons and the α Pa

rameter

. The results of

the study indicate that the introduction of sodium
(Na) into catalysts with compositions of 20%Co

–

20%Fe

–

5%B

–1.5%Zr/Al₂O₃ and 20%Co–

20%Fe

–

5%B

–

1.5%Zr/SiO₂ did not significantly affect the fractional

composition of the resulting liquid hydrocarbon

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products. This suggests that sodium modification
influenced the reactivity of the active centers rather
than

fundamentally

altering

the

structural

composition of the products. Despite the presence of
sodium in the catalyst formulation, the distribution of
the main hydrocarbon fractions remained nearly
unchanged.

At the optimal synthesis temperature, the chain

growth probability (expressed by the α value) was

found to be in the range of 0.86

–0.87 for Al₂O₃

-based

catalysts and 0.82

–0.832 for SiO₂

-based systems. This

α parameter, known as the Anderson–

Schulz

–

Flory

coefficient, reflects the likelihood of carbon chain
elongation in the hydrocarbon synthesis process.

Accordingly, Al₂O₃

-based catalysts are more likely to

yield longer-chain hydrocarbons, contributing to a
higher share of heavier fuel fractions such as gasoline
and diesel.

In the products synthesized using 20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃ catalysts, the gasoline

fraction (C₅–C₁₀) and the diesel fraction (C₁₁–C₁₈) were

generated in nearly equal amounts, accounting for
approximately 42

–

45% and 41

–

42%, respectively.

This balance indicates that these catalysts possess
high versatility and the capability to generate a broad
range of valuable liquid hydrocarbons. Additionally,

the C₁₉⁺ fract

ion (high molecular weight paraffins and

waxes) made up 14

–

16% of the total.

When the synthesis was conducted using SiO₂

-based

catalysts, a shift in the fractional distribution was

observed. The share of the C₅–C₁₀ fraction increased

to 52

–

54%, indicating a higher yield of lighter liquid

hydrocarbons. Meanwhile, the diesel fraction (C₁₁–
C₁₈) constituted 37–38%, and the C₁₉⁺ fraction

accounted for only 9

–

10%. This trend may be

attributed to the lower efficiency of the active sites

on the SiO₂ surface in prom

oting the formation of

long-chain hydrocarbons.

In general, these findings confirm that the

incorporation of sodium into Al₂O₃

-based catalysts

promotes a balanced distribution of gasoline and
diesel fractions, thereby enhancing their value as
fuels. In con

trast, SiO₂

-based catalysts are more

suited for synthesis processes aimed at producing
lighter liquid hydrocarbons, which may be preferable
for certain industrial and energy applications. Each of
these outcomes presents practical advantages
depending on specific operational and end-use
requirements.

Influence of Sodium on the Composition of Reaction
Products and Catalyst Activity

During the synthesis of liquid hydrocarbons from CO

and H₂ gases, the addition of 1% sodium (Na) to the

composition of the selected catalyst

—

characterized

by

high

catalytic

activity,

selectivity,

and

productivity

—

resulted in noticeable changes in the

group composition of the reaction products. The
introduction of the promoter led to a shift in the
distribution between unsaturated hydrocarbons and
paraffins, clearly demonstrating the interaction of

Na⁺ ions with the active sites on the catalyst surface.

Specifically,

for

Al₂O₃

-based

catalysts,

the

concentration

of

unsaturated

hydrocarbons,

including ethylene series (such as C₂H₄), dec

reased

from 12% to 3%. This reduction is attributed to the
suppression of surface acidity and dehydrogenation

reactions by Na⁺ ions. Simultaneously, the content of

paraffins increased from 67% to 72%, indicating a
shift in the reaction environment toward the
formation of saturated hydrocarbons.

In contrast, for SiO₂

-based catalysts, the trend was

less pronounced. The amount of unsaturated
hydrocarbons slightly increased up to 2%, while the
paraffin content rose from 64% to 68%. This indicates
weaker interactions between sodium and the metal-
complex active sites on the silica surface, resulting in
a less pronounced modification effect.

