CATALISIS 2
(2013) 1-13.
Transformation of methanol into
aromatic compounds over MFI type zeolites
Transformación de metanol en compuestos aromáticos
sobre zeolitas del tipo MFI
Albania Villarroel1, Djamal Djaouadi1,
FÉlix AvendaÑo1, Luis GarcÍa2
1Petróleos de
Venezuela (PDVSA)-Intevep, Gerencia General de
Refinación e Industrialización, Gerencia de Valorización de Corrientes,
Apartado 76343, Caracas 1070-A, República Bolivariana de Venezuela
E-mail: villarroelax@pdvsa.com
2Escuela de
Ingeniería Química, Facultad de Ingeniería, Universidad Central de Venezuela. A.P. 47102, Caracas 1020-A, Venezuela
.
ABSTRACT
A large amount of petcoke is produced in Venezuela due to the crude oil processing in
the refining industry. Gasification of this pet-coke could be an alternative to
its valorization. Thus the syngas produced can be
transformed in methanol and then in more added-value products such as aromatics
compounds (MOGD Process). This study has been done to
explore the transformation of methanol to aromatics compounds over medium pore
size zeolites (MFI type), taking into account that olefins compounds will be
also generate as intermediaries. For all used zeolites, at any operational
condition, high selectivity was found towards aromatics compounds - mainly
xylenes - , regarding with the olefins, ethylene and propylene were the most
favored. The overall conversion value was close to 90%, based on methanol
transformation, for all evaluated catalyst.
Keywords: Aromatics, methanol transformation, MFI type zeolites,
olefins.
RESUMEN
En Venezuela se produce una gran cantidad de coque de
petróleo (pet-coke) debido al procesamiento de crudo
en la industria de refinación. La gasificación de este pet-coke
podría ser una ruta alterna a su valorización. De esta forma, el gas de
síntesis producido se podría transformar en metanol y luego en productos de
mayor valor agregado, tales como los compuestos aromáticos (proceso MOGD). Este
estudio ha sido realizado para explorar la transformación de metanol en
compuestos aromáticos sobre zeolitas de tamaño de poro
medio (tipo MFI), tomando en cuenta que se generarán compuestos olefínicos como intermediarios. Para todas las zeolitas
empleadas, en cualquier condición operacional, se encontró alta selectividad
hacia compuestos aromáticos - principalmente xilenos -, mientas que en cuanto a
las olefinas, etileno y propileno fueron las más
favorecidas. El valor de conversión global estuvo cerca de 90%, basado en la
transformación del metanol, para todos los catalizadores evaluados.
Palabras clave:
Compuestos aromáticos, olefinas, transformación de metanol, zeolitas tipo MFI.
© Sociedad Venezolana de Catálisis. Todos los
derechos reservados. Para permiso, envíe un correo electrónico a: revista.catalisis@gmail.com
INTRODUCTION
The syngas obtained through gasification
of pet-coke can be used to produce methanol, which can
be transformed in more added-value products as aromatics compounds. Those
products are one of the basic raw materials in the refinery and petrochemical
industry; benzene, toluene and xylenes (BTX) are the most commonly used.
The
commercial processes for aromatics production are almost
based on processes such as reforming and cracking using crude oil.
However, it is important to explore cleaner technologies to produce
petrochemicals compounds, especially BTX.
Methanol
conversion to olefins over H-ZSM-5 zeolites (MTO Process) has been the subject
of many researches [1, 2, 3] and is considered the
first stage of MTA process [4], as can be seen in the following scheme:
Figure 1. General scheme of MTA Process.
Chang
and Silvestri [5] reported that at 644K and
atmospheric pressure, the aromatics fraction in the products was 41% by weight;
according to the accepted mechanism, the aromatic hydrocarbons are formed from olefins. The major secondary products that
result in MTA process are ethylene, propylene, butenes
and pentenes; the heavier compounds as gasoline
fraction C5-C10 were found in
lesser amount.
The
general aim of this research is to study the methanol conversion to aromatic
compounds over medium pore size zeolites (MFI type). It was examined the
influence of the following factors on selectivity and overall conversion: a)
modification of the Si/Al relation for two zeolites in order to vary the number
and strength of acid sites, b) modification of process conditions such as
reaction temperature and space velocity.
EXPERIMENTAL
Catalyst preparation. For evaluating
the effect of reaction temperature and space velocity, ZSM-5 commercial zeolite
catalyst was used.
