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ISSN 2522-1841 (Online) ISSN 0005-2531 (Print)
AZERBAIJAN CHEMICAL JOURNAL № 4 2019
65
UDC 541.128; 547.281
CONVERSION OF METHANOL TO DIMETHOXYMETHANE
L.G.Maharramova
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 04.04.2019
Investigation on conversion of methanol to dimethoxymethane in the presence of VOx/ZrO2 catalyst on the basis Zr/Vx alloys were carried out. Tests has been implemented both in the presence O2 and in anaerobic conditions. Selectivity of reaction products has been determined. A good efficiency of catalysts has been revealed in the presence of O2 - selectivity on dimethoxymetane makes-up 67.0 wt.%. For the purpose of investigating surface activity of catalysts the latter ones uere undercone to X-ray diffraction and X-ray photoclectron spectroscopy. The dependens of target products yields on condition of the proceding and a state of sursace layer of catalysts has been established.
Keywords: methanol, catalytic oxidation, dimethoxymethane, X-ray diffraction analysis, surface activity.
doi.org/10.32737/0005-2531-2019-4-65-69
Introduction
Recently, numerous efforts have been made to develop catalysts for selective oxidation of methanol to dimethoxymethane (DMM). One-step selective oxidation of methanol to DMM has attracted much attention for its economic and environmental advantages. DMM is an important chemical intermediate product that has been widely used as an organic synthesis reagent and an additive to diesel fuel. It also can also be used as an excellent solvent in pharmaceutical and perfume industries due to its low toxicity. Numerous studies have shown that catalysts such as Rhenium oxide [1], heteropoly acids [2], Cu-ZSM-5 [3], V2Os/TiO2 [4] and V-complex oxides catalysts were active in the receach [5, 6]. VOx-based catalysts showed a good performance in selective oxidation of methanol to DMM.
Experimental part
We have conducted an evaluation of methanol oxidation on bifunctional catalysts -alloys of Zr-Vx (x=0.2-0.6), the preparation method of which is described in work [7]. Initially, catalyst was oxidized by air flow at 873 K during 1 -2 hours, and then treated by hydrogen at the same temperature for one hour in the quartz reactor [8]. The reactor was loaded with catalyst sample and mixed with pounded glass in an amount of 0.2-0.3 g and particles size 0.06-
0.10 mesh. Experiments were performed at 403523 K in pulse and flow modes (carrier gas -helium). The products of reaction were analyzed by Gas Liquid Chromatography on Paropak-Q. Vanadium (optimum content of oxidized vanadium on the surface was ~16%) was selectively oxidizable component of the alloy. The desorption temperature of the methanol and formaldehyde (FA) was about 403-413 K. The methanol oxidation reaction is more easily performed in the presence of oxygen (~5-10 vol %. O2/He). At the same time, a small amount of dimethyl ether (DME), DMM and methyl formate (MF) were also produced. Maximum yield of DMM was noted at the temperature below 463 K, indicating that lower temperature is favour able for conversion, methanol to DMM but the formation of dimethyl ether and methyl formate was increased at higher temperature. Methanol conversion to various products in the presence of O2 and under anaerobic conditions was studied [9]. In the presence of oxygen, hydrogenated vanadium oxide catalyst exhibited a good performance with DMM selectivity 67.0 wt %. After about 2 hours, the reaction has reached a steady state, and has been maintaining a good stability for more than 20 hours. X-ray diffraction (XRD) analysis of catalyst samples (ZrV0.2-0.4; ZrV0.5-0.6; ZrV2-intermetallide) was carried out before and after oxidation-reduction (O2+H2)-treatment [10]. The composition of surface and a valence state
of catalysts components were studied by X-ray photoelectron spectroscopy (XPS) on AE/ES 200 B.
Results and discussion
Conducted X-ray diffraction analysis of Zr-V catalyst samples showed deep phase changes on the surface layer and a formation of oxides VOX, V2O3-x, V2O5-x etc., whose activity increases with the loss of oxygen atoms. Zirconium atoms also undergo oxidation, but vanadium oxides, as more active metal, segregate on surface of catalyst. Zirconium oxides remain in catalyst carrier, and it can be represented as VOx/ZrO2. The diffusion of oxygen in such crystal structures is facilitated. As shown by XPS results, the conditions of O2+H2-proces-sing may cause changes not only in the surface layer, but also in the valence state of the active component of alloy. Physico-chemical examination showed that redox treatment applies only to the surface of the alloy layer; however, the volume structure of the catalyst remains unchanged. It is confirmed by X-ray diffraction [10]. Increased activity of the catalyst is associated with the appearance of vanadium in intermediate oxidation state:
■t r5+ -t r4+ -t r3+
V V V .
