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35
Fayzullaev N. I., Shukurov B. Sh., Samarkand state university E-mail: [email protected]
CATALYTIC AROMATIZATION OF METHANE WITH NON-MO-CONTAINED CATALYSTS
Abtsract: In this aricle catalytic aromatization of methane was studied with (MoO3) x • (ZrO2) • (ZnO2)z content nanocatalyst. Effect of nature different d-elements and their promoter property of catalyst on catalytic dehydrogenaromatization reaction. Interaction between MoO3 and benton-ite acidic centers was investigated. Based on obtained results possible mechanism for preparation aromatic hydrocarbons.
Keywords: methane, dehydroaromtaization, bentonite, sol-gel technology, nanocatalyst, acidic center, mechanism scheme.
Introduction
Effective using of natural gas has been staying problematic for a long time.
Main component of natural gas is methane and its quantity reaches up to 95%. Today methane is used to heat houses, in production of ammonia and hydrogen. Researches to prepare benzene from methane directly started in 90th of previous century [1-4]. Product of dehydroaromatization of methane without oxidants are aromatic hydrocarbons. Arenes, mainly benzene, toluene, ethyl-benzene and xylene are important proucts in petroleum chemistry.
Today aromatic hydrocarbons are prepared from catalytic reforming of petroleum fractions. But as decreasing petroleum stock there is requirement to find alternative sources of the preparation of arenes. Alternative sources of preparation of aromatic hydrocarbons are natural gas and biogas. Aromatization of alkanes can be carried out for two different purposes: obtaining liquid fuel with high octane number and aromatic hydrocarbons for petroleum chemistry synthesis [5].
Benzene formation reaction from methane molecule 6CH4 - C6H6 + 9H2 is highly exothermic ( AH = 523 kJ/mol), and requires effective catalyst
and high temperature to go the process. As a main component of natural gas, methane is thermodynamic stable and durable effects of many reagents. Energy of C-H bond is 398 kJ/mol. Direct synthesis on the basis of methane is very difficult, but its products are more active than methane and reacts easily.
Today Mo-containing mono- and polymetal systems catalysts are good catalysts for methane dehydroaromatization reaction, and zeolites are used in the research to prepare motor oil and aromatic hydrocarbons from natural gas, petroleum. satellite gas and gas condensates [6]. Reaction takes place in presense of zeolite contained catalysts. No oxidant present in the process. Taking into account above mentioned current work aims to study direct catalytic aromatization of natural gas and effect of different factors.
Experimental part
Methane conversion absence of oxidants takes place at 600-800 °C and P = 0,1 Mpa pressure, volumetric rate of methane is 500-1500 hour-1. Ratio of methane: argon = 1:1 and in flow reactor.
Reactants and proucts of the reaction were studied with chromatography. Chromatogram of reaction mixture is given in (Fig. 2).
Figure 2. Chromatogram of liquid products of the catalytic aromatization of methane in the presence of selected catalysts
Today Mo-containing mono- and polymetal modified systems catalysts are good catalysts for methane dehydroaromatization reaction. Recently sol-gel method is used at the synthesis of organic-inorganic matrices at low temperatures. This method has advantage because of simplicity of apparatus, cost effectiveness, ecological safer, and adaptive technological features.
Nanocatalysts represent high catalytic activity, selectivity and stability. High effectiveness of nanocatalysts come from charge, energy, mass and information transfer and movement, and this process takes place in nanostructure and reaction in nano-systems. Application of novel nanocatalysts with high effectiveness made it possible improvement
of ecological characteristics of industry and technological processes, decrease of waste into the atmosphere, creation of new products and materials. Using nanoparticle catalysts in catalysis depends on the following two points. Firstly, as decreasing of the particles most atoms place on the surface, for this reason nanoparticle catalysts gain high surface area and show high activity in heterogeneous reactions. Secondly, many features of nanoparticles depend on their size (size effectiveness), because of this by changing size of nanoparticle we may change the activity as well as control selectivity. Decreasing size of nanoparticles increases the rate of the reaction.
