Phthalocyanine- and Calixarene-Templating Effect on the Catalytic Performance of Solid Supported...

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Catalysis Letters ISSN 1011-372X Catal LettDOI 10.1007/s10562-011-0662-7

Phthalocyanine- and Calixarene-Templating Effect on the CatalyticPerformance of Solid SupportedVanadates

Grigoriy Sereda, Taejin Kim, AubreyJones, Hari Khatri, ChristopherL. Marshall, H. Subramanian & RanjitT. Koodali

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Phthalocyanine- and Calixarene-Templating Effecton the Catalytic Performance of Solid Supported Vanadates

Grigoriy Sereda • Taejin Kim • Aubrey Jones •

Hari Khatri • Christopher L. Marshall •

H. Subramanian • Ranjit T. Koodali

Received: 22 April 2011 / Accepted: 28 June 2011

� Springer Science+Business Media, LLC 2011

Abstract Catalytic performance of solid supported mono-

vanadate catalysts toward oxidation of propane is signifi-

cantly affected by the templating effect of: (1) pretreatment

of the solid support with calixarene derivatives before

deposition of ammonium vanadate, (2) direct deposition of

vanadyl phthalocyanine as the vanadate precursor. This

effect is believed to be linked to the accessibility of the

active sites, and was studied in comparison with a reference

mesoporous support.

Keywords Supported catalysts � Oxidation �Heterogeneous catalysis � Catalysis

1 Introduction

Vanadium-based catalysts have been the center of attention

due to their activity toward industrially important synthetic

processes such as manufacturing sulfuric acid and alkenes.

The wide range of available coordination numbers and

oxidation states of vanadium (ranging from -3 to ?5)

significantly expands the potential of vanadium-based

catalysts toward various oxidative and reductive organic

transformations [1]. Catalysts containing vanadium oxide

supported on metal oxides, Al2O3, SiO2, TiO2, and ZrO2,

are employed in several industrial heterogeneous catalytic

processes, such as partial oxidation of methanol to form-

aldehyde, oxidation of o-xylene to phthalic anhydride,

ammoxidation of alkyl aromatics, oxidation of sulfur

dioxide to sulfur trioxide, selective catalytic reduction

(SCR) of nitrogen oxides (NOx) with ammonia to nitrogen

and water, and oxidative dehydrogenation of alkanes to

alkenes [2–7]. Mechanical strength, thermal stability, better

control of the catalyst’s structure, and therefore good

selectivity are the attributes of supported vanadium oxide

catalysts over bulk V2O5. The metal oxide-support inter-

actions in supported vanadium oxide catalysts affect both

the redox properties and the dispersion of the active sites

[8, 9]. Catalytic studies of vanadium oxide supported by

ZrO2, Al2O3, and SiO2 toward oxidative dehydrogenation

of methanol and alkanes have shown that the nature of the

solid support has a significant influence on the catalytic

performance of the material [10]. At low (\0.52 V/nm2)

catalyst loadings, vanadium oxide is more likely to be

highly dispersed, forming after calcination in air isolated

tetrahedral vanadate (V?5) species, while with the

increased loading, vanadium oxide species may form

polymeric (dinuclear, trinuclear, etc.) two-dimensional

networks of distorted tetrahedral and square-pyramidal

G. Sereda (&) � A. Jones � H. Khatri � H. Subramanian �R. T. Koodali

Department of Chemistry, The University of South Dakota,

414 E. Clark St., Vermillion, SD 57069, USA

e-mail: [email protected]

A. Jones


H. Khatri


H. Subramanian


R. T. Koodali


T. Kim � C. L. Marshall

Chemical Sciences and Engineering Division, Argonne National

Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA


C. L. Marshall


123

Catal Lett

DOI 10.1007/s10562-011-0662-7

Author's personal copy

oxovanadium clusters. At high loadings, three dimensional

V2O5 crystallites in octahedral coordination become abun-

dant [11]. It has been documented that monolayer- or sub-

monolayer of oxidized vanadium contains active sites

towards many reactions, whereas multilayer coverages are

inactive [12]. It was shown [13], that the V–O-solid support

bond is more significantly involved in the catalytic cycle of

oxidation, compared to V–O–V and V=O bonds. The dis-

persion of vanadium oxides in sub-monolayer to monolayer

quantities depends on the nature of solid support [13], nature

of sample preparation method (incipient wetness impreg-

nation [14, 15], grafting [16, 17], chemical vapor deposition

[18]), strength of support-vanadium oxide interactions,

loading density of vanadium oxide precursors [19], and the

type of ligand attached to the vanadium atom [20].

