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UDC 544.4:661.725/741.5
KINETICS AND MECHANISM OF SELECTIVE OXIDATIVE DEHYDROGENATION OF n-AMYL ALCOHOL INTO VALERALDEHYDE ON MODIFIED ZEOLITE
CATALYST CuZnPdCaA
A.M.Aliev, F.A.Agayev, V.Sh.Agayev, A.A.Saridjanov
Institute of Catalysis and Inorganic Chemistry named after Acad.M.Nagiyev H.Javid ave., 113, Baku AZ1143, Azerbaijan Republic; e-mail: itpcht@,lan.ab.az
Catalytic activity of zeolite CaA synthesized from zeolite NaA and modified by cations of metals Cu2+, Zn2+ and Pd2+ by the method of ion exchange in the reaction of partial oxidation of n-amyl alcohol into valeraldehyd, has been studied. It revealed that metal-zeolite catalyst CaA synthesized from zeolite NaA and containing 3.0 wt.% Cu 2+, 2.0 wt.% Zn2+ and 0.1 wt.% Pd2+ displays the highest activity and selectivity in the reaction. Also, kinetic regularities of the reaction on the said catalyst have examined. On the basis of the experimental data, a probable phasic mechanism has been suggested and a theoretically substantiated kinetic model of the reaction developed. Keywords: n-amyl alcohol, valeraldehyde, zeolite CaA, mechanism, kinetic model, kinetic costants.
INTRODUCTION
Valeraldehyde is a valuable raw material for food and chemical industries. It is used for the preparation of medicaments, food supplements and essences. Besides, it is used as an emulsifier, which is a part of synthetic materials and plasticizers. Note that industrially aldehydes are obtained by catalytic oxidation of aliphatic alcohols. Metals, transition metal oxides, mixed oxides and transition metal salts are used as the catalysts. Process proceeds in a temperature range (350-4500C) with a relatively low selectivity conformably to the desired product [1-4]. In the previous paper [5-10], we found that the
zeolites modified with metal cations by ion exchange display relatively high catalytic activity and selectivity in the oxidative dehydrogenation of aliphatic alcohols at relatively low temperatures (250-3500C).
The aim of this work is the synthesis of zeolite CaA from zeolite of NaA through the use of ion exchange method, its modification
with cations of Cu2 +, Zn2 + and Pd2 + by ion
exchange, the definition of its catalytically active composition in the oxidative dehydrogenation of n-amyl alcohol conformably to valeraldehyde and the analysis of the kinetics and mechanism of the reaction.
EXPERIMENTAL RESULTS AND DISCUSSION
Catalysts were prepared on the basis of synthetic CaA and zeolite CaA synthesized from NaA by ion exchange method. Inoculation of indicated zeolite with metal cations; Cu, Zn, and Pd was established by ion exchange in aqueous solutions of CuCl2, ZnCl2 and [Pd(NH3)4]Cl2 followed by drying (80-1200S, for 5 hours) and calcining in flowing air at a temperature of 3000C with space velocity of 2400 h-1 for 30 min. The amount of metal cations introduced into the zeolite was
established by mass spectrometry analysis on ICP-MS Agilent 7700 instrument.
Experiments were carried out on a flow unit directly connected with a gas-liquid chromatograph Agilent 7820A and a column DB-624 at the rate of carrier gas (He) 1,5 ml/min.
The test results in the catalytic activity of some samples, and unmodified forms of incorporation zeolites in the reaction of
oxidative dehydrogenation of n-amyl alcohol to valeraldehyde are shown in Table 1.