These analytical results confirm that the most
significant promoting effect of 1% Na addition was

observed for Al₂O₃

-supported catalysts. Sodium ions

alter the configuration and electron density of active
sites on the alumina surface, steering the reaction
pathway toward the formation of paraffinic products.

Additionally, in the synthesis process conducted at

200 °C, the sequenc

e of active metal incorporation

into

the

20%Co

–

20%Fe

–

5%B

–

1.5%Zr

catalyst

significantly

influenced

its

physicochemical

properties and the formation of active sites. The
adsorption sequence of metal ions, their dispersion,
and spatial distribution directly affect the selectivity,
activity, and productivity of hydrocarbon synthesis.

Therefore, the selection of modifiers and the method
of metal impregnation should be based on a detailed
understanding of the mutual interactions between
components and their behaviour in the reactive
environment. This approach is crucial for designing
high-performance, selective, and stable catalysts for
industrial-scale applications.

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Figure 4. Effect of sodium introduction order on the catalytic properties of 20%Co

–

20%Fe

–

5%B

–

1.5%Zr(0

–

2)%Na/Al₂O₃ catalysts during hydrocarbon synthesis from CO and H₂ at T = 200 °C.

The experimental results show that introducing
sodium into the 20%Co

–

20%Fe

–

5%B

–1.5%Zr/Al₂O₃

catalyst by different methods and in various
sequences significantly affects key technological
parameters such as activity, selectivity, and
productivity in the hydrocarbon synthesis process.

At 200 °C, the method of sodium addition influenced

the nature and distribution of active sites and their

interactions with CO and H₂ molec

ules:

•

Na + Co method (sodium and cobalt

introduced simultaneously):

CO conversion: 41%, C₅⁺ selectivity: 91%, C₁–C₄

selectivity: 7%.

This suggests effective formation of high molecular
weight hydrocarbons as the main products.

•

Co → Na method (cobalt ad

ded first, followed

by sodium):

CO conversion improved significantly to 59%, C₅⁺
selectivity: 81%, but C₁–C₄ selectivity increased to

17%, indicating enhanced formation of undesired
methane and light hydrocarbons.

•

Na → Co method (sodium added first, then

cobalt):

Achieved the highest CO conversion of 67%, C₅⁺
selectivity: 92%, and maintained low C₁–C₄ selectivity

at 7%.

These results confirm the efficiency of this order for
generating valuable products with high selectivity.

In summary, the sequence of sodium introduction

directly affects the formation of active sites and their

interaction with reactant gases. The Na → Co method

enables optimal structuring of active sites and
effective activation of cobalt, leading to improved
conversion, selectivity, and desired product
composition.

Influence of the Cobalt and Sodium Introduction
Sequence on Catalytic Properties: Synergistic and
Selectivity Effects

According to the results, the catalyst with the
composition

20%Co

–

20%Fe

–

5%B

–

1.5%Zr*Na,

synthesized using the simultaneous introduction of
sodium and cobalt, demonstrated moderate catalytic
performance despite its targeted high activity,
selectivity, and productivity. Under these conditions,
the CO conversion reached 41%, and the yield of
liquid hydrocarbons was 77 g/m³.

Notably, the C₅⁺ sele

ctivity remained high at 91%,

indicating a strong preference for valuable long-chain
products. In contrast, the selectivity toward low-

molecular gaseous hydrocarbons (C₁–C₄) was 7%,

suggesting their formation was minimal.

In follow-up experiments, the gradual addition of
sodium in concentrations ranging from 0 to 5% led to
a marked improvement in catalytic performance. The

CO conversion increased to 59%, while the yield of C₅⁺

hydrocarbons rose to 92 g/m³, indicating enhanced
productivity in hydrocarbon synthesis.

However, excessive sodium content promoted the
formation of undesirable light hydrocarbons

—

the

selectivity toward gaseous C₁–C₄ fractions increased

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up to 17%, which consequently decreased the

selectivity for C₅⁺ hydrocarbons down to 81%. This

indicates a shift in the reaction pathways favouring
lower molecular weight compounds.

In general, these findings confirm that controlled

sodium addition significantly boosts liquid product
yield, while excess sodium may lead to increased
formation of gaseous by-products. Therefore,
optimising sodium concentration is crucial for
balancing catalyst activity and selectivity.