The distribution of the deposited vanadium oxide clus-

ters over the surface of the solid support can be charac-

terized by UV–vis Diffuse Reflectance Spectroscopy

(DRS), based on the known quantitative dependence of the

effective band gap energy on the number of vanadium

atoms in the clusters [21], and 51V solid state NMR [14],

which provide detailed information about the local coor-

dination number of the two dimensional surface vanadate

species on an oxide support. Deconvolution of the UV–vis

DRS Kubelka–Munk function with Gaussian functions was

introduced as an additional valuable tool for studying the

nature of vanadium oxide and vanadate species on the

surface of silica and mesoporous silica supports [22, 23].

Distribution of vanadium oxide deposited on zeolites was

studied by Temperature Programmed Reduction (TPR)

with H2 [24], Electron Spin Resonance (ESR), X-ray

Absorption Near Edge Structure (XANES), and photolu-

minescence spectroscopy [25]. There have been several

reported attempts to control deposition of vanadium oxide

by varying the nature of the precursor employed. In 2001, a

series of vanadium–calixarene complexes was synthesized

as molecular models for supported vanadate [26]. Recently,

the group of Alexander Katz has grafted mono- and dinu-

clear calixarene-bound vanadates to silica [27]. Calcination

of these materials led to silica-supported vanadates, whose

‘‘Edge energy’’ determined by UV–vis DRS depended on

the number of vanadium atoms in the calixarene-ligated

precursor. Unfortunately, catalytic performance of these

materials was not tested. Though the ‘‘Edge energy’’ shift

was most likely driven by at least partial formation of

calixarene-templated formation of divanadates during cal-

cination, different vanadium loadings used in the study led

to some ambiguity to the interpretation of the true nature of

the supported vanadia species. Further, a study of possible

mobility of once-deposited vanadium oxide (especially, in

the presence of moisture [28]) suggests that the structure of

the final material does not depend on the vanadium oxide

precursor [29]. It inspired us to thoroughly explore vanadia

supported catalysts with the same vanadium loadings, but

prepared differently. A recent 51V-NMR study [14]

revealed that the vanadium precursor affects the strength of

interaction between c-Al2O3 and deposited vanadate spe-

cies at the density of 4 V-atoms/nm2. Interestingly, neither

the structure of the surface vanadate species nor their

reducibility with H2 was significantly affected. Unfortu-

nately, catalytic performance of the prepared series of

materials was not tested toward any other reactions. While

a large body of research is done on c-alumina, practical

application of its more thermally stable h-form as solid

support for catalysts is worthy of a detailed study.

In this paper, we report how deposition of vanadate on

h-Al2O3, TiO2, and SiO2 is affected by three modes of

organic templating: (I) Use of vanadyl phthalocyanine 1 as

the vanadate precursor. (II) Complexation of the NH4VO3-

borne vanadium with octa-tert-butylcalix[8]arene 2, pre-

physisorbed on a solid support, and (III) Complexation of

the NH4VO3-borne vanadium with sodium calyx[8]arene-

octa-sulfonate 3, covalently pre-attached to a solid support.

2 Experimental Section

2.1 Preparation of Catalysts

Vanadyl phthalocyanine 1, solid supports, other chemicals

and solvents were obtained from commercial sources.

Octa-tert-butylcalix[8]arene 2 and sodium calix[8]arene-

octasulfonate 3 were synthesized by known procedures [30,

31]. For the non-templated reference materials (A01C,

A02C, T01C, and S01C, Table 1), vanadate was deposited

on the corresponding solid support, using NH4VO3–

H2C2O4 as vanadate precursor, according to the known

technique [32].

Vanadium loadings of 0.25 or 0.5 V/nm2 were targeted.

The prepared materials were dried in air at 200 �C for 2 h

and calcined in static air at 500 �C for 4 h. The actual

vanadium loads were calculated on the basis of surface

analysis of the final catalyst and presented in Table 2.