From the data shown in Table 1, it follows that the unmodified forms of synthetic CaA and CaA zeolite synthesized from NaA show a lower catalytic activity in the reaction of oxidative dehydrogenation of n-amyl alcohol of the valeraldehyde. Over these zeolites the reaction of n-amyl alcohol into amylenes proceeded (exp. №1,9) with precedence dehydration reaction of n-amyl alcohol into amylenes, and the yields of amylenes over CaA zeolite synthesized from NaA were relatively higher than over synthetic CaA. For this reason, there is a relatively high catalytic activity of CaAzeolite synthesized from NaA and modified metal cations as compared to the synthetic CaA (exp. №2-8 and №10-16). Note that cation concentration varied in a wide range of values (4.1 wt.%). Table 1 shows the concentrations of those cations which are of interest from industrial practicability standpoint. It should also be
added that the yield of the products is dependent on distribution of acid sites; concentration and nature of the cation. Incorporation of copper cations in CaA (synthesized from NaA) raises the concentration of dissociatively adsorbed oxygen molecules, which leads to increased catalytic activity of the zeolite in the reaction (exp. №2, 10). As the concentration of copper cations rises from 1.0 wt.% to 3.0 wt.%, the yield of valeraldehyde increases from 53.3% to 58.9% (exp. №2, 3) for synthetic CaA, and from 56.6% to 61.1% for CaA zeolite synthesized from NaA. Subsequent rise in the concentration of copper cation (up to 4.0 wt.%) has an insignificant influence on the yield of valeraldehyde (exp. №4,12). It might be explained as being due to changes in the distribution of acid centres over the catalyst surface toward reducing the Bronsted acid centres average force at relatively high concentrations of copper cations.
№ Zeolite Composition in wt.% Conversion, X,% Yield A%
Cu2+ Pd2+ Zn2+ Valeraldehyde, A1 Amilenes, A2 CO2, A3
1 CaA (synth.) - - - 45.9 39.6 5.1 1.2
2 - 1.0 - - 57.8 53.3 2.7 1.8
3 - 3.0 - - 64.8 58.9 3.5 2.4
4 - 4.0 - - 64.2 57.1 4.2 2.9
5 - 3.0 0.1 - 80.3 70.1 5.9 4.3
6 - 3.0 0.2 - 79.4 68.4 6.2 4.8
7 - 3.0 0.1 2.0 85.7 76.8 5.3 3.6
8 - 3.0 0.1 3.0 83.5 72.3 6.9 4.3
9 CaA (synth-ed. from NaA) - - - 50.4 43.2 5.6 1.6
10 - 1.0 - - 62.1 56.6 3.4 2.1
11 - 3.0 - - 67.7 61.1 3.9 2.7
12 - 4.0 - - 68.2 60.4 4.6 3.2
Table l.Test results of the catalytic activity of the synthetic CaA and zeolite CaA are
2+ 2+
synthesized by ion exchange method from a zeolite NaA, modified metal cations Cu , Zn and Pd2+ in the oxidative dehydrogenation of n - amyl alcohol in valeraldehyde at a temperature 3200C, a space velocity V = 1600 h-1 and a molar ratio of reactants alcohol: O2: N2 = 1: 0.75:2.9
13 - 3.0 0.1 - 85.1 73.8 6.4 4.9
14 - 3.0 0.2 - 84.4 72.4 6.7 5.3
15 - 3.0 0.1 2.0 92.7 82.7 5.8 4.2
16 - 3.0 0.1 3.0 90.4 78.4 7.2 4.8
Incorporation of small amounts of palladium cations (0.1 wt.%) and zinc (1.0-2 wt.%) contributes to favorable distribution of acid centres on the surface of the catalyst in the reaction (exp. №5-8 and №13-16) and, moreover, the increased concentration of cations dissociatively adsorbed oxygen molecules.
As a result of data analysis as shown in Table 1, it may be concluded that the catalyst,
CaA, synthesized by ion exchange on the basis
2 +
of zeolite NaA and containing 3.0 wt.% Cu , 2.0 wt.% Zn2 + and 0.1 wt.% Pd2 + shows relatively high activity in the reaction of oxidative dehydrogenation of n-amyl alcohol in valeraldehyde (exp. №9).
-H2O
Kinetic regularities of the reaction were investigated over metal-zeolite catalyst CuZnPdCaA in the absence of diffusion inhibition. The kinetics of the reaction was carried out in a temperature range 280-3 400C, space velocities of 1600-2400 h-1, partial pressures of reactants P n - amyl = 0,2-0,3 atm
and P o = 0,1-0,20 atm under conditions that
allow the flow reaction in the kinetic area. The purity of the n - amyl alcohol of the brand is 99.8%.
