CC BY
20
UDC 541.128.13: 547.291
OXIDATIVE CONVERSION OF METHANOL OVER THE MODIFIED ZEOLITES
A.M .Aliyev, S.M .Mejidova, G.A.Ali-zade, R.Yu.Agayeva
Institute of Chemical Problems of the National Academy of Sciences of Azerbaijan
itpcht@jtpcht. ab. az
Received 25.12.2012
A number of zeolite catalysts synthesized on the basis of synthetic (NaA, NaX, NaY) and natural (mordenite, klinoptilolite) zeolites, modified by Pd and Cu ions have been tested in the combination reactions of oxidation of methanol into formaldehyde, formic acid and methylformiate. The highly-efficient catalysts have been chosen for oxidative conversion of methanol into formaldehyde (CuPdNaY) and direct oxidation of methanol into formic acid (Pd-mordenite) has been chosen. On the basis of the results of experimental investigation of kinetic regularities of proceeding the process of direct oxidation of methanol to formic acid over the catalyst Pd-mordenite (0.1% Pd), a probable stage mechanism of the process is presented and its probable kinetic model has been elaborated.
Keywords: oxidation, methanol, zeolite.
Oxidative dehydrogenation of aliphatic alcohols is a known reaction catalyzed by metals catalysts, metal oxides and number of complex oxide systems. Several materials such as Cu, Ag, Mo, V, Cd, Fe, Cr, Co, Sn, Ti, W [1-8], etc., have been proven to be catalysts in reaction of oxidation of methanol. The conversion of this alcohol was also studied on the modified zeolite catalysts [9-12]. These studies were focused mainly on the partial oxidation of the alcohol into formaldehyde.
One of major merits of polyfunctional catalytic systems is the possibility of realization on their surface of multistage reactions for which the conversion of reagents exceeds the product of conversions achieved in the separate stages. In our previous paper, a cite as an example ethyl alcohol we showed the development of high efficient polyfunctional metalzeolite catalyst for the multistage reactions of oxidative conversion of ethyl alcohol into ethylacetate [13]. It was established that the total scheme of vapor-phase oxidation reactions of lower aliphatic alcohols on the modified zeolite catalysts can be summarized as follows:
Cu , Pd / zeolite Cu2 , Pd2 / zeolite zeolite RCH2OH -'-► RCHO --7777;-► RCOOH-►RCOOC^R.
2 +1/2 02; -H20 +1/2 O2 +RCH2OH, -H2O
For all three stages of this mechanism one of the active adsorption sites are Bronsted acid sites.
It is known that the vapor-phase esterification of acetic acid, ethyl alcohol takes place on the Bronsted acidic zeolite catalysts secondary centers of power [14].
The role of surface acid sites in the reaction of partial oxidation of lower aliphatic alcohols on the modified zeolite catalysts we have studied on the example of the partial oxidation of isoamyl alcohol to isovaleric aldehyde by treating the catalyst CuPdCaA different amounts of pyridine, which blocks the acid sites [15]. It was established that treatment of the catalyst of pyridine reduces the yield of isovaleric aldehyde.
The role of Bronsted acidic sites in the mechanism of heterogeneous partial oxidation of aliphatic alcohols over the metal oxide catalysts (by Cr-Mo-O3, MoO3) have been investigated by IR spectroscopy [16]. It was found that the similarly liquid-phase version of the process that takes place in the acid environment (chromic acid) with participation of protons through formation of intermediate compound chromate-ether, the heterogeneous partial oxidation of aliphatic alcohols proceeds via the surface intermediate compounds of the alcoxide type, which are formed with adsorbed molecules of alcohols on the Bronsted acidic sites of medium strength. Reaction of heterogeneous partial oxidation of aliphatic alcohols proceeds as a result of interaction of these alcoxide compounds with surface nucleophilic oxygen. Ions of transitions metals participate in formation of surface nucleophilic oxygen [17].