The study of the influence of the Si/Al
ratio was carried out with MFI types zeolites
synthesized by Intevep and named INT-MFI [6]. Both catalysts (ZSM-5 and INT-MFI) were subjected to ion
exchange treatment with a solution of (NH4)2SO4,
followed by calcination at 500 ° C, to obtain the protonated zeolites.
For catalysts characterization it were used five
techniques: X-ray diffraction (XRD), pyridine adsorption by infrared
spectrophotometer, inductively coupled plasma optical emission spectrometry
(ICP-OES), BET analysis by nitrogen adsorption and temperature programmed
oxidation (TPO).
The crystallinity degree of the
catalysts was obtained by X-ray diffraction on a difractometer PANalytical X’Pert PRO model.
Brönsted and Lewis acidity was calculated
by pyridine adsorption with an infrared spectroscopy
Perkin-Elmer 1750 model. With the infrared spectra of samples containing
basic molecules such as pyridine, it can get information about the nature of
acid centers.
The ICP-OES was used to obtain
the elemental composition of a solid and thus the unit cell formulas of the
zeolites, which are shown in Table 1.
Table
1.
Chemical composition of zeolites.
Catalyst |
Ratio Si/Al (% molar) |
Unit cell formula |
|
ZSM-5 (11) |
10,85 |
H+8,10
[Al8,10 Si87,90 O192]·16H2O |
|
INT-MFI (12) |
11,83 |
H+7,48
[Al7,48 Si88,52 O192]·16H2O |
|
INT-MFI (35) |
35,45 |
H+2,63
[Al2,63 Si93,36 O192]·16H2O |
|
INT-MFI (100) |
100,1 |
H+0,95
[Al0,95 Si95,05 O192]·16H2O |
To determine textural properties, it was used BET
analysis by nitrogen adsorption with a Micromeritics equipment Tristar 3000 model.
TPO technique was used to obtain
the temperature profile of coke deposited on the zeolites surface, through a
thermal oxidation. The equipment used was a Micromeritics Autochem 2 model.
The catalysts used were ZSM-5 zeolite ratio Si/Al of 11 – ZSM-5 (11) – and INT-MFI
zeolites ratios Si/Al of 12, 35 and 100 – INT-MFI (12), INT-MFI (35), INT-MFI (100).
Experimental setup. For the catalytic tests, methanol was used as a reactive
(99% Aldrich) and nitrogen was used as a diluent (99%). It was used a
Pyrex glass reactor of 610mm long and 18mm wide, with
a bed catalyst of particle size between 355 and 425 µm. Reaction heat
was supplied by an infrared furnace Research
Inc. Box 24064 model connected to a Micristrar controller. Nitrogen flow was regulated by an
electrically valve and methanol
injection was done through a positive displacement
pump ISCO. Experimental setup used in
this research is shown in Fig. 2.
The temperature effect was studied
between 350 and
The study of relation Si/Al effect was carried out under
optimal operating conditions found in the catalytic tests. The reaction time
used for comparative purposes was about 200min.
Figure 2. Experimental setup used for MTA process.
Product
analysis.
Products at reactor outlet were flowing through a condenser connected to a
cryostat, which operated at
Two
phases were obtained into liquid stream: an aqueous phase (mainly water and
remaining methanol) and an organic phase composed by paraffins,
olefins and aromatics. This phases were separated and the organic compounds
were analyzed by gas chromatography using a Agilent
Technologies 6890N model with PIONA application and a WASSON×ECE×NP×3013KC 148
column of 100m in length.
Experimental data processing. It was obtained
the yield both in the organic phase as in the gas phase as through
chromatographic analysis. In addition, from a global balance and considering
that it was possible to measure the amounts of methanol fed and the resulting
liquid product for a given operating time, the amount of gaseous products were
determined.
With the global mass and the yields, it
was determined the mass of each component present in the mixture reaction,
using the following equations:
(1)
(2)
Once known the masses, it were calculated the
corresponding moles and therefore the conversion were determined. Assuming that
the stoichiometric ratio is met, it can establish a relationship between the
methanol moles that have to react to form the product i
and the carbon numbers in that product. For benzene:
(3)
This procedure it was repeated for all the
reaction products and the sum represents the moles of methanol that reacted:
(4)
Finally, methanol conversion was calculated using the following expression:
(5)
RESULTS AND
DISCUSSION
In this section the results obtained for
studying the effect of the variation of temperature, space velocity and Si / Al
ratio on product distribution and methanol conversion are shown.