On the basis of the experimental data the following kinetic scheme of reactions during the partial oxidation of n - amyl alcoholover catalyst CuZnPdCaA was suggested:
C5H11 OH
► C5H10
► C4H9CHO
+0.502, -H2O
%CO2+H2O
(1)
+O2
By means of the analysis of l iterary materials [9] the following stepwise mechanism of the formation of valeraldehyde (2) may be suggested. Adsorbed molecules on n-amyl alcohol were protonated with surface
k1
O2 + 2Z
ZO + C5H„OH
>2ZO k„
Brensted acid sites to eliminate water and form a surface alkoxide. Decomposition of surface alkoxide into valeraldehyde proceeds with nucleophilic oxygen. A simplified diagram of the staging mechanism is as follows: 1
>Z0(C5Hn0H)
Z0(C5H„0H) —3 > Z(C4H9CHO) + H2O
z(c4h9cho)—
^C4H9CH0 + Z
2
1
(2)
All the stages are practically irreversible. Assuming their simplicity, we find the following expressions for stage rates:
1 = k1PO2 ; r2 = k2Pn-amyl°2 ; r3 = k3°3 ; r4 = k4&4
(з)
where QX,Q2,Q3,Q4 are fractions of catalyst sites with adsorbed atom oxygen and
surface; sites with the ability to adsorb atomic molecules of n-amyl alcohol; sites with oxygen; sites with adsorbed stoms oxygen;
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adsorbed molecules of aldehyde; where k1, k2, of the respective index components; r1, r2, r3,r4 k3, k4 are rate constants of the respective stages are rates corresponding to index steps. of the index; p n-amyi , P O partial pressures
In stationary conditions it is as follows: r = r = = r3 = r4
(4)
where the above is total rate of the process. Based on these equations and total permanence of surface portions 61 + 62 +63 +64 = 1, one can find the concentration of intermediates 9i and the rate of formation valeraldehyde as shown below:
rv.ald: 61 =
k464 ,6 = k464 . 6 =
69 = -—--. 6 =
kiPo' 2 h Pn -
2rn - amyl
^464
(5)
Designating 04 = y , we get:
k4
k4
- + — +1 ^ k2 Pn - amyl k3 J
y2 +.
k4
y -1 = 0(6)
64 =
y =
k 4 + klpo2 \
f
k4
k 4 ,
- + — +1
v k2 Pn - amyl k3 j
k4
kiPo2
+
kA
k4 ,
+ — +1
^ k2 Pn - amyl k3 J
(7)
(8)
Substituting 04 in the corresponding rate equation, we get: - for the rate of formation of valeraldehyde
r4 = rv.a. = k
k4
kiPo,
+
k4
y k2 Pn - amyl
+ — +1 k3
A
J
(9)
Carbon dioxide is formed through the cording to the Langmuir-Hinshelwood me-reaction of weakly adsorbed molecules of n - chanism. Then the kinetic equation corres-amyl alcohol adsorbed oxygen molecules ac- ponding to this mechanism is as follows:
rCO2 =
k6K1K2 PO2 Pn - amyl
1 + KlPO2 + K 2 Pn - amyl f
(10)
2
2
2
2
2
where k6 - constant of forming reaction rate of carbon dioxide; K1, K2 - constants of adsorption of oxygen molecules and n - amyl alcohol at the active centers of the catalyst surface for reaction.
Note that amylenes are formed by dehydration of n - amyl alcohol, which is a reversible reaction. The observed rate of the reaction is as follows:
r = r - r = r
1 - n'
Where
r . Pc5H10 ' PH2O
1 - - = 1 -r
K P
P1 n - amyl
Y - irreversibility criterion, then:
r = rY
r -can be expressed by:
r = KP,
n - amyl
= Y
(11) (12)
(13)
(14)
Substituting (12) and (14) into (13) we have the following:
f
r = kP,
n - amyl
1 -
pc5h10 ■ ph2o ^ f
K P
^ P rn - amyl y
=k
Pn
n - amyl
K
pc5 h10 ■ pho
P
(15)
In considering the reaction rates of inhibition adsorbed molecules of alcohol and
water, the observed rate of the reaction is as follows:
p,
1
rc5 h10 = k7
n - amyl
K
pc5 h10 ■ p
P
H 2O
K 3 Pn - amyl + K 4 PH 2o
(16)
where Kp is an equilibrium constant dehydration n - amyl alcohol
where A and B are empirical constants identified from experimental data (A=2.39 and B=2010.7); 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 n - amyl alcohol, respectively; k7 - constant of dehydration reaction rate. (To calculate the constants k and K in equations (9), (10) and (15) we used a E
formula k = K0 ■ e RT , K = K0 ■ eRT ).