Figure 5.

Effect of sodium introduction order on C₅⁺ hydrocarbon yield during Fischer–

Tropsch synthesis at

200 °C using 20%Co–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃ catalysts.

During Fischer

–Tropsch synthesis at 200 °C, the yield

of C₅⁺ liquid hydrocarbons was found to be highly

dependent on the sequence in which sodium was
introduced into the catalyst composition. All tested
catalysts were prepared to exhibit high catalytic
activity, selectivity, and productivity. Among the
parameters evaluated, the sodium incorporation
method was a key determinant of catalyst
performance.

•

Na + Co sequence (sodium and cobalt

introduced simultaneously):

Yield of C

₅⁺ hydrocarbons was 77 g/m³, indicating

suboptimal distribution of active sites and moderate
catalytic efficiency.

•

Co → Na sequence (cobalt introduced first,

then sodium):

Yield increased to 92 g/m³. This was attributed to the
initial formation of cobalt active sites, later enhanced
by sodium's promoting effect.

•

Na → Co sequence (sodium introduced first,

then cobalt):

Achieved the highest yield of 127 g/m³, suggesting
that this approach enables better formation and
dispersion of active sites, and optimally channels the
reaction toward long-chain hydrocarbon products.

Overall, the sequence of sodium incorporation affects

not only the chemical composition but also the
structure of active sites and the electronic
environment on the catalyst surface. Specifically,
preliminary sodium introduction allows the surface to
be more precisely structured, greatly enhancing the

C₅⁺ hydrocarbon yield.

These trends underscore the strategic importance of
modifier implantation order in catalyst synthesis. It is
a critical consideration in the design of high-
performance Fischer

–

Tropsch catalysts.

Furthermore,

applying

a

reverse

sequential

impregnation

method

—

introducing

modifier

elements (Na, Zr/B) first, followed by active metals
(Co and Fe)

—

enabled the synthesis of a catalyst with

superior performance. This order facilitated the
formation of well-dispersed and optimally structured
active sites, with improved mutual interactions.

Experimental data confirmed that the CO conversion
reached 67%, which is significantly higher than that
observed in conventional impregnation methods
(e.g., simultaneous or reversed sequences). This
improvement is linked to the increased number and
enhanced activity of active sites, as well as more
efficient adsorption

–

activation of gas molecules (CO

and H₂).

Thus, the reverse sequential deposition technique
positively influenced the textural, structural, and
electronic properties of the catalyst, leading to

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improvements in catalytic activity, selectivity, and C₅⁺

hydrocarbon yield. It is a promising strategy for the
development of advanced catalysts in Fischer

–

Tropsch synthesis.

Table 1.

Catalytic performance indicators for hydrocarbon synthesis from CO and H₂ using 20%Co–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃ catalysts at optimal temperature T = 210 °C.

Introduction Sequence

X_CO
(%)

Yield
(g/m³)

Selectivity
(%)

Composition of C₅⁺

hydrocarbons (%)

CH₄

C₂–C₄

C₅⁺

C₂–C₄

20%Co

–

20%Fe

–

5%B

–

1.5%Zr*Na

72

21

15

99

Na + Co

69

20

14

103

Na → Co

82

16

10

138

These data demonstrate that sodium addition in
different amounts (0

–

2%) has a clear effect on

catalyst performance. Specifically:

•

CO conversion increased as sodium content

rose. The enhanced interaction between active sites
and reactant gases resulted in more effective CO
adsorption and dissociation.

•

The yield of C₅⁺ hydrocarbons also improved

with sodium addition, attributed to modification of
the metal

–

complex environment, which promotes

chain-growth reactions.

•

Selectivity shifted in favour of C₅⁺ p

roducts,

suggesting surface conditions that suppress the
formation of gaseous by-products.

•

However, at excessive Na concentrations,

selectivity declined, and the yield of C₁–C₄

hydrocarbons increased, likely due to unfavourable
changes in active site structure.

In conclusion, optimizing the amount and order of
sodium addition provides an effective route to
enhance both overall catalyst activity and product
selectivity. This serves as a crucial foundation for the
rational design of advanced catalysts.