The study of the reaction oxidation of n-propyle alcohol over modified zeolites by IR spectroscopy showed that on these catalysts also are observed surface intermediate compounds of the alcoxide type,
which are formed with adsorbed molecules of alcohols on the Bronsted acidic sites of a surface of the modified zeolite [18].
The role of ions of copper in reactions of partial oxidation of aliphatic alcohols over modified zeolites on an example of partial oxidation of «-propyle alcohol on the CuCaA catalyst have been investigated by EPR [18]. It is established that copper participates in reaction of oxidation of aliphatic alcohols and introduction of cations of copper in zeolite leads to formation the additional the Bronsted acidic sites.
It was found that the zeolites, exhibiting a relatively high catalytic activity in the esterification reaction in the modification of Cu2+ and Pd2+ ions by ion exchange are active and selective catalysts for the reactions of partial oxidation of lower alcohols to the corresponding aldehydes and ketones [19]. Zeolites, which have relatively lower catalytic activity in the esterification reaction, at the modifying by Cu2+ and Pd2+ exhibit high activity and selectivity in the reactions of partial oxidation of lower aliphatic alcohols to the corresponding carboxylic acids [20]. Zeolites that exhibit relatively weak catalytic activity in the vapor-phase esterification reaction after modification Cu+2- and Pd2+-forms are active and selective catalysts for the oxidative conversion of lower aliphatic alcohols to the corresponding esters [21, 22].
In the present paper it has been given the results of the investigation on selection of the high efficiency catalysts for the vapor-phase oxidation of methanol over modified by Cu2+ and Pd2+ ions zeolite catalysts into formaldehyde and formic acid and also the results of a study of kinetics and mechanism of oxidation reaction of this alcohol into formic acidic on selected catalyst.
EXPERIMENTAL PART
The synthetic zeolite NaA, CaA, NaX, NaY (with ratio of silica and aluminium oxides is equal to 1.9, 2.0, 2.9, 4.2 respectively) and natural zeolites - clinoptilolite (ratio of silicon and aluminium oxide is equal to 9.6) of Aidag field and mordenite (ratio of silicon and aluminium oxide is equal to 8.68) of Chananab field (Azerbaijan) have been used in the present investigation. These zeolites, modified by cations of Cu+2 and Pd2+, were synthesized by ion-exchange method. For preparing of catalyst on the basis of natural zeolites at first the dealumination of ones is carrying out by means of acid treatment. The quantity of incorporating metal was 0.05-6.0 wt. %. After incorporating of ions, all of the specimens of modified catalysts were activated by air at temperature 3500C and space velocity 2400 h-1, during 30 min.
The test of the activity of the prepared catalysts was carried out in a flow apparatuses with reactor, connected directly to the gas chromatograph. The reactor was placed inside a thermostated chamber. The temperature of heating is measured by thermocouple and regulated with KVP-503. The feeding of reagents was realized by saturator. A fraction of granulated modified zeolites of about 0.25-0.63 mm of equivalent diameter was used as catalysts. The analyses of the products of the reaction were performed by gas chromatography, using a column filled with polysorb-1 (length 3 m), helium as the carrier gas, hot wire detector and program control of the temperature. Runs performed at several feed rates and using granules of the catalyst of different sizes showed that external and internal mass transfer effects were negligible under the studied experimental conditions.
The activities of the specimens of the catalysts were tested in range of temperatures of 95-1800C, mole ratios of (C^OH^:^) = 1:(0.33-3):(1-3), space velocity of the reaction mixture 900-1200 h-1 at atmospheric pressure.
Reaction kinetics was studied at the temperature range 85-1250C, the space velocity of the reaction mixture 900-3200 h-1, the partial pressures of reactants Po2 = 0.15-0.6 am, Pch3oh = 0.09-0.43 am. The experiments used methyl alcohol with a degree of purity of 99.0%. Volume amounted to 2.0 ml catalyst, particle size of catalyst 0.40-0.63 mm.