2.1.1 Deposition of Vanadyl Phthalocyanine

For the catalysts, templated by vanadyl phthalocyanine 1, a

calculated (for 0.25 or 0.5 V/nm2) amount of 1 in a frit

funnel was placed inside a Soxhlet extraction chamber. The

appropriate solid support (1 g) in 100 mL of chloroform

was placed in the evaporation part of the Soxhlet extractor.

Due to the low solubility of 1 in chloroform, the full

extraction typically required about 200 cycles (100 h). The

prepared materials were dried in air at 140 �C until the

constant weight (typically 1 h) and finally calcined in air at

500 �C for 4 h.

G. Sereda et al.

123


2.1.2 Calixarene-Templated Deposition of Ammonium

Vanadate

For the catalysts, templated by calixarene derivatives 2 and

3 (Fig. 1), the derivative 2 in chloroform or derivative 3 in

ultrapure water were deposited on the appropriate solid

support by incipient wet impregnation, targeting the den-

sity of 1 molecule of the template per 4 nm2 of the solid

support surface. The prepared materials were dried at

140 �C until the constant weight (typically 1 h). Finally,

vanadate was deposited on the corresponding solid support,

using NH4VO3–H2C2O4 as vanadate precursor, according

to the known technique [32] keeping the V to calixarene

ratio of 1:1. The prepared materials were dried in air at

200 �C for 2 h and calcined in air at 500 �C for 4 h.

2.1.3 Impregnation of Vanadium into Mesoporous Silica

V-MCM-48 cubic mesoporous material was prepared using

a modified Stober’s synthesis developed previously for

siliceous MCM-48 at room temperature [33]. In a typical

synthesis 1.2 g of hexadecyltrimethylammonium bromide

(CTAB) (Alfa Aesar) was placed into a polypropylene

bottle (150 mL). To this 50 mL of deionized water and

25 mL of ethanol (AAPER 200 proof) was added and

stirred well. Then, 0.0037 g of VOSO4�3H2O (Aldrich) was

added slowly. To this solution, 6 mL of NH3 (*25 wt%,

Fisher) was added followed by 1.8 mL of tetraethoxysilane

(TEOS) (Aldrich). The resulting suspension was stirred for

4 h at room temperature at a stirring rate of 300 rpm. The

precipitate obtained was filtered and washed with deion-

ized water extensively. The solid was dried at 80 ± 10 �C

overnight in an oven. The dried powder was ground to a

fine powder and calcined in a muffle furnace at 550 �C in

static air at a heating rate of 3 �C/min for 6 h to remove the

surfactant molecules. This procedure was a significant

improvement over the tedious and lengthy hydrothermal

technique reported previously for a similar material [22,

23, 34].

2.1.4 Notation of the Catalysts

Each material mentioned in this manuscript except meso-

porous V-MCM-48 is assigned a four-digit code according to

Table 1. For instance, a sample, prepared by vanadyl phtha-

locyanine-templated deposition of vanadate (0.25 V/nm2)

on Al2O3 followed by calcination, will be coded as AP1C. Its

uncalcined precursor will be AP1U.

2.2 BET Specific Surface Area

N2 adsorption–desorption studies were carried out at liquid

N2 temperature (77 K) using a NOVA 2200e series appa-

ratus. The surface areas were calculated by using the

Brunauer–Emmett–Teller equation in the relative pressure

range (P/P0) of 0.05–0.30. The samples were degassed at

100 �C for at least 1 h prior to the isotherm measurements.

2.3 Raman-Spectroscopy

The Raman spectra of the dehydrated supported vanadate

catalysts were obtained with UV (325 nm) excitation. The

UV laser excitation was generated from a He-Cd laser

(Kimmon, Model IK3401R-F). The scattered photons were

directed into a triple spectrometer (Princeton Instruments

Acton, Trivistra 555) and focused onto a liquid-nitrogen-

cooled CCD detector (Princeton Instruments, Model

Table 1 Notation of the catalysts

First digit (type of support) Second digit (organic template) Third digit (vanadium load) Fourth digit (preparation)

A (h-Al2O3) O (no template) 1 (0.22–0.34 V/nm2) C (calcined)

T (TiO2) P (vanadyl phthalocyanine 1) 2 (0.52–0.53 V/nm2) U (uncalcined)

S (SiO2) G (calixarene 2)

A (calixarene sulfonated 3)

Table 2 Preparation and surface analysis of supported vanadate

catalysts

Catalyst Surface

area

(m2/g)

