- 1. Reac Kinet Mech Cat (2012) 105:469481DOI
10.1007/s11144-011-0383-3V-Mn-MCM-41 catalyst for the vapor phase
oxidationof o-xyleneC. Mahendiran T. Maiyalagan P. VijayanC. Suresh
K. ShanthiReceived: 4 May 2011 / Accepted: 1 October 2011 /
Published online: 21 October 2011 Akademiai Kiado, Budapest,
Hungary 2011 Abstract The role of V and Mn incorporated mesoporous
molecular sieves wasinvestigated for the vapor phase oxidation of
o-xylene. Mesoporous monometallicV-MCM-41 (Si/V = 25, 50, 75 and
100), Mn-MCM-41 (Si/Mn = 50) and bime-tallic V-Mn-MCM-41 (Si/(V ?
Mn) = 100) molecular sieves were synthesized bya direct
hydrothermal (DHT) process and characterized by various techniques
suchas X-ray diffraction, DRUV-Vis spectroscopy, EPR, and
transmission electronmicroscopy (TEM). From the DRUV-Vis and EPR
spectral study, it was found thatmost of the V species are present
as vanadyl ions (VO2?) in the as-synthesizedcatalysts and as highly
dispersed V5? ions in tetrahedral coordination in the
calcinedcatalysts. The activity of the catalysts was measured and
compared with each otherfor the gas phase oxidation of o-xylene in
the presence of atmospheric air as anoxidant at 573 K. Among the
various catalysts, V-MCM-41 with Si/V = 50exhibited high activity
towards production of phthalic anhydride under the exper-imental
condition. The correlation between the phthalic anhydride
selectivity andthe physico-chemical characteristics of the catalyst
was found. It is concluded thatV5? species present in the MCM-41
silica matrix are the active sites responsible forthe selective
formation of phthalic anhydride during the vapor phase oxidation
ofo-xylene.C. Mahendiran (&)Department of Chemistry, Anna
University of Technology Tirunelveli,University College of
Engineering, Nagercoil Campus, Nagercoil 629004, Indiae-mail:
[email protected]. MaiyalaganSchool of Chemical and Biomedical
Engineering, Nanyang Technological University,Singapore 639798,
SingaporeP. Vijayan C. Suresh K. ShanthiDepartment of Chemistry,
Anna University, Chennai 25, India123
2. 470C. Mahendiran et al.Keywords V and Mn-MCM-41 Vapor phase
Oxidation o-xylene Phthalic anhydrideIntroductionHeterogeneous
catalyzed gas phase oxidation plays a vital role in the
chemicalindustry. In fact, selective oxidation is the simplest
functionalization method; inparticular, more than 60% of products
synthesized by catalytic routes in thechemical industry are
obtained by oxidation reactions [1]. From the standpoint
ofenvironmental friendliness, much attention has been paid to the
development ofmetal catalysts for the selective oxidation using
molecular oxygen as an oxidant[25]. The oxidative product pthalic
anhydride is a commercially important andversatile intermediate in
organic chemistry. The primary use of phthalic anhydride(PA) is as
a chemical intermediate in the production of plastics from vinyl
chloride.Phthalate esters, which function as plasticizers, are
derived from phthalic anhydride.Phthalic anhydride has another
major use in the production of polyester resins andother minor uses
in the production of alkyl resins used in paints and lacquers,
certaindyes (anthraquinone, phthalein, rhodamine, phthalocyanine,
uorescein, andxanthene dyes), insect repellents, and urethane
polyester polyols. It has also beenused as a rubber scorch
inhibitor [6].A method for converting naphthalene to phthalic
anhydride using sulfuric acid asthe oxidizing agent in the presence
of mercury salt as the catalysts was discoveredby E. Sapper, and
was patented by the Badische Anilin and Soda Fabrik in 1896.During
the last decade of the nineteenth century, the growing demand for