RESULTS AND DISCUSSION
Table 1 shows the experimental data on the oxidation of methanol in the presence of oxygen on synthesized polyfunctional metalzeolites catalysts at a space velocity 1200 h-1, molar ratios of alcohol:oxygen:nitrogen, equal 1.0:(0.33-3):(1-3), and optimum temperatures for each catalyst. The products of oxidation of methanol on the investigated catalysts are formaldehyde, formic acid, methylfor-miate and carbon dioxide. At the same time on some catalysts observed the formation of the methyl ether.
Table 1. The results of the test of the activity of the modified zeolite catalysts in the reaction of oxidation of methanol
No exp. Zeolite Contain, wt.% С О 0х Selectivity, % Yield, %
Cu Pd Conversion of alcohol, mol formaldehyde formic acid methylfofmiate carbon dioxide methyl ether formaldehyde formic acid methylfofmiate carbon dioxide methyl ether
v=1200 h-1, CH3OH:O2:N2=1:1:2
1 СаА 3.0 - 160 20.5 - 32.7 12.7 54.6 - - 6.7 2.6 11.2 -
2 СаА 3.0 1.0 160 22.6 - 33.2 20.3 46.5 - - 7.5 4.6 10.5 -
3 СаА 3.0 1.0 180 54.7 - 6.8 17.0 76.2 - - 3.7 9.3 41.7 -
4 NaA 3.0 0.4 140 53.7 - 14.0 - 86.0 - - 7.5 - 46.2 -
5 NaA - 0.4 150 86.8 - trac. - 99.0 - - trac. - 86.8 -
6 NaX 0.5 0.1 100 26.5 67.2 16.2 - 16.6 - 17.8 4.3 - 4.4 -
7 NaX 3.0 0.1 100 45.2 58.4 7.1 - 34.5 - 26.4 3.2 - 15.6 -
8 NaX 6.0 1.5 100 62.7 47.4 3.8 - 48.8 - 29.7 2.4 - 30.6 -
9 NaY 0.5 0.1 135 64.7 40.2 - - 59.8 - 26.0 - - 38.7 -
10 NaY 3.0 0.1 130 32.6 37.1 - - 31.3 31.6 12.1 - - 10.2 10.3
11 NaY 2.0 0.5 150 98.5 63.5 - - 36.5 - 62.5 - - 36.0 -
12 mordenite - 0.025 170 38.6 27.2 17.4 7.0 9.1 39.3 10.5 6.7 2.7 3.5 15.2
13 а и - 0.05 95 50.5 36.5 40.4 - 23.1 - 18.4 20.4 - 11.7 -
14 и и - 0.1 95 53.7 - 74.0 - 26.0 - - 39.7 - 14.0 -
15 и и - 0.2 95 50.6 - 62.9 - 37.1 - - 31.8 - 18.8 -
16 и и 0.5 - 160 30.5 33.2 3.6 - 22.3 40.9 10.1 1.1 - 6.8 12.5
17 и и 0.5 0.1 140 25.2 19.8 69.5 - 10.7 - 5.0 17.5 - 2.7 -
18 clinopti-lolite 0.5 0.1 160 35.7 - 59.7 - 40.3 - - 21.3 - 14.4 -
v = 1200 h-1, CH3OH:O2:N2= 1:0,33:1
19 NaY 2.0 0.5 150 92.5 85.5 - - 14.5 - 79.1 - - 13.4 -
v=1200h4, CH3OH:O2:N2=1: 3:3
20 clinopti-lolite 0.5 0.1 160 55.5 trac. 9.0 5.6 42.4 43.0 trac. 5.0 3.1 23.5 23.9
It can be seen from the date in Table 1 that the catalysts synthesized on the basis of zeolites X and Y, having a more open structure and containing a large effective size of zeolites windows 8-llA [23], possess a high catalytic activity in comparison with other catalysts in the oxidation of methanol in formaldehyde (Table 1, exp. 6-11, 19).
In the modified forms of zeolite NaX the formation of small amounts of formic acid was observed (Table 1, exp. 6-8).
