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CHEMICAL PROBLEMS 2020 no. 4 (18) ISSN 2221-8688
477
UDC 541.128.547.264
SYNTHESIS OF TARGETED CATALYST FOR THE OXIDATIVE DEHYDRATION OF SEC-BUTANOL AND STUDY OF THE KINETICS AND MECHANISM OF THE PROCESS
F.A. Agayev
Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry, National Academy of Sciences of Azerbaijan e-mail: [email protected]
Received 07.11.2020 Accepted 25.12.2020
Abstract. The catalytic activity of CaA zeolite modified with metal cations (Cu, Zn, Pd) by means of ion exchange in the reaction of partial oxidation of sec-butanol to methyl ethyl ketone was studied at atmospheric pressure in the temperature range of 240-350° C, space velocities 1500-4000 h-1 and partial pressures of butanol-2 p 2 = 0.12-0.36 atm, oxygen p = 0.12-0.24 atm. It found that the metal zeolite catalyst CuZnPdCaA
containing 3.0 wt% Cu2+, 2.0 wt% Zn2+ and 0.1 wt% Pd2+ exhibited the highest activity and selectivity in the reaction under consideration. Also, kinetic regularities of the reaction on the indicated catalyst were studied. On the basis of experimental data, a probable stepwise mechanism of the reaction was proposed and a theoretically substantiated kinetic model of the process developed.
Keywords: butanol-2, zeolite catalyst, oxidative dehydrogenation, kinetics, methyl ethyl ketone, ion exchange DOI: 10.32737/2221-8688-2020-4-477-484
Introduction
Methyl ethyl ketone (MEK) is used as solvent and extractant, in some cases superior to acetone, it proved not so volatile and safer in terms of fire. On industrial scale, methyl ethyl ketone is produced by oxidation of butylenes on PdCl2 a two-stage method which consists of hydration of n-butylene into butanol-2 with the participation of sulfuric acid as a catalyst and oxidative dehydrogenation of sec-butanol in MEK on mixed oxide catalysts [1,2].
The main disadvantages of the first method are irretrievable consumption of a certain part of expensive catalyst PdCl2 and the formation of byproducts - chloroketone and crotonaldehyde. Disadvantages of the process of oxidative dehydrogenation of sec-butanol in MEK are a relatively high temperature (400-500°C) and a low selectivity for methyl ethyl ketone [3-5].
For the production of methyl ethyl ketone, the method of oxidative dehydrogenation of sec-butyl alcohol is used more [1]. However, due to
these disadvantages, this method is characterized by low economic efficiency.
In works [6-12], we found that zeolites modified with metal cations by ion exchange exhibit high catalytic activity and selectivity in the reactions of oxidative dehydrogenation of aliphatic alcohols at relatively low temperatures (250-3500C).
Based on the analysis of the literature and the results obtained using physicochemical analysis methods, the following mechanism of methylethylketone formation can be proposed: During the absorption of sec-butyl alcohol, the catalyst is protonated in the presence of Bronsted acid sites, which subsequently separates water and leads to the formation of surface alcohol. The transformation of surface alcohol into methyl ethyl ketone is possible by obtaining a surface ketone-like compound formed by combining it with surface nucleophilic oxygen. The ketone-like surface
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CHEMICAL PROBLEMS 2020 no. 4 (18)
compound is then decomposed into methyl ethyl ketone and the catalyst is restored to its initial state.
The reaction of heterogeneous partial oxidation of aliphatic alcohols on the surface of metal zeolite catalysts occurs as a result of the interaction of these alkoxide formations with surface nucleophilic oxygen [13-15].
The aim of this work is to select an active and selective catalyst for the reaction of partial oxidation of butanol-2 into one of the most important products of the chemical industry, methyl ethyl ketone, as well as to study the kinetics and mechanism of the reaction with the participation of active metal zeolite catalyst.
Experimental part
The catalysts were prepared on the basis of CaA zeolite synthesized by ion exchange from NaA. Modification of the specified zeolite with metal cations; Cu, Zn, and Pd were carried out by ion exchange in aqueous solutions of CuCl2, ZnCl2, and [Pd (NH3M Ch, followed by drying (80-1200C, 5 h) and calcining it in an air stream at a temperature of 3000C, a space velocity of 2400 h-1 within 30 minutes. The amount of metal cations introduced into the zeolite composition was determined by means of mass spectrometric analysis on an ICP-MS Agilent 7700 instrument.