1. Temperature effect over product
distribution and conversion.
Space velocity and nitrogen flow were kept
constants at 10h-1 and
10 cc/min respectively. Methanol
conversion is about 13% for all temperatures studied, as shown in Table 2.
Table 2. Methanol
conversion over ZSM-5 (11)
(200 min, 10 h-1, 10 cc/min
N2) |
||||
Temperature (°C) |
350 |
400 |
450 |
500 |
Conversion
(% mol) |
13,6 |
13,1 |
13,5 |
13,5 |
Selectivity to paraffins,
olefins and aromatics for different temperatures is shown in Fig. 3. It can be
seen a drop in the olefins production and a growth in aromatics formation, when
the temperature rises.
It is clearly observed the
behavior of the olefins as intermediates for the production of aromatic
compounds.
As for the paraffinic compounds, it was
observed a slight decrease with temperature. These results
indicate that oligomerization and cyclization
reactions were favored with temperature increases. The appearance of paraffins implies the existence of hydrogen transfer
reactions. Hydrocarbons distribution for all temperatures studied can be
seen in Table 3.
Figure 3. Selectivity
to paraffins, olefins and aromatics at different
temperature
Table 3. Temperature
effect over hydrocarbon distribution using ZSM-5 (11) zeolites
(200min, 10h-1, 10cc/min N2) |
||||
Temperature (°C) |
350 |
400 |
450 |
500 |
molar % |
|
|
||
Methane |
1,44 |
3,52 |
7,71 |
10,88 |
Ehtylene |
17,02 |
15,16 |
11,51 |
9,51 |
Ethane |
0,32 |
0,53 |
0,66 |
0,41 |
Propylene |
8,22 |
9,40 |
9,09 |
6,15 |
Propane |
6,08 |
10,39 |
4,41 |
2,74 |
Isobutane |
9,97 |
15,38 |
7,29 |
5,23 |
n-Butane |
0,00 |
0,00 |
0,00 |
0,00 |
t-2-Butene |
2,15 |
1,86 |
1,61 |
1,51 |
1-Butene |
0,18 |
0,00 |
0,00 |
0,28 |
Isobutene |
1,62 |
2,74 |
1,36 |
0,80 |
c-2-Butene |
0,00 |
0,00 |
0,00 |
0,00 |
Isopentane |
0,01 |
0,03 |
0,03 |
0,02 |
n-Pentane |
0,00 |
0,00 |
0,00 |
0,00 |
t-2-Pentene |
5,41 |
6,61 |
3,25 |
2,94 |
1-Pentene |
0,00 |
0,00 |
0,00 |
0,00 |
c-2-Pentene |
0,03 |
0,04 |
0,05 |
0,04 |
Benzene |
0,73 |
0,20 |
0,11 |
0,05 |
Toluene |
3,32 |
3,53 |
3,72 |
4,32 |
Ethylbenzene |
1,14 |
0,75 |
1,14 |
1,42 |
Xylenes |
29,13 |
20,79 |
32,70 |
39,35 |
1,2,4-TMB |
16,28 |
7,58 |
14,94 |
17,79 |
According to mechanisms proposed to explain
the transformation of mixture DME / Methanol into light olefins and aromatics
compounds, the first product formed is ethylene [7, 8, 9]. The results observed in Table 3 are consistent with these theories.
It found a higher selectivity to
aromatics compounds, principally to xylenes. Aromatics
compounds can be formed by ethylene and propylene oligomerization
reactions, followed by ciclization and aromatization
processes. TMB formation is affected by the shape selectivity of zeolites. It
was observed the formation of 1,2,4-TMB
and not the
other isomers, this
is because the Van Der Waals diameter (7,4 Å) is similar to the o-xylene
molecule.
As to the olefins, the ethylene selectivity is higher
than for the propylene for all temperatures tested, however, the difference
between both molar percentages decreased with the increasing of reaction
temperature. This is an indicative that the alkylation of ethylene with
methanol, that could be one way for the propylene formation, is favored with increasing
temperature.
At temperatures studied, propylene
has higher selectivity than pentenes, especially at
high temperatures (450 –
2. Space velocity effect over product
distribution and conversion. Temperature (
Table 4. WHSV effect
over product distribution using ZSM-5 (11) zeolite.