The concentration of entered into zeolite of copper and palladium cations influences on the yields of formaldehyde and formic acid. Increasing the concentration of copper cations in the zeolite NaX from 0.5 to 6.0% and palladium cations from 0.1 to 1.5% the yield of formaldehyde increases from 17.8 to 29.7% and decreases in the yield of formic acid from 4.3 to 2.4%.
In the modified forms of zeolite NaY is observed the formation of formaldehyde and carbon dioxide (Table 1, exp. 9-11, 19) and methyl ether (Table 1, exp. 10), and these catalysts compared with catalysts CuPdNaX more effective in the oxidation of methanol to formaldehyde.
With introduction into the zeolite the cations of transition metal the concentration and strength of acid sites of the catalyst is changing [23, 24]. Increase or decrease in the concentration of copper and palladium cations in zeolites X and Y leads to a change in the distribution of acid sites. It changes the activity and selectivity of the investigated catalysts in the oxidation of methanol.
It is known that the addition of acidic compounds doesn't always lead to an increase in the acidity of the catalyst and the acidity of the catalytic systems containing two cations of transition elements, generally isn't a simple sum of these acid components [25].
NaY zeolite, containing optimum amount the cations of copper and palladium the equal 2.0 wt.% Cu and 0.5 wt.% Pd (Table 1, exp.19) is the efficiency catalyst in the formation of formaldehyde, where the highest yield of aldehyde is 79.1%.
The catalysts, synthesized on the basis of synthetic zeolites NaA, CaA and natural clinoptilolite, are characterized by intensive formation of carbon dioxide and low selectivity of catalytic action in formation of formic acid and metilformiate (Table 1, exp. 1-5, 18 and 20). The quantity of cations of transition metals, introduced in these catalysts weakly influences on the yield of methylformiate and formic acid.
Selectivity of the conversion of methyl alcohol is influenced also by a geometrical constitution of the porous structure of the zeolites. In the case of narrow-porous zeolites NaA, CaA and clinoptilolite, of which the critical diameter of the entrance window of channels are respectively 4.2, 5.0, and 4.4-4.9 A, the product of partial oxidation of methanol - methylformiate have no time to be desorbed into the gas phase and the reaction is accompanied by decomposition of methylformiate on methyl alcohol and carbon oxide on reaction
HOCOCH3 ^ CH3OH + CO
and the subsequent oxidation of oxide carbon to dioxide carbon.
The modified zeolite NaA, which has in comparison with other investigated catalysts the smallest size of the input windows is not active in the oxidation of methanol to methylformiate, because the effective diameter of his molecules are large compared with the other products of oxidation.
Catalysts, prepared on the basis natural mordenite, modified by cations of palladium, are most active in the reaction of partial oxidation of methanol to formic acid (Table 1, exp. 12-15).
On the mordenite, modified with 0.5 wt.% cations of copper (strong oxidant with respect to palladium cations), the reaction of oxidation go with the formation of formaldehyde, small quantities formic acid (1.1%), carbon dioxide and methyl ether (Table 1, exp. 16).This catalyst shows the highest selectivity in the formation of formaldehyde and methyl ether (Table 1, exp. 16). Introduction to the catalyst 0.1% Pd leads to a change in the selectivity of the process and increases in the reaction products of formic acid (17.5%) (Table 1, exp. 17).
Figure 1 shows the dependences of conversion of alcohol and of yields formic acid and carbon dioxide from the concentration of a palladium in the Pd-mordenit catalyst at the temperature 950C, space velocity 900 h-1, molar ratio of alcohol:oxygen:nitrogen, equal 1.0:1.0:2.0.
100
60 -
Figure 1. Dependences of conversion of alcohol (x, %) and yields (A, %) of formic acid and carbon dioxide from the concentration of a palladium in the Pd-mordenit catalyst: 1 - conversion of alcohol; 2 - yield of formic acid, 3 - yield of carbon dioxide; T= 950 C, v=900 h-1, CH3OH:O2:N2=1.0:L0:2.0.