The experiments were carried out on a flow-through installation directly connected to an Agilent 7820A gas-liquid chromatograph with a DB-624 column, a gas carrier (He) flow rate of 1.5 ml / min and a pressure of 7.5 psig. A Pyrex glass-based reactor was loaded with 2 cm3 of a catalyst with a particle size of 0.23-0.63 mm, activated in a nitrogen flow at a temperature of 4000C (3h), then the temperature was lowered to
the reaction temperature, and the reaction was fed at a certain volumetric rate mixture. The feedstock was fed using an NE-1600 micro-dosing device into a mixer located in a thermo-stated cabinet equipped with an electric heater and a fan; a stable temperature in the thermostat was maintained using a "Micromax" micro-electric thermostat. The mixer also receives oxygen and a diluent gas, nitrogen. The reactor is placed in a thermostated cabinet. The reaction products at the reactor outlet were taken for analysis from a sampling loop connected to a six-way valve.
The kinetics of the reaction was studied in the temperature range of 280-340° C, space velocities of 2000-4000 h-1, partial pressures of the reagents = 0.12-0.36 atm. and ^ = 0.120.24 atm. in terms to ensure the flow reactions in the kinetic region. The purity degree of butanol-2 (B-2) grade "analytical grade" was 99.5%.
Results and discussion
On the basis of CaA zeolite and metal cations Cu2+, Zn2+, and Pd2+, catalyst samples containing different amounts of these cations were synthesized by ion exchange. The results of testing of catalytic activity of some of these samples, as well as unmodified zeolite, CaA in the oxidative dehydrogenation of butanol-2 (B-2) into methyl ethyl ketone (MEK) are presented in Table 1. From the data given in Table 1 it follows that the reaction of oxidative dehydrogenation of 2-butanol to methyl ethyl ketone on these catalysts is accompanied by deep oxidation and
dehydration of 2-butanol with the formation of carbon dioxide and butylenes. The yield of individual products is significantly influenced by the distribution of acid sites (when metal cations are introduced into the zeolite, the distribution of acid sites on the surface changes), as well as by the concentration and nature of the cation. From Table 1 it follows that the reaction of oxidative dehydrogenation of butanol-2 to methyl ethyl ketone on zeolite, CaA proceeds with a relatively low conversion (experiment No.1) which can be explained by insufficient concentration of
dissociatively adsorbed oxygen molecules. The introduction of copper cations into the zeolite raises the yield of methyl ethyl ketone (experiments No.2-4). As the concentration of copper cations increases from 0.5 wt% to 3.0 wt%, the yield of methyl ethyl ketone significantly increases from 45.0% to 58.0%, a
further rise in the concentration of copper cations (up to 4.0 wt%) (experiment No.5) slightly affects the IEC output. This can be explained by changes in the distribution of acid sites on the catalyst surface towards a decrease in the number of Bronsted acid sites of medium strength at relatively high concentrations of copper cations.
Table 1. Results of testing of catalytic activity of CaA zeolite (synthesized on the basis of NaA zeolite) modified with metal cations Cu2+, Zn2+, and Pd2+ in the oxidation of butanol-2 to methyl ethyl ketone at a temperature of 3000C, a space velocity V = 2500 h-1 and a molar ratio of reagents alcohol: O2: N2 = 1:
0.69: 2.5
№ Zeolite Composition in wt.% Conversion, X, % Yield, A %
Cu2+ Pd2+ Zn+2 MEK, A1 Butylenes, A2 CO2, A3
1. CaA (synthesized on the basis of NaA zeolite) - - - 18.4 12.1 6.0 0.3
2. 0.5 - - 48.1 45.0 2.5 0.6
3. 1.0 - - 58.5 54.6 3.0 0.9
4. 3.0 - - 63.8 58.0 4.3 1.5
5. 4.0 - - 64.1 57.6 4.6 1.9
6. 0.5 0.1 - 80.4 74.7 3.7 2.0
7. 3.0 0.1 - 84.9 78.8 4.0 2.1
8. 3.0 0.1 1.0 88.0 82.4 3.4 2.2
9. 3.0 0.1 2.0 91.1 86.9 2.0 2.2
10. 3.0 0.1 4.0 87.2 80.0 4.4 2.8
11. 3.0 1.0 4.0 95.3 81.8 9.1 4.4
The introduction of small amounts of palladium cations (0.1 wt.%) and zinc (1.0- 2.0 wt.%) leads to favorable distribution of acid sites on the catalyst surface in the reviewed reaction (experiments No. 6-9). In addition, these cations raise the concentration of dissociatively adsorbed oxygen molecules.