WHSV (h-1) |
10 |
24 |
50 |
|
molar % |
||
Paraffins C1-C5 |
22,96 |
35,77 |
12,06 |
Olefins C2-C5 |
27,51 |
43,43 |
32,81 |
Aromatics (BTX) |
48,38 |
20,80 |
55,13 |
The higher selectivity to aromatics
was obtained operating to 50h-1. It was found a maximum in
selectivity to paraffins and olefins when the space
velocity was between 10h-1 and 24h-1.
3. Ratio Si/Al effect over product distribution and conversion. Temperature (
Selectivity to paraffins,
olefins and aromatic is reported in Table 5. It was observed that molar
percentage of olefins and aromatics increases with the ratio Si/Al, while the
molar percentage of paraffins decreases.
(200min, |
|||
Ratio
Si/Al |
12 |
35 |
100 |
|
molar
% |
||
Paraffins
C1-C5 |
45,61 |
14,68 |
8,13 |
Olefins
C2-C5 |
20,13 |
41,69 |
44,59 |
Aromatics
(BTX) |
31,86 |
43,78 |
47,24 |
Table 5. Effect of ratio
Si/Al over product distribution using the INT-MFI zeolite
Paraffins
formation involves hydrogen transfer reactions (bimolecular reactions); the
increase of the ratio Si/Al implies an active sites separation, so the
bimolecular reactions are unfavorable. These effects explain the trend observed
in Table 5.
The tendency resulting in olefins
and aromatics formation can be explained by the increases in acid sites
strength that occur product of the dealumination
process, coupled with the fact that cracking reactions for olefins formation
and cyclization do not require two adjacent active sites to take place.
Aromatics compound formation is
favored at high Si/Al ratio, however, these conditions also favor the catalyst
deactivation.
4. Brönsted
and Lewis acidity by pyridine adsorption. It was
determined Brönsted and Lewis acidity for ZSM-5 (11) and INT-MFI (12) by pyridine
adsorption; the results are shown in Tables 6. INT-MFI (12) zeolites have
largest number of strong Brönsted acid sites than
ZSM-5 (11).
Table
6. Brönsted and Lewis
acidity for ZSM-5 (11) and INT-MFI (12) zeolites
Sites Type |
N° Brönsted |
N° Lewis |
(μmol/gr) |
(μmol/gr) |
|
ZSM-5 (11) |
||
Weak |
91 |
118 |
Media |
77 |
79 |
Strong |
127 |
84 |
|
INT-MFI (12) |
|
Weak |
23 |
84 |
Media |
215 |
131 |
Strong |
249 |
74 |
Acidity analysis was done in order
to compare zeolites with a similar Si/Al relation, which is why the results for
INT-MFI (35) and INT-MFI (100) were not presented.
5 Characterization of zeolites by
Temperature Programmed Oxidation (TPO). Using this technique it could be
observed the signals of coke deposited on the catalyst surface after had been
used. Fig. 4 shows the spectra of the coke oxidation as a function of
temperature for ZSM-5 (11) and INT-MFI (12) zeolites.
Figure 4. Thermograms of coke oxidation for spent zeolites ZSM-5 (11)
and INT-MFI (12).
The thermogram
of ZSM-5 (11) zeolites has a maximum signal at
6. Surface area determination by BET
analysis with nitrogen adsorption. Nitrogen adsorption isotherms for ZSM-5
(11) and INT-MFI (12) zeolites fresh and spent are shown in Fig.5.
In the isotherm of zeolites fresh
ZSM-5 (11) a hysteresis is observed at a relative pressure range of 0,45 to
0,9, this phenomenon is
characteristic of mesoporosity
materials. The isotherm of INT-MFI (12) without using reflects microporosity.
In zeolites with coke impregnated on
the surface, the isotherm of ZSM-5 (11) shows hysteresis due to the mesoporosity, which is smaller than observed to the same
zeolites before use. The isotherms of INT-MFI (12) zeolites are similar in both
cases.
Figure 5. Nitrogen adsorption isotherms of ZSM-5 (11) and INT-MFI
(12) zeolites before and after use.
Surface area values of zeolites used
in this research are show in Table 7. Both zeolites - ZSM-5 (11) and INT-MFI
(12) - without using have a similar surface area. Values of fresh INT-MFI
zeolites are bounded in a range of approximately 306 to 426 m2/gr.
Comparing the values of fresh and
spent zeolites, it was observed that surface area of used INT-MFI (12) zeolite
was 92% lower than the surface area of the same fresh zeolites. In the case of
ZSM-5 the reduction was 69%.