phthalicanhydride for use in the preparation of xanthene and the
indigoid dyes led toresearch toward the discovery of cheaper
processes for its manufacture [7]. In thiscontext, Dias et al. [8]
found that V2O5 supported TiO2 as an efcient catalyst forphthalic
anhydride production. However, due to its poor mechanical strength
ofV2O5/TiO2 and low surface area, attention has been focused on the
development ofcatalysts with high mechanical strength and high
surface area. In this context,mesoporous MCM-41 molecular sieves
with high surface area and tunable pore sizecame into existence
[9]. The unique physical properties have made these materialshighly
desirable for catalytic applications [10, 11]. Isomorphous
substitution ofsilicon with other elements is an excellent strategy
in creating active sites andanchoring sites for active molecules in
the design of new heterogeneous catalyst.Many metals, e.g., Al, Ti,
Mn, Fe, B, Ni and V, have been incorporated into thesilica matrix
of MCM-41 [1215]. Molecular sieves containing redox active
metals,like Ti, V, Cr, Fe, or Co, are increasingly used as
heterogeneous catalysts for theselective oxidation of organic
compounds. Among the metals, particularlyvanadium and manganese
were found to have remarkable catalytic activity forthe selective
oxidation of various organic molecules when incorporated into
silicatemolecular sieve [1621]. In this paper, the vapor phase
oxidation of o-xylene tophthalic anhydride over V-MCM-41, Mn-MCM-41
and V-Mn-MCM-41 catalystshas been investigated and correlated with
the structural, electronic and surfaceresults obtained.123 3.
V-Mn-MCM-41 catalyst 471ExperimentalSynthesis of V-MCM-41V-MCM-41
(Si/V = 50) was synthesized by hydrothermal method
reportedelsewhere [21] using sodium metasilicate (CDH) as silica
source, cetyl trimethylammonium bromide (CTAB, OTTO Chemie) as the
structure-directing agent withthe following molar gel composition
SiO2:0.02 (VOS4H2O):0.2 CTAB:0.89H2SO4:160 H2O. In a typical
synthesis, 21.32 g of sodium metasilicate and0.63 g of vanadyl
sulfate monohydrate were dissolved in 60 g of water. Thereaction
mixture was stirred for 2 h. Meanwhile, CTAB (5.47 g) was dissolved
in20 g of water. Then, the resultant mixture of sodium metasilicate
and vanadylsulfate monohydrate was added dropwise into the CTAB
solution. The nal mixturewas stirred for 1 h. The pH of the gel was
adjusted to 10.511 using 2 M sulfuricacid followed by stirring for
3 h. The obtained gel was placed into an autoclave andheated to 413
K under static conditions for 12 h. The resultant precipitate
wasltered, washed with deionized water and dried in air at 375 K
and then nallycalcined at 773 K for 1 h in N2 ow and for 12 h in
CO2-free air ow. The catalystsV-MCM-41 (Si/V = 25, 75,100),
Mn-MCM-41 (Si/Mn = 50) and V-Mn-MCM-41(Si/(V ? Mn) = 100) were also
synthesized in a similar manner wherein only theratio of vanadyl
sulfate monohydrate for vanadium source and manganese acetatefor
manganese source was adjusted.Characterization of the
catalystsInductively coupled plasma (ICP) optical emission
spectroscopy was used for thedetermination of the metal content in
each sample synthesized above. Themeasurements were performed with
a Perkin-Elmer OPTIMA 3000 and the samplewas dissolved in a mixture
of HF and HNO3 before the measurements. XRDanalysis was performed
on Rigaku Miniex X-ray diffractometer. A germaniumsolid state
detector cooled in liquid nitrogen with Cu Ka radiation source was
used.The samples were scanned between 0.5 and 8.5 (2h) in steps of
0.02 with thecounting time of 5 s at each point. N2 adsorption
studies were carried out toexamine the porous properties of each
sample. The measurements were carried outon a Belsorpmini II (BEL