This method has advantage because of simplic- Nanocatalyst preparation for methane catalytic
ity of apparatus, cost effectiveness, ecological safer, aromatization with sol-gel technology is given in
and adaptive technological features. For this reason (Fig. 3).
we chose nanocatalyst with several polyfunctional Synthesis scheme ofcore-shell(MoO3)^ (ZrO2) •
properties for methane dehydroaromatization pro- . (ZnO ) nanoparticle: cess prepared with sol-gel technology with the following content (MoO3)x • (ZrO2)y • (ZnO2)z.
Figure 3. Synthesis scheme of core-shell (MoO3)x • (ZrO2)y • (ZnO2)z nanoparticle
Structure of catalyst active site and state were characterized with electron microscopy and electron diffraction.
Experimental results and their discussion
One of drawbacks of non-oxidative methane aro-matizion reaction at 700-800 °C is deactivation of catalysts because of coke formation. To keep stability of catalysts different metals are added (Cu, Zr, Pt, Zn, Fe, Co Ba ^.3) as promoters.
It is known from the literatures that Mo-contained catalysts show high activity in methane aromatization reaction. Core-compound (carrier)
2'V
of the catalyst made from local raw material - ben-tonite. Preliminary experiments shown that pure bentonite at the following methane aromatization reaction conditions at 750 °C and V , = 1000
methane
hour-1 has no catalytic effect and no arenes formed. As adding Mo-nanoparticles to bentonite methane conversion to aromatic hydrocarbons observed. Experiments show that optimal concentration of Mo in bentonite is 50% and such catalyst show selectivity and high throughput.
In the formation of aromatic hydrocarbons catalyst with 5% Mo and modification it increases
its catalytic activity. For this reason we studied ef- catalysts increase the yield of methane aromatization
fect of copper, zinc, gallium, zirconium, manganese, reaction. Experiments shown that zirconium doped
iron, nickel and cobalt metals on the catalyst. Experi- catalyst has highest result. Experimental results is
ments shown that Zr, Zn and Ga modified bentonite shown in (Fig. 4).
5.0%Mo 1,0?bGa l,0%Zn lt0%2i 5,0°/<Mo 5,0%Mo 5,0%Mo
Figure 4. promoter effect of different metals on molybdenum based catalyst
As seen from (Fig. 4), bentonite based catalyst studied Ga and Zn doped catalysts. Doping Zr-Mo content 1.0% Zr and 5.0% Mo has high catalytic contained catalysts with zinc and gallium increases activity. Moreover increase of zirconium concentra- their catalytic activity. Yield of aromatic hydrocar-tion from 0.25% to 2%, the optimal concentration bons is highest with (MoO )x • (ZrO2) • (ZnO2)z/ found 1.0%. To achieve maximum reaction yield we bentonite catalyst.
Table 1. - Effect of temperature on Mo, Zr and Zn nanoparticle modified catalysts for methane catalytic
aromatization reaction (V CH = 1000 hour-1)