Table 7. Surface area of
used zeolites.
Catalyst |
BET Surface
area |
(m2/gr) |
|
Fresh ZSM-5
(11) |
418 |
Fresh INT-MFI
(12) |
407 |
Fresh INT-MFI
(35) |
306 |
Fresh INT-MFI
(100) |
426 |
Spent ZSM-5
(11) |
130 |
Spent INT-MFI
(12) |
33 |
The difference in percentages may be
due that INT-MFI (12) zeolite presents
higher microporosity, so coke formation on the
surface is more favored than in ZSM-5 (11) zeolite. The oxygen treatment (TPO)
for used zeolites allowed to remove certain amount of the coke deposited on the
solids. The superficial area of treated catalyst was also calculate, for ZSM-5 (11) the area was 366 m2/gr
while for the INT-MFI (12) was 403 m2/gr.
Unlike what was found for ZSM-5 (11)
zeolites, the superficial area of INT-MFI (12) treated is slightly lower than
the area of the same zeolites before use; this difference indicates that this
solid has a higher regenerability.
7. Mechanism of methanol transformation into
aromatics compounds. As shown in Figure 1, to
convert methanol into aromatic compounds must pass through an intermediate
stage of production of olefins. First light olefins are obtained and then, by
alkylation or oligomerization reactions are obtained
heavier olefins which are the precursors of aromatic compounds.
Kaeding
and Butter [8] proposed a carbocations mechanism to
explain the formation of light olefins from methanol.
Figure
6.
Light olefins formation from carbocations as
intermediates.
Olefin chain
growth was explained by Cormerais [9] through oligomerization reactions:
Figure 7. Oligomerization reactions of light olefins.
Finally, the olefins are converted into
aromatic compounds by a mechanism that includes, besides the cyclization, a
series of reactions of hydrogen transfers:
Figure
8.
Aromatization reactions of light olefins.
CONCLUSIONS
INT-MFI zeolite is a catalyst with high
potential for used in the transformation of methanol into olefins and aromatics
compounds.
The optimum conditions found in this
research to the transformation of methanol into olefins and aromatics compounds
over medium pore zeolites are:
Si/Al ratio is a variable that
determine the catalyst deactivation, and high temperatures decrease the life of
the solid because it promotes the coke formation. On the other, the increase of
Si/Al ratio leads to more stable catalysts.
TPO analysis showed that INT-MFI
(12) zeolites recover most of the initial textural properties.
GLOSSARY
: Total mass of
methanol fed at a given time t
: Total mass of
liquid product obtained at a given time t
: Total mass of gas
product obtained at a given time t
: Mass of component i at stream j
: Total mass of
stream j
: Mass fraction of
component i at stream j
REFERENCES
1. C. Chu, C. Chang. J. Catal. 86 (1984) 297.
2. G. J. Hutchings, R. Hunter. Catal. Today 6 (1990) 279.
3. F. Patcas.
J. Catal. 231 (2005) 194.
4. S. L. Meisel. Chemtech, 1 (1988) 32.
5. C. Chang, A. Silvestri. J. Catal. 47 (1977)
249.
6.
N.
Martinez and coll. US Patent 5,254,327; Oct. 19, 1993. (7)
7.
W. Zatorski, S. Krzyzanowski. Acta
Phys. Chem. 24 (1978) 347.
8. W. Kaeding,
S. Butter. J. Catal. 61 (1980) 155.
9.
F. Cormerais. “Transformation
de méthanol et le diméthyléther en oléfines légères”. Doctoral Thesis.
Poitiers University. 1981.
10. M. Bjørgen, S. Svelle, F. Joensen, J. Nerlov, S. Kolboe, F. Bonino, L. Palumbo, S. Bordiga, U. Olsbye. J. Catal. 249 (2007) 195.
11. J. R. Anderson, T. Mole,
V. Christov. J. Catal. 61 (1980) 477.
12. M. R. F. Siggel,
T. D. Thomas. J. Electron Spectrosc. Relat. Phenom. 48 (1989) 101.
13. M. A. Fahim,
T. A. Alsahlaf, A. Elkilani.
“Fundamentals of Petroelum
Refining.” Elsevier, Amsterdam (2010). 95-152.
14. Y. M. Ni, A. M. Sun, X. L. Wu, J. L. Hu, T. Li, G. X. Li. Chin. J. Chem. Eng. 19 (2011) 439.