Japan. Inc) instrument. All the samples were pre-treatedin vacuum
at 573 K for 12 h in owing N2 at a ow rate of 60 mL/min. The
surfacearea and pore size were obtained from these isotherms using
the conventional BETand BJH equation. The coordination environment
of vanadium and manganesecontaining MCM-41 catalysts was examined
by diffuse reectance UV-visspectroscopy. The spectra were recorded
between 200 and 800 nm on a ShimadzuUV-vis spectrophotometer (Model
2450) using BaSO4 as the reference. Further-more, the coordination
environment of vanadium and manganese was conrmed byEPR (Varian
E112 spectrometer operating in the X-band 9.2 GHz frequency) atroom
temperature. Transmission electron microscopy (TEM) images were
obtainedby using a JEOL electron microscope with an acceleration
voltage of 200 kV.123 4. 472C. Mahendiran et al.Experimental
procedure for the oxidation of o-xyleneThe oxidation of o-xylene
was carried out in a xed bed down ow quartz reactor atatmospheric
pressure in the temperature range of 473623 K with air ow of0.02
mol h-1. Prior to the reaction, the reactor packed with 0.3 g of
the catalyst waspreheated in a tubular furnace equipped with a
thermocouple. The reactant (o-xylene) was fed into the reactor
through a syringe infusion pump at a predeterminedow rate. The
product mixture was collected at the time interval of 1 h and
analyzedby a gas chromatograph (GC-17A, Shimadzu) equipped with a
ame ionizationdetector. The gaseous products were analyzed by a TCD
detector using an SE-30column. After every run, the catalyst was
regenerated to remove the coke deposit,by passing a stream of pure
dry air at a temperature of 773 K for 6 h. The effect ofvarious
parameters, viz., temperature, weight hourly space velocity and
time onstream was studied on the regenerated catalyst.Results and
discussionXRDThe XRD patterns of calcined V-MCM-41 materials with
an atomic ratio of(Si/V = 100, 75, 50, and 25), Mn-MCM-41 (50), and
V-Mn-MCM-41(Si/(V ? Mn) = 100) recorded at low diffraction angles
are shown in Fig. 1 andits inset. A strong intense peak observed in
the 2h range between 2 and 38 for all thesamples is due to the
reection from (100) plane of MCM-41. Apart from this, lowintensity
peaks in the 2h range 35, corresponding to the higher order
reections Intensity (a.u.)Intensity (a.u.)ab1 3 5 79112 (Deg.) a b
c d 0 2 46 810 2 (Deg.)Fig. 1 X-ray diffraction patterns of (a)
V-MCM-41 (100), (b) V-MCM-41 (75), (c) V-MCM-41 (50) and(d)
V-MCM-41 (25) catalysts. Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41
(Si/(V ? Mn) = 100)123 5. V-Mn-MCM-41 catalyst473Table 1
Physico-chemical characteristics of the catalystsCatalystV Mnd-Unit
cell Surface Pore PoreWallcontent content spacing parameter
areadiameter volumethickness(wt%)a(wt%)a (A) a0 (A)(m2/g)
(A)(cm3/g) (A)V-MCM-41 (25) 1.14 40.01
46.2081428.100.7618.10V-MCM-41 (50) 0.58 39.60
45.7389328.400.7917.33V-MCM-41 (75) 0.38 39.50
45.6197628.600.8117.00V-MCM-410.24 39.30 45.38 1,013 28.700.8216.68
(100)V-Mn-MCM-41 0.570.8641.30 47.6986328.480.8019.21 Si/(V ? Mn) =
100Mn-MCM-41 0.8840.75 47.0690427.750.8119.31 (50)aResults obtained
from ICP-AES analysissuch as (110) and (200) planes, were also
observed, which conrms the mesoporousnature of the samples. Higher
angle XRD (not shown) does not show any peaks forextra framework
vanadium oxide. The unit cell parameter (a0) calculated using
theformula, a0 = 2d100/H3, and d spacing values obtained using the
Braggsequation 2dsinh = nk, where k = 1.54 A for Cu Ka radiation
are presented inTable 1. Upon introduction of V into the MCM-41, a
slight decrease in the unit cellparameter value was observed.