Obtained results revealed that catalytic systems based on Zr-V alloys exhibit high activity of the catalyst at the lower temperatures. Zr-V0.3 sample showed the greatest activity in
oxidation of methanol reaction among evaluated catalysts, where as ZrV2 showed the lowest. Thus, at 473 K reaction temperature conversion of methanol was 76.6-80.0%. Reaction conditions (T, P) and the influence of alcohol/air ratio on the output of final products were studied.
The dependence of final products yield on the reaction temperature in the presence of O2 is presented in Figure 1.
As we can see, peak of DMM is below 473 K (about 403-423 K) temperature. In this temperature range, the influence of alcohol/air ratio on the yield of final products was investigated. As shown in Figure 2, with increasing alcohol/air ratio, selectivity of DMM decrease, and at the same time - DME and MF selectivity increase. As shown in Figure 3, air pressure in the system showed great influence on the process of methanol oxidation and DMM output. With increasing pressure up to 0.25 MPa, selectivity for DMM increases. The methanol conversion and selectivity of the products was conducted both in the presence of O2 and in anaerobic conditions. As shown in Figure 2, in the presence of oxygen catalyst, VOx/ZrO2 exhibited a good performance in methanol oxidation with DMM selectivity of 67.0% wt. The catalyst showed a good activity in initial stage, when reaction carried out in anaerobic conditions, and conversion of methanol was ~16.2%. Simultaneously, DMM selectivity also maintained above 90%, as shown in Figure 4.
a,% 80
60 40
20
0 383 423 463
503
a,% 80
60 40
20 543 T, K
S,%
Fig.1. Catalytic activity of VOx/ZrO2 in presence of O2 reaction on temperature: 1 - DMM, 2 - DME, 3 - FA, 4 - MF.
80
60
40
20
S,%
80 60 40 20
0 20 40 60 80 %
Fig.2. Dependence of selectivity products of reaction on alcoh/air ratio (7=423 K): 1 -DMM, 2 - DME, 3 - FA, 4 - MF.
The corresponding catalysts exhibited both acidic and redox properties, which are necessary for the oxidation of methanol to formaldehyde with subsequent condensation of the latter with the excess of methanol to final DMM yield:
2CH3OH+CH2 ^ CH3-O-CH2-O-CH3+H2O
What comes to catalytic properties, the best balance between acid and redox properties was observed in samples, containing 8-9 wt.% V, that corresponds to a theoretical coverage of VOX as a monolayer (sample Zr-V0.3).
The acidity of active centers and their distribution at the surface of catalyst was studied by temperature programmed desorption of ammonia NH3-TPD. The experiment was conducted by step-by-step adding of CH3OH and
O2; besides this, the carrier gas for temperature programmed surface reaction was 10 vol %. O2/He. The results are shown at Figure 5. The products distribution of the reaction is similar as it was in anaerobic condition. However, the desorption temperature of methanol and formaldehyde is about 403-413 K, the desorption peak of DMM is about 403-423 K, which shows that lower temperature may be suitable for the conversion of methanol to DMM. Furthermore, the simultaneous desorption of these species is an indication that they could be originated from the same surface intermediate. It also proves that the methanol oxidation and dehydration-condensation process take place at different active sites.
%
80 60
40 20
%
80 60
40 20
0
0.1 0.15 0.2 0.25 0.3 P, MPa
Fig. 3. Dependence of selectivity products reaction on air pressure (T =423 K): 1 - DMM , 2 - DME, 3 - FA.
s
a.
E 8
a
C A
sg 4
a, % -a
80
60
40
20
0.5 1.0
1.5
2.0
5, % 80 60 40 20
2.5 t, r
Fig. 4. Catalytic activity of VOx/ZrO2 without O2. (T=423 K):1 - CH3OH, 2 - DMM, 3 -DME, 4 - MF.
\ 2
1
3
45
Fig. 5. Temperature programmed desorption (TRD) over VOx/ZrO2: 1 -CH3OH, 2 - FA, 3 - DME, 4 -DMM, 5 - MF.
373 423 473 523 573 623 T, K
1
From the experiment results above, the process oxygen and acidic, which provide DMM gener-of the reaction can be speculated as follows: ation reaction even in anaerobic condition the catalyst itself has two active centers - (Figure 4) .
CH3OH
//%//O///O/
ZrO2
CH3OH
O
//%/%///// ZrO2
CH3OH
4-
-17-5+
//A///y///V/
ZrO2
H2C
oh
O
\!
V
■4+
//%//O///O/
ZrO2
H2C^
H OH
V4+
* /l\ //%/%///O/
ZrO2 -H :
H2C
O
OH
1 1
\
V
4+
//%/%///// ZrO2
H3C
L-O.