Achieving High Selectivity and Productivity in
Hydrocarbon Synthesis via Sequential Introduction
of Catalyst Components

The results of this study demonstrate that the
sequential introduction of active metals and
modifiers during the preparation of the 20%Co

–

20%Fe

–

5%B

–

1.5%Zr*Na

catalyst

enabled

the

development of a material with high catalytic activity,
selectivity, and productivity suitable for hydrocarbon
synthesis.

Under these conditions, CO conversion reached 82%,

and the productivity of liquid C₅⁺ fraction

products

increased to 138 g/m³. This indicates that the
composition and surface structure of the active sites
were optimised for the reaction and that a synergistic
effect emerged due to the interaction between the
active metals and modifiers.

Selectivity analysis showed that the formation of light

gaseous hydrocarbons (C₁–C₄) decreased to 14%,

indicating a restructuring of the catalyst surface that
suppressed the formation of low-molecular products.

At the same time, the selectivity toward C₅⁺ products

increased to 81%, confirming that the reaction
pathway was directed toward the formation of target
products.

The hydrocarbon chain growth probability (α), a key

parameter for evaluating the fractional activity of the
catalyst, increased proportionally with liquid
hydrocarbon productivity

—

from 0.84 to 0.87. This

value expresses the efficiency of the chain growth
reactions. In particular:

•

At α = 0.84–

0.85, the fractional composition

consisted of 47

–50% gasoline (C₅–C₁₀), 39–

40% diesel

(C₁₁–C₁₈), and 11–

13% h

eavy fraction (C₁₉⁺);

•

At α = 0.87, gasoline and diesel fractions were

produced in nearly equal proportions (42%), while the

C₁₉⁺ fraction increased to 16%.

An analysis of the group and structural composition
of the products showed that the highest proportion
of unsaturated ethylene-type hydrocarbons (mainly

C₂H₄) —

6%

—

was observed with the 20%Co

–

20%Fe

–

5%B

–

1.5%Zr*Na catalyst. This indicates that the

catalyst also contains active sites capable of partially
promoting dehydrogenation reactions.

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American Journal of Applied Science and Technology (ISSN: 2771-2745)

Additionally, the paraffin content in the products
reached 75%, signifying the dominance of long
saturated chains and confirming the high energy
potential of the fuel, particularly for diesel and
aviation fuel applications.

Based on the data in Table 2, the catalytic properties
of the 20%Co

–

20%Fe

–

5%B

–

1.5%Zr catalysts under

the influence of different sodium promoter
compounds

—

sodium chloride (NaCl) and sodium

hydroxide (NaOH)

—

were evaluated during the

hydrocarbon synthesis process. All experiments were
conducted under optimal temperature conditions

(Tₒₚₚ

), and the activity, selectivity, and productivity of

the catalysts during synthesis from CO and H

₂

were

analysed.

The NaCl-modified catalyst showed the lowest
catalytic activity in the series, despite optimal
temperature:

•

Although CO conversion reached 89%, the

productivity of liquid C₅⁺ fractions remained low, at

only 60 g/m³;

•

Simultaneously, the productivity of C₁–C₄

gaseous fractions rose to 28 g/m³, indicating reduced

selectivity toward desired liquid products;

•

Methane selectivity also increased to 11%,

pointing to intensified methanation reactions.

NaCl, as an ionic chlorine-containing compound, likely
exerts a negative effect on the active sites of the
catalyst surface, partially deactivating them and
suppressing radical steps in the reaction mechanism,
resulting in lower liquid product yield.

Conversely, when sodium hydroxide (NaOH) was used
in the preparation of 20%Co

–

20%Fe

–

5%B

–

1.5%Zr*Na

catalysts:

•

CO conversion increased to 68%;

This improvement is attributed to the stronger basic
environment on the catalyst surface, better structural
formation of the active sites, and enhanced
adsorption

–

activation of gas components.

In a NaOH-

modified environment, the introduced Na⁺

ions regulate the interaction between the carrier
surface and metal centers. This allows for increased
productivity of the liquid fractions while reducing the

formation of gaseous fractions (methane and C₂–C₄).