However, when the metal content was increased,the intensity of the
diffraction peaks decreased, indicating that it may be due
tostructural irregularity of the mesopores at high metal content as
reported inliterature [22].Nitrogen adsorptiondesorption
isothermsThe adsorptiondesorption isotherms of the catalysts V, Mn
and V-Mn-MCM-41are illustrated in Fig. 2. A typical type IV
isotherm as dened by IUPAC formesoporous material was obtained. The
adsorption isotherm exhibits a sharpincrease in the P/Po range from
0.2 to 0.3 which is obviously characteristic ofcapillary
condensation within mesopores [23]. The P/Po position of the
inectionpoints is clearly related to the diameter in the mesopore
range, and the stepindicates the mesopore size distribution. N2
adsorbed volumes at P/Po = 0.3, forSi/V = 100, 75, 50, 25, Si/Mn =
50 and Si/Mn ? V = 100 are 350, 330, 315, 295,275 and 250. The BET
surface area, pore volume, and pore diameter, as a functionof V, Mn
and V-Mn content are shown in Table 1. The increase in the
vanadiumcontent slightly decreased the surface area, pore volume as
well as pore diameter.From the results, the N2 adsorption studies
clearly indicate the successful incor-poration of V and Mn. When V
and Mn are used together, partial amorphization is123 6. 474 C.
Mahendiran et al.Fig. 2 N2 adsorptiondesorption isotherms of
catalysts (a) V-MCM-41 (100), (b) V-MCM-41 (75),(c) V-MCM-41 (50)
and (d) V-MCM-41 (25) Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41
(Si/(V ? Mn) = 100)occurs. It may be due to metal oxides blocking
the molecular sieves pore or partialcollapse of pore
structure.DR-UV-vis spectroscopyThe DRUV-Vis spectra of V-MCM-41
(Si/V ratio = 25, 50, 75 and 100) catalystsshowed the presence of
two shoulder peaks at 260 and the other at 340 nm (Fig. 3ad). These
correspond to the tetrahedral V5? ions inside the wall and the
tetrahedralV5? ions on the surface of the wall, respectively [24].
The intensity ratio of thesetwo peaks seems to be relatively high
for a catalyst with high vanadium loading. It isalso evident from
the spectra that as the ratio of Si/V increased, there is
acorresponding decrease in the intensity of the peaks due to
decrease in the numberof vanadium ions. These bands were attributed
to the low-energy charge transfertransition between tetrahedral
oxygen ligands and a central V5? ion [25, 26]. Such atetrahedral
environment was typical for silica matrix V5? ions. A typical
spectrum is123 7. V-Mn-MCM-41 catalyst475 Absorbance (a.u)abcde180
280380480580680Wavelength (nm)Fig. 3 DR-UV-vis spectra of catalysts
(a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM-41 (75),(d)
V-MCM-41 (100) and (e) spent V-MCM-41 (50)recorded for the spent
V-MCM-41 (50) catalyst (Fig. 3e). There are no absorptionbands and
this indicates that V5? species are absent in spent catalyst.EPRThe
EPR spectra of the as-synthesized V-MCM-41 samples with varying
valuesof Si/V atomic ratio (25, 50, 75, and 100) were recorded at
room temperature andare shown in Fig. 4ad. The source for the
synthesis of vanadium containingmesoporous materials was vanadyl
sulfate (V4?, d1), and all the as-synthesizedV-MCM-41 exhibits its
characteristic EPR signal. In comparison, the EPR spectraof
as-synthesized and spent V-MCM-41 (Si/V = 50) are also shown in
Fig. 5a, b.It is interesting to note the absence of the EPR signal
in the spent catalyst (Fig. 5b)which may be the indication for the
complete utilization of the V5? species foroxidation
reaction.TEMTEM images of the calcined V-MCM-41 samples with Si/V
atomic ratios of 100,75, 50, and 25 are shown in Fig. 6ad. A highly
ordered mesoporous frameworkwith hexagonal arrays of cylindrical
channels of the synthesized samples isconrmed by TEM images [27].