0.10 0.20 0.330 0.40 0.50 Concentration wt.% of Pd2+
As can be seen from the figure, increasing the concentration of palladium cations from 0.025 to 0.2% leads to higher yields of formic acid, carbon dioxide and the conversion of alcohol. The conversion of alcohol reaches a maximum at a concentration of palladium cations in the zeolite 0.1-0.2% and equal to 49. 0-56.5%. Yields of formic acid thus equal 33.0-37.5 %. A further increase in the concentration of palladium in the catalyst leads to decreasing of alcohol conversion, and yields of products of oxidation -formic acid and carbon dioxide. Falling of yield formic acid at concentrations of palladium cations higher than 0 . 1-0.2 % connected with decreasing of the concentration of active Bronsted acid sites on the catalyst surface. Palladium cations on a surface of this catalyst in the presence of the adsorbed oxygen serve as the centers of formation of the surface nucleophilic oxygen and increasing of their concentrations ranging from 0.025 to 0.2% leads to an increase in catalytic activity. Further increasing of the palladium concentration
1
from 0.2 to 0.5% leads to inaccessibility of centers on the catalyst surface, on which the alcohol adsorption goes and, consequently, the catalyst activity and conversion of the alcohol are decreasing.
Thus, the study of the catalytic activity of synthetic zeolite A, X, Y and natural zeolites - clinop-tilolite and mordenite, modified by cations of copper and palladium in the oxidation of methanol showed that in the presence of oxygen on these contacts takes place: 1) the reaction of oxidative dehydrogenation of methanol into formaldehyde, 2) the oxidation of formaldehyde into formic acid, 3) the reaction of esterification of formic acid into methylformiate.
The results of the test of catalytic activity of these specimens showed their selectivity in the oxidation of methyl alcohol with a predominant formation of one of these products is depends from the concentration of the introduced into these zeolites the cations of metals and the nature of the zeolite. Catalysts, prepared on the basis of zeolites with a relatively strong acidic properties (such as Y), shows relatively high activity in the oxidative conversion of methanol into formaldehyde, the catalysts with medium acidic properties (mordenite) - in the oxidation of alcohol into formic acid. On the catalysts prepared on the basis of zeolites with weak acidic properties (type A) - formation small amounts of methylformate and formic acid. This experimental fact can be explained with the numerical values of reaction rates of formation: formaldehyde, formic acid and methylformiate.
It has been studied kinetics of the reaction of partial oxidation of methanol to formic acid over the natural mordenite containing 0.1 wt.% of palladium ions.
Dependences of influence of the partial pressure of methanol on conversion of alcohol and yields of the products of the reaction can be seen from Fig.2.
75
Figure 2. The influence of the partial pressure of methanol on conversion of alcohol (x, %) and yields of the reaction products (A, %): 1 - conversion of methanol; yields: 2 - formaldehyde, 3 - formic acid, 4 - car-
bon dioxide; ^=1500 h-1, 7M150C.
Ро2=0.27 атм,
0.2 0.3 0.4 0.5
/choh , atm
The figure shows that at space velocity 1500 h-1, temperature 1150C and partial pressure of oxygen 0.27 atm with increasing the partial pressure of methanol from 0.09 to 0.43 atm, the conversion of methanol and yield of carbon dioxide is continuously decreasing. The curve of dependence of the yield of formic acid on the partial pressure of methanol under these conditions has an extreme nature. At partial pressure of methanol 0.21 atm the yield of formic acid reached 40.8% and then decreased to 35.2% (partial pressure of methanol 0.43 atm). Reduced yields of formic acid and carbon dioxide is accompanied by an increase in yield of formaldehyde from 0.9% (at -PCh30h = 0.09 atm) to 7.6 (at -PCh30h = 0.43 atm). This indicates to that there is implemented the consecutive scheme of formation of formic acid from formaldehyde. Decrease the yield of products of oxidation - formic acid and carbon dioxide as well as the conversion of alcohol due to the fact, that for a given partial pressure of oxygen relatively high partial pressures of methanol prevents the coordination of oxygen to the active centers metalzeolites catalyst.