Surface area

of solid support

(m2/g)

Vanadium

source

Vanadium

load

(V/nm2)

A01C 73.3 78.4 NH4VO3 0.27

A02C 75.6 78.4 NH4VO3 0.52

AP1C 65.3 78.4 1 0.30

AP2C 75.1 78.4 1 0.52

AC1C 57.9 78.4 NH4VO3 0.34

AA1C 45.1 55.2a NH4VO3 0.31

T01C 39.5 56.8 WH4VO3 0.32

TP1C 48.4 56.8 1 0.29

S01C 482 419 NH4VO3 0.22

SP1C 423 419 1 0.25

V-MCM-48 1,719 – – 0.057

a Surface area after deposition of the organic template

Phthalocyanine- and Calixarene-Templating Effect

123


7439-0002). The laser power delivered to the sample was

2 mW. The catalysts were maintained in loose powder

form in a fluidized bed reactor. The catalysts where ini-

tially dehydrated at 500 �C for 1 h in flowing 5% O2/N2

(60 mL/min, Linde gas) and the Raman spectra of

the dehydrated samples were collected after cooling the

catalysts back to room temperature in the flowing He

(60 mL/min, Airgas, ultrapure carrier grade). Due to the

low vanadium loads in studied materials, we increased the

signal/noise ratio by performing 6 scans at the acquisition

time of 10 min.

2.4 Powder X-ray Diffraction

The X-ray diffraction studies were performed on a Scintag

Pad V X-ray diffractometer with DMSNT data acquisition

and analysis software. The diffractometer was operated at

40 kV and 40 mA and the low angle regions were scanned

from 2 to 6� (2h) with a step size of 0.02�. XRD-analysis of

TP1C and TP1U showed that the ratio of TiO2 anatase to

rutile is 7.14:1. The h-phase of the Al2O3 and structure of

V-MCM-48 (Fig. 2) were also confirmed by XRD using a

Rigaku Ultima IV XRD instrument (voltage = 44 kV,

current = 40 mA, scan speed = 1�/min and scan width of

0.02�). Raman spectroscopy has also provided additional

proof for the h-phase of Al2O3 support. The powder XRD

pattern of the calcined V-MCM-48 mesoporous material in

Fig. 2 is consistent with previous reports [34]. The XRD

patterns indicate two peaks below 2h\ 3.5� that are due to

the d221 and d220 reflections. Additional proof for the

existence of the cubic phase comes from the presence of

three peaks in the region of 2h = 3.5–6� due to the pres-

ence of d420, d332, and d422 reflections. The Si/V ratio was

estimated to be 200.

2.5 Steady State Propane Oxidation

Steady state propane oxidation was studied in a plug flow

reactor. The catalysts were pre-treated in flowing 20% O2/

Ar (15 mL/min) at 150 �C for 2 h and 500 �C for 2 h.

Propane (3% C3H8/Ar) and oxygen (20% O2/Ar) were

simultaneously passed through the reactor. The total flow

rate was 5.5 mL/min for catalysts TP1C, T01C, and AA1C,

and 6.9 mL/min for all other samples. The O2:C3H8 molar

ratio was adjusted to 2:1 by gas chromatography (GC)

analysis. All catalysts were tested at 350, 400, 450, and

Fig. 1 Chemical structures of organic templates: vanadyl phthalocyanine 1, octa-tert-butylcalix[8]arene 2, and sodium calix[8]areneoctasulf-

onate 3

G. Sereda et al.

123


500 �C. Reactions were carried out in a quartz tube and the

reactor was packed with quartz wool above and below the

catalyst bed. The catalyst bed temperature was maintained

with external temperature control and the catalysts bed

temperature was measured with a thermocouple in contact

with the sample. The reaction products were analyzed

using a GC equipped with a TCD or FID. For all samples

the space velocity was maintained in the range of

0.011–0.013 mol C3H8 mol V-1 s-1. For samples TP1C,

T01C, and AA1C, the propane flow rate was 0.16 mL/min.

For all other samples, propane flow rate was 0.21 mL/min.

The kinetic and selectivity data of the studied catalysts are

summarized in Table 3.