These are virtually regular hexagonal arrays of nepore arrangement
existing in these samples. This ordered arrangement, typical forthe
MCM-41 materials, conrms the XRD data.123 8. 476 C. Mahendiran et
al.abcd 0 2000 4000 6000 8000 Magnetic field strength (Gauss)Fig. 4
EPR spectra of as-synthesized catalysts (a) V-MCM-41 (25), (b)
V-MCM-41 (50), (c) V-MCM-41 (75), (d) V-MCM-41 (100)Fig. 5 EPR
spectra of catalysts (a) as-synthesized V-MCM-41 (50) and (b)
V-MCM-41 (50) spentActivity of V-MCM-41 catalystThe activity of
V-MCM-41 (50) catalyst was studied for the vapor phase oxidationof
o-xylene at 573 K with the ow rate of o-xylene 5.87 h-1 (WHSV) and
CO2-freeair 0.02 mol h-1 over a period of 7 h. The results are
illustrated in Fig. 7. Thepercentage conversion and selectivity
increased from 1 to 2 h and then decreased upto 5 h. Beyond 5 h,
the catalyst attained steady state activity. The initial increase
inconversion from 1 to 2 h is attributed to oxidation of V4? to
V5?, which is necessaryfor oxidation. The decrease in trend up to 5
h may be due to some carbondeposition. Under this steady state
reaction conditions, the activities of the catalystswith varying
Si/V ratios are compared.123 9. V-Mn-MCM-41 catalyst477Fig. 6 TEM
pictures of a V-MCM-41 (100), b V-MCM-41 (75), c V-MCM-41 (50), d
V-MCM-41 (25) In order to nd out the optimum vanadium content, the
vapor phase oxidation ofo-xylene was carried out at 573 K on
V-MCM-41 catalyst with varying vanadiumcontent (Si/V ratio 25, 50,
75 and 100) and the results are given in Fig. 8. It isobserved that
the conversion o-xylene increases with Si/V ratio till
V-MCM-41(50). Obviously, more vanadium loading can increase
o-xylene conversion becauseof the increased amount of available
active sites. This is revealed from the lowintensity of DRUV-Vis
spectral bands of V-MCM-41 (25) catalyst around 260and 340 nm
corresponding to V5? (Fig. 3a, b) compared to that of V-MCM-41
(50).The optimum ratio is around 50. V-MCM-41 (50) exhibited the
maximum catalyticactivity. However, beyond the Si/V ratio 50, there
is a decrease in trend observedwith respect to its conversion. This
may be because of the lack of dispersion ofvanadium even though
available in large quantity. The decrease in conversion athigh Si/V
value may be attributed to the decrease in the concentration of V5?
activesites as it is evident from the DRUV-Vis spectra where a
decrease in the absorbanceintensity is noticed with increase in
Si/V ratio from 50 to 100. Hence, the highactivity of V-MCM-41 (50)
may be attributed to the availability of higher numberof V5? in
V-MCM-41 (50) than in V-MCM-41 (75) and V-MCM-41 (100). 123 10.
478C. Mahendiran et al.Fig. 7 Effect of reaction time on the
oxidation of o-xylene. Reaction conditions: temperature = 573
K,weight of V-MCM-41 (Si/V = 50) = 0.3 g, WHSV = 5.87 h-1 and ow
rate of air 0.02 mol h-1The dispersion and amount of V5? become
important in order to account for highconversion of o-xylene. The
same trend was registered for the selectivity of phthalicanhydride.