Analysis of the Table 1 data concludes that the catalyst synthesized on the basis of CaA zeolite by the ion exchange method, containing
3.0 wt% Cu2+, 2.0 wt% Zn2+ and 0.1 wt% Pd
2+
2+
CH3—CH(OH)—CH2-CH3;
k4 -+o2 ki
exhibits the highest activity in the reaction of oxidative dehydrogenation of butanol-2 to methyl ethyl ketone.
The kinetic regularities of the reviewed reaction were studied on a CuZnPdCaA metal zeolite catalyst of the indicated composition in the absence of diffusion inhibition.
The experiments make it possible to offer the following kinetic scheme of reactions in the process of partial oxidation of 2-butanol on the catalyst CuZnPdCaA: CO2
+ ^202 k
■ CH3-CO-CH2—CH3 + H20
(1)
X4H8
Based on the analysis of literary materials [4-6] one can suggest the following mechanism of the MEK. In the adsorption of 2-butanol is its protonation with Bransted acid sites of the catalyst to the further elimination of water and formation of a surface alkoxide. Conversion of surface alkoxide MEK occurs when it interacts
with the surface through the formation of a nucleophilic oxygen ketonopodobnogo surface compound. Then there is the collapse ketonopodobnogo surface connection to the MEK and restores the initial state of the catalyst. A simplified diagram of the staging mechanism is as follows:
O2 + 2Z-^2ZO
ZO + B-2
k-
■ZOB-2
ZOB-2 -^Z + MEK + H2O
1 O2 + B-2 = MEK + H2O
All these stages are practically irreversible. expressions for stage rates: Assuming their simplicity, we find the following
ri = k1PO, 01 ; r2 = k2PB-202 ; r3 = k303
(2)
where 01, 02, 03 - is the proportion of vacant respective stages of the index; PG , p_2 partial
lots modified zeolite-coated atomic oxygen pressures of the respective index compon ents; r2,
molecules of 2-butanol and ketones like surface n, r3 - speed corresponding to index steps. intermediates; k1, k2, k3 are rate constants of
r=ri=r2=r3
where, r-total rate of formation of MEK. Considering these equations and the constant qi of the total formation of surface areas and the
In stationary conditions:
(3)
total reaction rate as a function of the concentration of reagents:
01+02+03=1
(4)
Q _ k1PO2
V P
k2P B-2
93 = ^ 9i2
k1Po2012=k2PB-202; k1Po2012=k303;
Substituting expression 02, 03, in the equation (4) we obtain:
f
VP VP
k1PO2 k1PO2
A
V P
V k2P B-2
k
92 + 9 -1 = 0
3 y
Solving the equation (5), we obtain an expression for 01.
(5)
Substituting 01 in equation (2) we obtain
r = if p <
*MEK 1 O2
1 + 4
K P K P
'M^ O2 02
V 2k2PB-2
2k
-1
3 y
K Po
V 2k2 P B-2
+
K1 Pç 2k
3 y
(6)
1
2
2
Carbon dioxide is formed by reacting weakly adsorbed molecules of 2-butanol adsorbed oxygen molecules, according to the Langmuir-
r =
CO2
v KKP P
v4K1K2P 02P B-2 (1 + K1P02 + K2 Pb-2 )
Hinshelwood mechanism. Kinetic equation corresponding to this mechanism is as follows:
(7)
where k4 - forming reaction rate constant of Butylenes are formed by dehydration of 2-
carbon dioxide, K1, K2 - constant of adsorption of butanol which is a reversib le reaction. The
oxygen molecules and 2-butanol at the active observed rate of the reaction is as follows: centers of the catalyst surface for reaction.
r=r — r = r
"1-
V
r J
Where
(8)
P • P
! — I = ! — C4H8 H2O = y F KPPB-2
(9)
Y - irreversibility criterion, then: r -can be expressed:
r = ry
(10)
r = KP
B-2
(11)
Substituting (9) and (11) into (10) we have:
r
r = kp
B-2
A
P • P
I C4H H2O
V — KpPB-2 J
f
1
A
P--P • P
B-2 TT" C4H H2O V KP J
(12)
In view of the reaction rate of inhibition observed rate of the reaction is as follows: adsorbed molecules of alcohol and water, the
P--— P • P
1 D O J-r'XJ 1 T
K 5
C4H8
C4H0 HO
K P + K P
K3P B-2 ^ K4P H2O
(13)
where Kp - equilibrium constant dehydration sec-butanol
lgKp=-A+B/T
(14)
k
where A and B are empirical constants determined from experimental data; K3 and K4 are constants of equilibrium adsorption of molecules of water and alcohol in the active centers of the catalyst for dehydration reaction of 2-butanol, respectively; k5 - dehydration reaction rate constant. (To calculate the constants a and K in equations (6), (7) and (13) used the formula
k = k 0 • e
RT
K = Kn • e
RT
).