The results of investigation of the effect of partial pressure of oxygen on the yield of the reaction products at a space velocity 1500 h-1, partial pressure of methanol 0.21 atm and a temperature 1050C are presented on Fig.3. The partial pressure of oxygen was varied in the range of 0.13-0.6 atm. The increase of partial pressure of oxygen from 0.13 to 0.28 atm leads to an increase in yield of formic acid from 5.9 to 37.1%, that is due to the increase of surface concentration of nucleophilic oxygen. With further increase the partial pressure of oxygen observed reduction in the yield of formic acid. In all the studied range of increasing partial pressure of oxygen the yield of carbon dioxide is continuously increasing. Increase in the yield of formic acid in the range of partial pressures of oxygen of 0.13-0.28 atm is accompanied by a
decrease of yield of formaldehyde that can be explained by the consecutive formation of formic acid from formaldehyde.
: 40 -
Figure 3. The influence of the partial pressure of oxygen on conversion of alcohol (x, %) and yields of the reaction products (A, %): 1 - conversion of methanol; yields: 2 - formaldehyde, 3 -formic acid and 4 - carbon dioxide; v = 1500 h-1, PCh3oh =0.21 aTM, 7=105°C.
Pc2 , atm
The results of studies on the influence of temperature on the course of the reaction shown in Fig. 4.
Figure 4. The influence of temperature on the conversion (x, %) and yields of the reaction products (A, %): 1 - conversion of methanol; yields: 2 - formaldehyde, 3 - formic acid, 4 - carbon dioxide; v=1200 h-1, CH3OH:O2:N2=1:1:2.
90
100
110
120 130
T 0C
As can be seen from the figure, increasing of temperature from 85 to 1050C at a space velocity 1200 h-1 leads to an increase in yield of formic acid from 26.4 to 40.7% and further increasing of temperature up to 1250C reduces the yield of formic acid to 34.8%. The yield of carbon dioxide in the entire investigated temperature range is increasing in this case from 9.8 to 36.0% and the yield of formaldehyde is decreasing. The observed regularity of reaction with increasing temperature is explained by a parallel mechanism of formation of carbon dioxide from formaldehyde.
Increase of space velocity from 900 to 3200 h-1 at 1050C and the molar ratio of alcohol:oxygen: nitrogen = 1:1:2 leads to a decrease in yields of formic acid, carbon dioxide, the conversion of alcohol and increase the yield of formaldehyde (Fig. 5).
80
70
60
0х
50
*
40
30
200
1
3
4
Figure 5. The influence of space velocity on the conversion (x, %) and yields of the reaction products (A, %): 1 - conversion of methanol; yields: 2 - formaldehyde, 3 - formic acid 4 - carbon dioxide; CH3OH:O2:N2=1:1:2, 7=105°C.
500 1000 15000 2000 2500 3000 3 5000
V, h-1
Reduction of conversion of alcohol is related to a decrease in contact time, and the increase in the yield of formaldehyde with the fact, that the decrease in contact time prevents additional oxidation of the product in formic acid.
The influence of formaldehyde on the oxidation of methanol was studied on the catalyst Pd-mordenite. For this purpose, was conducted a series of experiments with the addition of formaldehyde in the initial reaction mixture. The results showed, that the addition of formaldehyde to the reaction mixture leads to an increase in yields of carbon dioxide and formic acid. These data, as well as the results of experiments on the influence of space velocity on the course of the reaction showing that with increase the volume velocity yield of formaldehyde increases, shows about the mechanism of formation of formic acid from formaldehyde.
On the basis of analysis of literature material and experimental data it has been suggested the mechanism of the reaction oxidation of methanol to formic acid over the catalyst Pd-mordenite:
H
I
H-£-OH
i h !