The selectivity of o-toluic acid (OTA) remained reverse trend to
thatof phthalic anhydride selectivity; hence it might be considered
as the majorintermediate for phthalic anhydride formation as showed
in the reaction scheme.Based on the activity study and
characteristics of catalysts, the vapor phaseoxidation of o-xylene
is proposed to take place as suggested in reaction scheme(Scheme
1). According to the scheme, molecular oxygen is activated by
frameworkvanadium. The activated O2 is inserted between carbon and
hydrogen bond of themethyl group in o-xylene. The resulting alcohol
is rapidly oxidized to o-tolaldehydewhich is also subsequently
oxidized to o-toluic acid. The same process is alsorepeated on
adjacent methyl group to yield phthalic acid. The product
issubsequently oxidized to phthalic anhydride.Comparison of the
catalyst supportsThe activity of V-MCM-41 (50), Mn-MCM-41 (50) and
V-Mn-MCM-41(V:Mn = 50:50), was measured at 573 K with the WHSV of
o-xylene 5.87 h-1(WHSV). The results are compared under the
optimized reaction conditions tounderstand the inuence of various
metals on the oxidation reaction and presentedin Fig. 9. Among the
three catalysts, it is the V-MCM-41 (50) catalyst that
exhibitedmaximum activity. The reason for the high activity of
V-MCM-41 (50) may be dueto the availability of silica matrix V5? in
MCM-41which is evident from DRUV-Vis123 11. V-Mn-MCM-41 catalyst
479Fig. 8 Effect of Si/V ratio on the oxidation of o-xylene over
V-MCM-41. Reaction conditions:temperature = 573 K, catalyst weight
= 0.3 g, WHSV = 5.87 h-1 and ow rate of air 0.02 mol h-1;reaction
time = 120 minO O . CH2 HOOV O2V CH2 OH OO O O CH3O
OSiSiSiSiCH3SiSi fast CHO CH3OC COOHCOOHRepeated O fast CCOOHCH3
OScheme 1 Vapor phase oxidation of o-xylene to phthalic anhydride
123 12. 480 C. Mahendiran et al.100% of Conversion &
Selectivity80 60Conversion 40Selectivity 200V-MCM-41- Mn-MCM-41-
V-Mn-MCM-41-(50+50)Fig. 9 Comparison of activity of the catalysts
for the oxidation of o-xylene. Reaction conditions:temperature =
573 K, weight of the catalyst = 0.3 g, WHSV = 5.87 h-1 and ow rate
of air0.02 mol h-1; reaction time = 120 minspectra (Fig. 3).
Further evidence of the elemental analysis results also (Table
1,ICP-AES) reveals that decrease the Si/V ratios (from 100 to 25)
there is increase theincorporated metal content into the silica
matrix. Hence, it is concluded that silicamatrix V5? was shown to
be more active for the oxidation of o-xylene to phthalicanhydride
[28]. Manganese (Mn) incorporated into the MCM-41 is expected
tosupport oxidative dehydrogenation of hydrocarbons because of the
presence ofsuccessive acidic and redox sites. However, during the
oxidative dehydrogenationof o-xylene, the strong aromaticity will
be lost signicantly. Hence, Mnincorporated MCM-41 does not support
the oxidation reaction of o-xylene tophthalic anhydride under these
experimental conditions. Finally, based on theliterature, it can be
understood that the poor activity of V-Mn-MCM-41 may be dueto the
presence of lower number of silica matrix V5? in V-Mn-MCM-41
catalyst.ConclusionsFrom the scrutiny of the above work, the
following conclusions can be drawn:1.Mesoporous V-MCM-41 molecular
sieves with Si/V ratio 25, 50, 75 and 100contains vanadyl ions
(VO2?) in the as-synthesized form, whereas oncalcination, vanadyl
ions (VO2?) is converted into highly dispersed V5?species with
tetrahedral coordination.2.Enhancement of the activity of MCM-41
for the vapor phase oxidation of o-xylene is achieved by
incorporating vanadium. The high activity of V-MCM-41(50) for
phthalic anhydride formation could be accounted due to the presence
oflarge amount of well dispersed V5? on V-MCM-41. Both UV-Vis DRS
and123 13. V-Mn-MCM-41 catalyst 481 EPR spectroscopies provide
valuable information about the surface structure of V-MCM-41
catalysts.3. When the activity of vanadium loaded MCM-41 is
compared with Mn and bimetal (V&Mn) loaded MCM-41, it is the
vanadium that is the most preferred metal for oxidation
reaction.Acknowledgments The authors would like to thank the
Defence Research and DevelopmentOrganization (DRDO) of India for
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