Note that equations (9) and (10) and (15) form a kinetic model of the n - amyl alcoholoxidation.
lg Kp =- A + B/T (17)
Statistically analyzed kinetic model of the reaction was based on kinetic data. Calculation of pre-exponential factors of reaction constants ln k° (ln K®), activation energies (E°) and heat
of adsorption (q0 ) have been carried on by
methods of "rolling admission" and with the help of the Powell software system "Search".
A kinetic model of the reaction of oxidation of n - amyl alcohol to valeraldehyde provides an appropriate description of experimental data. Calculations showed that a relative error in the experimental and calculated data does not exceed 5%.
1
REFERENCES
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MODiFiKASiYA OLUNMU§ CuZnPdCaA SEOLiTKATALiZATORU UZdRiNDd AMiL SPiRTiNiN VALERiAN ALDEHiDiNd SELEKTiV OKSiDLd^DiRiCi DEHiDROGENLd$MdSiNiNKiNETiKA Vd MEXANiZMi
A.M.dliyev, F.A.Agayev, V.§.Agayev, A.A.Saricanov
AMEA-nin akad. M.Nagiyev adina Kataliz vd Qeyri-uzvi Kimya institutu AZ1143, Baki, H.Cavidpr., 113; e-mail: [email protected]
NaA seoliti dsasinda sintez olunmu§,ionmubadild usulu ild muxtdlif metal kationlari ild (Cu, Zn vd Pd) modifikasiya edilmi§ CaA seolitinin amil spirtinin valerian aldehidind oksidld^dirici dehidrogenla^masi reaksiyasinda katalitik aktivliyi oyrdnilmi§dir. Mudyydn olunmu§dur ki, NaA seolitinddn sintez edilmi§ vd tdrkibindd 3.0% Cu2+, 2.0% Zn2+ vd 0.1% Pd2+ionlari saxlayan CaA metal-seolit katalizatoru baxilan reaksiya ugun yuksdk aktivlik vd selektivliyd malikdir. Bu
katalizator uzsrinds reaksiyanin getmssinin kinetik qanunauygunlugu oyrsnilmigdir. Tscrubi nsticslsr dsasinda reaksiyanin getmssinin mumkun msrhslsli mexanizmi verilmi§ vs prosesin nszsri ssaslandirilmi§ kinetik modeli hazirlanmi§dir.
Agar sozlw. amil spirti, valerian aldehidi, CaA seoliti, mexanizm, kinetik model, kinetik sabitlsr.
КИНЕТИКА И МЕХАНИЗМ СЕЛЕКТИВНОГО ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ н-АМИЛОВОГО СПИРТА В ВАЛЕРИАНОВЫЙ АЛЬДЕГИД НА МОДИФИЦИРОВАННОМЦЕОЛИТНОМКАТАЛИЗАТОРE CuZnPdCaA
А.М.Алиев, Ф.А.Агаев, В.Ш.Агаев, А.А.Сарыджанов
Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143 Баку, пр.Г.Джавида, 113; e-mail:[email protected]
Исследована каталитическая активность цеолита CaA, синтезированного из цеолита NaA и модифицированного катионами металлов (Cu, Zn, Pd) методом ионного обмена, в реакции парциального окисления н-амилового спирта в валериановый альдегид. Установлено, что металлцеолитный катализатор CaA,синтезированный из цеолита NaA и содержащий 3.0 мас. % Си2+, 2.0 мас.% Zn и 0.1 мас. проявляет наибо-
лее высокую активность и селективность в рассматриваемой реакции. Изучены кинетические закономерности протекания реакции на указанном катализаторе. На основе экспериментальных данных предложен вероятный стадийный механизм протекания реакции и разработана теоретически обоснованная кинетическая модель процесса. Ключевые слова: н-амиловый спирт, валериановый альдегид, цеолит CaA, механизм, кинетическая модель, кинетические константы.
Поступила в редакцию 21.10.2016.