Equations (6) and (7) and (13) form a
kinetic model of the sec-butanol oxidation.
A kinetic model of the reaction is subject to
statistical analysis based on kinetic data.
Calculation of pre-exponential factors of reaction
. . ln k0 (ln K0) .. .. constants ^ ' ', activation energies
(e 0) (o0)
^ 1 ''and heat of adsorptiono ' by means of "rolling admission" and Powell using software system "Search".
E
Q
Table 3. Numerical values of kinetic model constants for oxidative conversion of sec-butanol alcohol
to MEK.
ln k 0,ln K0 e, q, kkal/mol
ln k 0 7.19 E1 5.15
ln k 2 4.03 E2 6.47
ln k 0 7.67 E3 5.28
ln k 4 20.91 E4 21.27
ln k0 9.98 E5 16.40
ln K0 -3.21 Q1 1.98
ln K20 4.08 Q2 4.00
ln K0 -4.67 Q3 1.50
ln K40 4.54 Q4 4.19
A kinetic model of the reaction of oxidation of secondary butyl alcohol MEK adequately describes the experimental data above.
Calculations have shown that a relative error of the experimental and calculated data does not exceed 7%.
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BUTANOL-2 SPlRTlNlN OKSlDLe^DlRlCl DEHlDROGENL3§M3 PROSESl UCUN M3QS3DYONLU KATALlZATORUN SlNTEZl V3 PROSESlN KlNETlKA V3 MEXANIZMInIN
T3DQ!Q1
F.A. Agayev
AMEA-nin akad. M.Nagiyev adina Kataliz vd Qeyri-uzvi Kimya institutu ^Z 1143, Baki, H. Cavidpr., 113; e-mail: [email protected]
ion-mubadila usulu ila muxtalif metal kationlan (Cu, Zn va Pd) ila modifikasiya olunmu§ CaA seolitinin katalitik aktivliyi 240-350°C temperatur intervalinda, 1500-4000 s-1 hacmi suratda va butanol-2=0.12-0.36 atm., oksigen=0.12-0.24 atm parsial tazyiqi §araitinda butanol-2-nin metiletilketona qadar qisman oksidla§masi prosesinda tadqiq olunmu§dur. Muayyan olunmu§dur ki, tarkibinda 3.0% Cu 2+, 2.0% Zn2+ va 0.1% Pd2+ saxlayan CuZnPdCaA metal-seolit katalizatoru sozugedan reaksiya u^un yuksak aktivlik va selektivlik gostarir. Bu katalizator uzarinda reaksiyanin getmasinin kinetik qanunauygunluqlari oyranilmi§dir. Tacrubi naticalar asasinda reaksiyanin getmasinin mumkun marhalali mexanizmi verilmi§ va prosesin nazari asaslandirilmi§ kinetik modeli hazirlanmi§dir.
A^ar soztar: butanol-2, seolit katalizatoru, oksidla§dirici dehidrogenla§ma, kinetika, metiletilketon, ion mubadila.
ЦЕЛЕНАПРАВЛЕННЫЙ СИНТЕЗ КАТАЛИЗАТОРА ДЛЯ ПРОЦЕССА ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ ВТОР-БУТАНОЛА И ИССЛЕДОВАНИЕ КИНЕТИКИ И МЕХАНИЗМА
ПРОЦЕССА
ФА. Агаев
Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143 Баку, пр. Г. Джавида, 113; e-mail: agayev.fuad@asoiu. edu. az
Изучена каталитическая активность цеолита CaA, модифицированного катионами металлов (Cu, Zn, Pd) методом ионного обмена, в реакции парциального окисления втор -бутанола до метилэтилкетона при атмосферном давлении в интервале температур 240-350°С, объемных скоростях 1500-4000 ч-1 и парциальных давлениях бутанола-2=0.12-0.36 и кислорода=0.12-0.24 атм. Установлено, что металл-цеолитный катализатор CuZnPdCaA, содержащий 3.0 мас.% Cu2+, 2.0 мас.% Zn2+ и 0.1 мас.% Pd2+, проявляет наибольшую активность и селективность в рассматриваемой реакции. Изучены кинетические закономерности реакции на указанном катализаторе. На основе экспериментальных данных предложен вероятный ступенчатый механизм реакции и разработана теоретически обоснованная кинетическая модель процесса.
Ключевые слова: бутанол-2, цеолитный катализатор, окислительное дегидрирование, кинетика, метилэтилкетон, ионный обмен.