O2
///V//////V/ ZrO2
H3C O
V4
//%//O///O/
ZrO2
H3C O
O2
V
4+
//%/%///O/
ZrO2
C - O - CH2 - O - CH3
O
4+
ZrO2
O2
4+
V
//%/%///// ZrO2
5+
4+
4+
Scheme. Methanol oxidation model over VOx/ZrO2 .
Thus, as the reaction proceeds, active oxygen sites in the catalyst are gradually consumed while the acidic sites are not. So, the DMM selectivity decreases while selectivity of acidic reaction products, such as methyl formate and dimethyl ether, increase. Finally, when the active oxidation sites are depleted, DMM and MF selectivity is minimized, but DME can be generated since it only relies on dehydration-condensation at separate acidic centers. However, due to the consumption of the active sites, the overall activity of the catalyst decreases until complete inactivation.
X-ray photoelectron spectroscopy (XPS) was carried out to investigate the product distribution of the methanol oxidation and their adsorption-desorption process in active centers of the catalysts. It was also established, that the methanol oxidation and dehydration-condensation process take place at different active sites. Different valences of vanadium on catalyst surface was analyzed by XPS, and presence of two
V4+
-V ions promoted selective oxidation of methanol. Methanol was absorbed on VOx at the surface of the catalyst with formation of meth-oxy group, and then dehydrogenated to produce
formaldehyde. Methoxy group and adsorbed formaldehyde generate DMM on the acidic sites of catalyst.
References
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2. Briand L.E., Bonetto R.D., Sanchez M.F. Structural modeling of coprecipitated VTiO catalysts. Catalysis Today. 1996. V. 32. No 1-4. P. 205-213.
3. Zhang Y., Drake I.J., Briggs D.N., Bell A.N. Synthesis of dimethyl and dimethoxymethane over Cu-ZSM-5. J. Catalysis. 2006. V. 244. No 1. P. 219-229.
4. Zhao H., Bennici S., Shen J., Auroux A.l. Nature of surface sites of V2O5-TiO2/SO42- catalysts and reactivity in selective oxidation of methanol to DMM. J. Catalysis. 2010. V. 272. No 1. P. 176-189.
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METANOLUN DiMETOKSiMETANA ÇEVRÎLMOSÎ L.G.Maharramova
Metanolun muxtalif birlaçmalara: formaldehida, dimetoksimetana, dimetilefirim cevrilma reaksiyalannin selektivliyi oxsigen içtirakinda texnoloji parametrlarin qeniç dayiçma çaraitinda ôyranilmiçdir. Metanolyn dimetoksimetana cevrilmasi Zr-Vx arintisi asasinda alinan VO/ZrO2 rfnfkizatorunun içtirakinda aparilmiçdir. Muayyan edilmiçdir ki, O2 muhitinda vanadium oksid katalizatoru DMM qora yuksak selektivlik (67.0%) gostarir. Bu katalizatorun sathi RFA va RFES analizlari ila tadqiq edilarak muayyan edilmiçdir ki, asas maqsadli mahsullarin çiximlari reaksiya çaraitindan va katalizatorun aziyyatindan asili olaraq dayiçir.
Açar sozlar: metanol, Zr-Vx arintisi, aktivhçma, oksidh§ma reaksiyasi, dimetoksimetan.
КОНВЕРСИЯ МЕТАНОЛА В ДИМЕТОКСИМЕТАН Л.Г.Магеррамова
Проведены исследования по конверсии метанола в диметоксиметан в присутствии VOx/ZrO2 - катализаторов на основе сплавов Zr-Vx. Эксперименты осуществлены как в присутствии O2, так и в анаэробных условиях. Определены селективности реакции. Выявлена хорошая эффективность катализаторов в присутствии О2 - селективность по диметоксиметину составила 67 вес %. С целью исследования поверхностной активности катализаторов последние были подвергнуты рентгенофазовому анализу и рентгенофотоэлектронной спектроскопии. Установлена зависимость выходов конечных продуктов реакции от условий ее проведения и состояния поверхностного слоя катализаторов.
Ключевые слова метанол, катализаторы, Zr-Vx сплавы, активация, окисление, диметоксиметан.
6. Meng Y., Wang T., Chen S., Zhao Y. et al. Selective oxidation of methanol to DMM on V2O5-MoO3/y-Al2O3 catalysts. Appl. Catalysis B: Environmental. 2014. P. 161-172.
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9. Aliyeva A.M., Efendi A.J., Gadjiyeva S.R., Sha-milov N.T., Tagiyev D.B., Maharramova L.G., Tagiyev D.B. Ecologically clean fuels on basis of methanol. The 6th Inter. Conf. "Ecological & Environmental Chemistry". Moldova. 2017. P. 161.
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