Table 2. Results of hydrocarbon synthesis from CO and H₂ over 20%Co–

20%Fe

–

5%B

–

1.5%Zr(0

–2)%Na/Al₂O₃

catalysts under optimal temperature (Tₒₚₚ

) conditions

Sodium
Compound

Tₒₚₚ

(°C)

CO Conversion
(%)

Productivity (g/m³)

Selectivity
(%)

α

C₂–C₄

C₅⁺

C₂–C₄

NaNO₃

190

72

15

112

12

NaCl

210

82

10

138

8

Na₂CO₃

210

86

17

140

9

NaOH

220

87

14

147

6

The results in the table indicate how the introduction
of various amounts of sodium promoter (0

–

2%) to

20%Co

–

20%Fe

–

5%B

–

1.5%Zr catalysts, supported on

Al₂O₃, affects the main technological parameters of

the synthesis process. All tests were conducted under
optimal temperature conditions, with key indicators
such as CO conversion, gas and liquid product
productivity, and selectivity assessed.

The analysis confirms the following:

•

Sodium

introduction

leads

to

the

restructuring of active sites on the catalyst surface.
These changes enhance the adsorption and activation

of CO and H₂ molecules, improving the efficiency of

sequential reaction stages;

•

CO conversion increased compared to

sodium-free catalysts. Specifically, the addition of a

precise concentration of Na⁺ ions optimised reaction

efficiency and improved conversion rates;

•

Liquid hydrocarbon (C₅⁺ fraction) productivity

increased proportionally with sodium concentration.
This reflects the activation of chain growth reactions
and the predominance of target high-molecular-
weight fractions in the product mixture;

•

Simultaneously, the productivity of gaseous

fractions (C₁–C₄) and methane selectivity decreased

with the optimal amount of sodium, indicating a shift
in the reaction toward higher selectivity.

In summary, the data show that introducing an

optimal amount of Na⁺ ions on an Al₂O₃

-supported

catalyst surface not only enhances catalytic activity
and selectivity but also significantly increases the

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American Journal of Applied Science and Technology (ISSN: 2771-2745)

yield of liquid products. However, excessive sodium
may passivate active sites and shift the reaction
toward methanation. Therefore, careful control of
sodium concentration, combined with structural
optimisation, is necessary.

Ultimately, the proper order of component
introduction during synthesis plays a decisive role in
determining the configuration of active sites, the
probability of hydrocarbon chain growth, and the
composition and quality of final products. This makes
it a key strategic approach for achieving maximum
selectivity and high productivity in industrial
applications.

CONCLUSION

1.

It was determined that the activity of

catalysts for the synthesis of liquid hydrocarbons

from CO and H₂ is strongly influenced by the nature

of the support and the presence of a sodium modifier.

The addition of Na to Al₂O₃

-based supports

significantly increased both the CO conversion and

the productivity of C₅⁺ liquid fractions.

2.

Under the influence of the Na modifier, the

structure of active sites on the Al₂O₃ surface

impr

oved, the carbon chain growth probability (α

parameter) increased, and a balanced distribution of
gasoline and diesel fractions was achieved.

3.

In SiO₂

-based catalysts, the influence of

sodium was comparatively weaker. An increase in the
proportion of light hydrocarbons in the product
composition and a decrease in low-molecular product
formation were observed.

4.

The type of sodium source had a significant

effect on the catalytic properties: while the addition
of NaCl reduced catalytic activity, catalysts prepared
with NaOH demonstrated high activity and selectivity.

5.

The order of metal introduction into the

support also influenced catalyst performance: the
Na

–

Co scheme (introduction of sodium before cobalt)

yielded the best CO conversion and C₅⁺ product

productivity.

6.

The findings of this study establish the

practical basis for developing high-performance and
selective catalysts for the Fischer

–

Tropsch process

through the optimal selection of sodium modifiers
and support materials.

7.

The use of different Na sources (NaCl and

NaOH) significantly affects catalyst characteristics.
The NaCl additive reduces catalyst activity and
promotes gaseous product formation, whereas NaOH
enables the formation of a selective and productive
system by properly organizing the reactive active
sites. Therefore, the choice of sodium source is one of

the key factors in catalyst design.

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