T. °C X .% gas Reaction products Y % Ap. S % aP.
H2 CH , n 2n+2 C H n 2n C6H6 C7H8 C8H10 C10H8
1 2 3 4 5 6 7 8 9 10 11
5.0% Mo/bentonite
650 18.6 2.45 85.27 3.58 3.53 0.01 0.10 5.07 8.7 46.8
675 26.4 3.95 76.74 2.41 5.67 0.05 0.30 10.88 16.9 64.0
700 31.5 4.97 72.60 1.03 6.99 0.10 0.60 13.71 21.4 67.9
750 38.3 8.68 65.69 0.73 9.92 0.11 0.90 15.97 26.9 70.2
5.0% Mo-I 0.5% Zr/ bentonite
650 21.8 1.79 84.47 2.84 3.58 0.01 0.20 7.11 10.9 50.0
675 32.6 4.49 74.29 2.22 4.71 0.06 0.50 13.73 19.0 58.3
700 39.4 6.52 66.51 1.77 8.17 0.10 0.80 16.13 25.2 64.0
750 49.7 8.81 50.91 1.08 12.6 1.25 1.5 17.35 32.4 65.2
5.0% Mo-1.0% Zr/ bentonite
650 31.6 1.87 79.25 2.38 6.69 0.11 0.23 9.47 16.5 52.2
675 43.8 5.20 65.55 2.25 10.6 0.75 0.59 15.07 27.0 61.6
1 2 3 4 5 6 7 8 9 10 11
700 48.6 7.72 54.72 2.06 15.5 1.08 1.22 17.65 35.5 73.0
750 52.5 10.8 47.85 0.92 15.2 1.32 1.87 21.98 40.4 77.0
5.0% Мо-1.5%Zr/ bentonite
650 21.6 1.57 85.82 2.21 3.87 0.12 0.20 6.21 10.4 48.2
675 28.3 2.86 75.27 2.07 8.65 0.59 0.30 10.26 19.8 70.0
700 36.7 5.27 66.63 2.20 9.8 0.72 0.50 14.88 25.9 71.0
750 44.6 6.57 59.57 2.06 11.4 0.96 0.70 18.76 31.8 71.3
Note: T - reaction temperature; Xconversion; YAn - yield of aromatic hydrocarbons; SAn - selectivity of aromatic hydrocarbon formation
Effect of temperature on the rate ofmethane non-oxidative aromatization reaction, yield of the reaction, selectivity and conversion of reactants were studied. Results of the experiment is given in (Table 1).
Maximal amount of aromatic hydrocarbons was obtained with the catalyst of following content: (MoO3)x • (ZrO2)y • (ZnO2)z/bentonite. Yield od aromatic hydrocarbons is 40.4% with the catalyst
It can be seen from the (Table 2) that increase in volumetric rate from 1000 to 1200 hour1 conversion of reactants and yield ofaromatic hydrocarbons decreases.
Catalytic activity of the catalyst depends on not only its content, but also thermometric preparation process. When working on catalyst thermomechani-cally high disperse particles with excess energy form. During in-process for 2-3 hour at 500-600 °C on 1.0% zirconium contained catalyst no change observed in aromatization characteristics. Increase in temperature at 700-750 °C improves aromatization quality of the catalyst. Moreover, cracking and de-hydrogenation products formation also decreased.
In addition to catalyst activity and selectivity, another important parameter is its stability of working
at VCHi = 1000 hour1, T = 750 °C. Effect of several factors (volumetric rate, temperature, contact time, height of catalyst layer, mass relationship of catalyst active components) on selected catalysts has been studied. Effect of volumetric rate on the rate of aromatization reaction with the (MoO3)x • • (ZrO2) • (ZnO2)z/bentonite catalyst is given in (Table 2). z
without lost of high aromatization quality. During aromatization process of hydrocarbons coke forms on the surface of catalyst, and this negatively effects efficiently and its activity gradually decreases. For this reason it requires regeneration of catalysis. Regeneration of catalysis carried out at 650°C and under air stream for 8 hours with gradual increase of oxygen content. After regeneration the catalyst gains its initial full activity.
In lower alkanes aromatization reaction catalysis preparation technology also effects catalyst activity and effectiveness. After zirconile nitrate sorption of bentonite, it thermally worked at 500-650 °C for 2 hours and its catalytic activity studied. At that stage its catalytic activity has not changed. When catalyst
Table 2.- Effect of volumetric rate on the rate of aromatization reaction with the (MoO3)x-(ZrO2) y-(ZnO2)z/bentonite catalyst is given in Table 2. (T = 750 °C)
VCH 4 Reaction products.% C.%
H2 Alkane Alkene C6H6 C7H8 C8H,0 C,A
500 56.8 32.65 12.15 12.01 9.68 0.73 1.01 16.06 27.48 48.38
1000 52.5 10.83 47.85 0.92 15.23 1.32 1.87 21.08 40.4 77.0
1500 49.6 8.55 50.32 1.15 15.71 1.27 2.05 17.44 7.5 73.5
worked at 700-750 °C for 3 hours its aromatization activity dramatically increased. At 750 °C yield of aromatic hydrocarbons reached to 5.2%.