The energy of activation and pre-exponential coefficient

were calculated from the Arrhenius plot. The turnover

frequency (TOF) was calculated based on the observed

extent of propane conversion, surface area of the catalyst,

and the vanadium load as follows:

Conversion ð%Þ ¼ # of moles of C3H8 consumed

# of moles of C3H8 fedð1Þ

TOF ðs�1Þ ¼ # of C3H8 molecules reacted

# of active sites � sð2Þ

2.6 Temperature Programmed Reduction

TPR was carried out in Altamira system. Samples of

*40–90 mg were loaded into a quartz tube and calcined

in air at 500 �C for 1 h. The samples were then cooled to

room temperature in air (50 mL/min). The H2-TPR pro-

files were monitored by then flowing H2–Ar gas (2.88%

H2/Ar) at a flow rate of 20 mL/min with a heating rate

8 �C/min in the temperature range 25–900 �C. The ther-

moconductivity detector (TCD) signal was recorded and

plotted against temperature, and the sample was cooled to

room temperature in helium. The temperature maxima

were determined from the TCD-signal using the Screen-

reader option of Origin software. The TPR-data are

summarized in Table 4.

2.7 UV–Vis Diffuse Refluctance Spectroscopy (DRS)

UV–vis Diffuse Refluctance Spectroscopy was performed

on a Varian Cary 5000 UV–vis-NIR absorbance spectro-

photometer with a Praying Mantis diffuse reflectance

accessory. The baseline was corrected for the bare support

material, and the Kubelka–Munk function was used to

convert reflectance data into absorption spectra, which

were deconvoluted with the Lorentz functions using Origin

software. Samples were kept at 150 �C overnight prior to

analysis. The data were collected shortly after dehydration

under ambient conditions.

2 3 4 5 6

d332d

420

d422

d220

d221

2Θ (Degrees)

Inte

nsity

(ar

bitr

ary

units

)

Fig. 2 Powder X-ray diffraction of V-MCM-48 mesoporous mate-

rial. Deconvolution lines are shown in grey

Table 3 Kinetic parameters

and reactivity data for propane

oxidation

Catalyst Eact, kJ/mol Ko Selectivity at 450 �C (%) Propane

conversion

at 450 �C (%)

TOF at 450 �C,

Propanemol

Vat-1 S-1CO2 CO Propene

A01C 14.0 1.5 * 100 100 0 0 49 3.3 * 10-3

A02C 113.5 5.7 * 105 100 0 0 2 5.2 * 10-5

AP1C 112.6 1.3 * 107 33 53 14 27 1.2 * 10-3

AP2C 108.0 5.1 * 105 100 0 0 4 1.2 * 10-4

AC1C 37.6 6.5 * 100 100 0 0 30 3.1 * 10-3

AA1C 79.5 4.6 * 104 100 0 0 27 1.3 * 10-3

T01C 60.4 4.8 * 102 100 0 0 9 4.5 * 10-4

TP1C 58.4 2.0 * 103 48 47 5 45 4.0 * 10-3

S01C 62.0 1.9 * 102 100 0 0 3 1.5 * 10-4

SP1C 70.5 1.3 * 104 100 0 0 32 2.3 * 10-3

V-MCM-48 19.0 3.2 * 100 100 0 0 36 4.5 * 10-3


123


2.8 Elemental Analysis

Elemental analyses were performed on the Exeter Analyt-

ical, Inc CE-440 elemental analyzer configured for the C,

N, S mode (oxygen combustion oven at 1,080 �C and

copper reduction oven at 820 �C).

3 Results and Discussion

3.1 h-Al2O3-Supported Vanadate

The specific catalytic activity, expressed as TOF, product

selectivity, and kinetics parameters for steady state pro-

pane oxidation over the V2O5/h-Al2O3 catalysts are

summarized in Table 3. First, we found that catalytic

activity and selectivity of supported vanadate catalysts at

very low vanadium loading (0.27–0.30 V/nm2) for the

oxidation of propane significantly depends on the vana-

date precursor used for the deposition. A reference sample

(A01C) prepared by incipient wet impregnation of aque-

ous NH4VO3–H2C2O4 effectively catalyzed full combus-

tion of propane to CO2 with high conversion (49%). Such

behavior corresponds to low values of apparent activation

energy (*14 kJ/mol) and low pre-exponential coefficient

(*1.5) characteristic for the presence of highly active

metal sites with low accessibility. Presence of different

types of Al–OH binding sites on the surface of h-Al2O3 is

consistent with a reported theoretical and experimental

study [35], which suggested that at the surface density of

0.16 V/nm2, vanadium tends to bind with the terminal

Al–OH configuration, which is the most basic among all

possible AlmOH configurations (m = 1, 2, 3). The possi-

bility of different modes of vanadate binding to c-Al2O3

at higher surface densities (4 V/nm2) was also proved by

the 51V-NMR [14].