H
H-C-OH2 H
I I
'P/rfss/Z
-H2O IV
H
H-C-H I
0
1
+ -О2
V
H
H-C-H 1: I
Oi
¿Pd2+ ON- ##
N
SC-1
VI
Pd2+ H+ ON-
+
H-C
H
O
VII
H-C-H II I
0 1
1 I -I—'—
YSSfSfSSJ
Pd2+ h+ ON-N
SC-2
+ -О2 VIII
/
H
, + H-C v
In accordance with the above given scheme in the beginning occurs a methanol adsorption and then its protonization with the participation of Bronsted acid sites of the catalyst with fUrther removal of water and the formation of surface alcoholate. Transformation of the surface alcoholate into formaldehyde occurs in the interaction with surface nucleophilic oxygen through a stage of formation of aldehyde like intermediate surface compound (SC-1). Then aldehyde like intermediate surface compound (SC-1) decays on formaldehyde and restores the initial state of the catalyst. The formed aldehyde is adsorbed on the catalyst surface interacts with the surface nucleophilic oxygen to form surface compounds (SC-2).
Next from the surface compound SC-2 is cleaved formic acid and restores the initial state of the catalyst.
Rates of the elementary stages III, V, VI, VII of mechanism of catalysis is much more than the rates of elementary stages I, II, IV, VIII. Therefore, in the kinetic scheme of stage mechanism of the reaction the formation of formic acid can be enabled only I, II, VII and VIII stages of the mechanism of catalysis:
O2 + 2Z ZO + CH3OH ZOCH3OH
2ZO
ky
ZOCH3OH
k
ZO + ZOCH2
^ ZOCH2 + H2O
k4
2Z+CHOOH
(I)
(II) (IV) (VIII)
All these stages are virtually irreversible. Assuming that their elementary, we find the following expressions for the rates of stages:
r 1 = kXPo2 01 ; r2 = £202pCH3OH; r3 = k303 ; r4 = £40204 , here 01, 02, 03, 04 - percentage of free sites of the modified zeolite capable of adsorbing oxygen; sites covered by adsorbed atomic oxygen; areas covered by atomic oxygen and molecules of methanol; and areas covered aldehyde intermediates surface compounds OCH2. P02, pCh3oh - partial pressure of the corresponding components index; k1, k2, £3, k4 - the rate constants of the corresponding stages index; rb r2, r3, r4, - rates of the corresponding stages index. Under the conditions of stationarity r = r1 = r2 = r3 = r4 , where r - the overall rate of the process.
Based on these equations and the constancy of the total number of surface sites 01+02+03+04=1, we can find the concentration of intermediates 0, and the overall reaction rate as a function of the concentrations of reactants
V02
kxPO2 0 1 k202PCH3OH ,
kxP0l 02 = k303 ,
k40204 = k202PCH3OH ,
кЛ,- , kp
02=-
01
к P
k21 CH3OH
к P
03= ^ 02.
к '
04= kl P,
01+-
k2 PCH3OH
02+-
02
k
02 + — p,
CH3OH
CH3OH,
-1=0;
k1PO2 k P
V k21 CH3OH
+
kiP
02
02+01+
3 у
V k4
2 P - i
1 CH30H 1
=0,
01=
1 - 4
kiP02 ^ k2 PCH30H
kiP
02
k3 у
V k4
2 P -1
PCH30H 1
-1
k1P02 . k1p
02
к P
V k21 CH30H
3 у
r = r1 = rCHOOH= k1PO2
1 - 4
k1P02 к P
V k21 CH30H
kP
1 02
— 1 - 1
. 1 CH30H 1
V 4
— 1
k1F02 к P
V k21 CH30H
k1P02 ^
(1)
Because there is the low yields of formaldehyde to its formation is adopted by the brutto-mechanism: formaldehyde formed by the interaction of molecularly adsorbed oxygen with adsorbed molecules of methanol. This is the formal mechanism corresponds to the equation:
k
4
4
2
2
_ k5KlK2PO2 PCH3OH ...
roH2o= 1 TT • (2)
(l + KlPO2 + K2 PCH3OH )
Part of the formed molecules of formaldehyde adsorbed on the strong acid sites of Bronsted and interacting with the adsorbed oxygen molecules leads to the formation of carbon dioxide.
The equation of the rate of formation of carbon dioxide corresponding to this mechanism is as follows:
k6K3K4 PO2 PCH2O
(i+K3PO2 + K4PCH2O y where K\, K2, K3, K4 - adsorption equilibrium constants.