Experiments shown that increase in temperature and volumetric rate leads more coke formation, furthermore because of covering of catalytic active sites with coke catalytic activity decreases. In order to hinder coke formation and decreasing we added 0.2% cobalt to Mo contained bentonite and coke formation dropped. As a result catalysts working time and stability increased.
Increasing volumetric rate of reactants causes conversion and benzene and naphthalene content in products decrease. Moreover C2 - C5 - olefin and alkylaromatic hydrocarbons content increases. Decrease in volumetric rate follows increase in oligo-merizarion and dehydrocyclization reaction activity of olefins and this increases the yield of aromatic
hydrocarbons. Increase in methane partial pressure also causes increase of aromatic hydrocarbons yield. From this conclusion, we may propose that aromatic hydrocarbon formation has unique mechanism and not depend on used catalyst content.
Aromatic hydrocarbon formation from methane has very complex mechanism and process takes place with several steps. Calculations shown that at the first step adsorption complex from methane and active site interaction (Fig. 5 a) and it dissociates methyl and hydrogen radical. Adsorption complex does not change methane molecule geometry but C -H bonds polarizes. 'at transition state (Fig. 5b) hydrogen atom of methane places between carbon and molybdenum linked oxygen atom. Dissociated hydrogen atom form OH gropu with oxygen atom and methyl radical interacts with molybdenum atom forming chemical bond.
a) 6) b)
Figure 5. Interaction of active site with methane molecule forming a) adsorption complex; b) transition state and; c) covalent bond
OH amd CH3 groups makes stable methane addi- num atom (Fig. 5c). There MoO3 interacts with acidic tion products and forms covalent bond with molybde- centers ofbentonite forming following complex ions:
M0O3 + O
O.
2
л/ \i
>5000C
Al
MoO2OH
Si
MoO2OH
O
2
500-7000C
Al
Si
O
Al
4 ___O-
Mo6+
O
O
O
Si Al
O
Si
H2O
Scheme 1. Formation if [Mo2O5]2+ dimers at Bronsted acidic surfaces
MoO2OH
O
УЧ
2_
500-7000C
OO
M+
Al Si Al Si
H2O
Scheme 2. Formation of [MoO2]2+ cation at Bronsted bridge area
H
+
O
O
+
H
+
+
Scheme 3. Benzene formation scheme
Scheme 4. Toluene formation scheme
Scheme 5. Xylenes formation schemes
Figure 6. Schematic representation of methane dehydroaromatization
Based experiment results and references we propose the following scheme aromatic hydrocarbon formation form methane:
Based on knowledge about reaction mechanism, reaction elementary steps, rate constants and kinetic equations containing absorption coefficients can be drawn. Application knowledge about reaction mechanism one can draw different kinetic equations for one reaction. After identification of kinetic parameters conclusion can be made about satisfactory equation.
Conclusions
1) Appropriate polyfunctional nanocatalyst (MoO3)x • (ZrO2)y • (ZnO2)z with sol-gel technology methane dehydroaromatization process.
2) With selected (MoO3)x • (ZrO2)y • (ZnO2)z/ bentonite catalyst yield of aromatic hydrocarbon at VCHi = 1000 hour1, T = 750 °C found 40.4%.
3) Different d-metals nature and their promoter characteristics for methane catalytic dehydroaromatization reaction catalyst activity.
4) Based on experimental results mechanism of aromatic hydrocarbon formation from methane proposed.
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