The equations (1), (2) and (3) constitute the kinetic model of oxidative conversion of methanol to formic acid.
The numerical values of kinetic constants of the model were determined by minimizing the objective function
_ ц 3 4 °2 CH2°
'CO2 / s7 , (3)
F=min zz
m n (AexP - ACa1^2
i=1 i=1
л calc
(4)
where - A®*p A;c*c - the experimental and calculated values of the yields of z-th component in the 7-th
experiment; m - number of experiments; n - the number of components.
Table 2 shows the numerical values of the parameters found by the kinetic model.
Table 2. Values of calculated constants of kinetic model of methanol oxidation reaction
Activation energy Et and heat of adsorption Qi, kkаl/mole Logarithms of preexponential factor in kinetic law lnk° and adsorption equilibrium ln K0
E1 = 14.0 ln k 0 =4.35
E2 = 11.5 ln k 0 =5.55
E3 = 12.9 ln k 0 =8.21
E4 = 10.8 ln k 0 =11.17
E5 = 21.6 ln k 0 =24.98
E6 = 20.4 ln k 0 =31.09
Q1= 7.0 ln K 0 = -13.31
Q2= 2.1 ln K0 = -0.340
Q3= 7.0 ln K0 = -16.31
Q3= 6.8 ln K0 = -1.87
K0 - preexponential factor in kinetic law;
The relative error of the experimental and calculated data for the numerical values of the kinetic constants given in Table. 2, did not exceed 8.0%.
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MODiFiKASiYA OLUNMU§ SEOLiTLOR UZORINDO METANOLUN OKSiDLO^DiRlCi
CEVRlLMOSi
A.M.Oliyev, S.M.M3cidova, G.A.Oli-zada, R.Y.Agayeva
Pd va kationlan ila modifikasiya olunmu§ sintetik (NaA, NaX, NaY) va tsbii (mordenit, klinoptilolit) seolitlar asasinda sintez olunmy§ bir sira katalizatorlann metanolun formaldehida, qan§qa tur§usuna va metilformiata oksidla§dirici gevrilma reaksiyalarinda aktivliyi tadqiq olunmu§dur. Metanolun formaldehida oksidla§dirici gevrilmasi (CuPdNaY) va metanolun qan§qa tur§usuna birba§a oksidla§masi (Pd-mordenit) reaksiyalari ugun yuksak effektiv katalizatorlar segilmi§dir. Pd-mordenit (0.1% Pd) katalizatorun uzarinda metanolun qan§qa tur§usuna birba§a oksidla§ma prosesin kinetik qanunauygunluqlarinin tadqiqinin tacrubi naticalari asasinda prosesin ehtimal olunan marhalali mexanizmi verilmi§ va onun asasinda kinetik model tartib olunmu§dur.
Agar sozlzr: oksidh§m3, metanol, seolit.
ОКИСЛИТЕЛЬНОЕ ПРЕВРАЩЕНИЕ МЕТАНОЛА НА МОДИФИЦИРОВАННЫХ ЦЕОЛИТАХ
А.М.Алиев, С.М.Меджидова, Г.А.Али-заде, Р.Ю.Агаева
Ряд цеолитных катализаторов, синтезированных на основе синтетических (NaA, NaX, NaY) и природных (морденит, клиноптилолит) цеолитов, модифицированных катионами Pd и Cu, был исследован в совмещенных реакциях окисления метанола в формальдегид, муравьиную кислоту и метилформиат. Подобраны высокоэффективные катализаторы для реакций окислительного превращения метанола в формальдегид (Cu Pd NaY) и прямого окисления метанола в муравьиную кислоту (Pd- морденит). На основе результатов экспериментального исследования кинетических закономерностей протекания процесса прямого окисления метанола в муравьиную кислоту на катализаторе Pd- морденит (0.1% Pd) представлен вероятный стадийный механизм процесса и разработана его кинетическая модель.
Ключевые слова: окисление, метанол, цеолит.