3.1.1 Vanadyl Phthalocyanine Precursor

We selected to deposit a monomolecular or submonomo-

lecular density layer of this precursor to prevent formation

of vanadium clusters during calcination and focus on the

interaction of the isolated vanadate species with the solid

support. Elemental analysis did not reveal any significant

amounts of residual carbon and nitrogen in the catalysts.

Specifically, for sample AP1C (%C: 0.09, 0.03; %N: 0.01,

0.00), and AP2C (%C: 0.11, 0.12; %N: 0.01, -0.04).

Organic template effect of the phthalocyanine ligand led to

dramatic increases in both the activation energy and the

pre-exponential coefficient with corresponding decreases in

the propane conversion and TOF of the calcined sample

(sample AP1C, Table 3). Most importantly, the templating

effect of the phthalocyanine ligand significantly changed

the selectivity for propane oxidation. While A01C cata-

lyzed only combustion to CO2, at 450 �C, AP1C showed

53% and 14% selectivity for CO and propene, respectively.

Reducibility of the sample was affected as well. Figure 3

and Table 4 show that while most of A01C reduction

occurred at 687 �C, this maximum shifted to 607 �C with

AP1C.

However, the observed shift could not be explained by

oligomerization of monovanadate, which is usually attrib-

uted to this change. Combination of the UV–vis DRS and

UV-Raman spectroscopy techniques allow for the under-

standing of the surface species molecular structure. Fig-

ure 4 and Table 5 show the UV–vis DRS spectra of A01C

and AP1C catalysts.

Although catalytic activity, selectivity, and reducibility

were affected by organic template, the UV–vis DRS

Table 4 Maxima TPR peaks for supported vanadate catalysts

Catalyst Peak #1

Max. (�C)

Peak #2

Max. (�C)

Peak #3

Max. ( �C)

Peak #4

Max. (�C)

A01C 435 687 - -

A02C 422 653 - -

AP1C 435 607 - -

AP2C 422 564 - -

AC1C 441 679 - -

T01C 386 512 687 752

TP1C 422 538 567 708

S01C 378 419 597 -

SP1C 409 582 - -

V-MGM-48 331 414 660 -

200 300 400 500 600 700 800 900

679

441

(c)653

422(d)

(e)

564

422

607435

435 687

(a)

(b)

TC

D s

ign

al, a

.u.

Temperature, oC

Fig. 3 TPR profiles of the supported vanadate on h-alumina

catalysts. (a) A01C, (b) AP1C, (c) AC1C, (d) A02C, and (e) AP2C

G. Sereda et al.

123


spectra of both catalysts are very similar. This result sug-

gests that the strength of interaction between vanadium and

support was affected by organic template, while the surface

vanadium species’ coordination number was not. Both

A01C and AP1C spectra consist of a single ligand-to-metal

transition (LMCT) at 270–280 nm which is assigned to

isolated tetrahedral oxovanadate species on the surface

[36].

Lack of the absorption band at 600 nm characteristic for

the V(IV) species [36, 37] for all analyzed catalysts points

at the complete oxidation of vanadium to V(V) under the

calcination conditions (500 �C in air). However, according

to Ref. [36], traces of V(IV) may remain at the non-

accessible sites of vanadium-impregnated mesoporous sil-

ica, such as inside the walls. Further, due to the apparent

involvement of V(IV) in the catalytic cycle of propane

oxidation, presence of its trace amounts in the material is

unlikely to significantly affect the catalytic performance.

Unfortunately, attempts to employ 15V NMR spectroscopy

to interrogate the deposited vanadium species for their

oxidation state and aggregation were unsuccessful due to

the low vanadium loadings in the explored materials.

Raman spectroscopy is one of the most powerful tools

for the investigation of heterogeneous catalyst surface

structure [38]. The in situ Raman spectra of the dehydrated

A01C and AP1C catalysts are shown in Fig. 5. The Raman

band at *1,008 cm-1 arise from the stretching vibration of

the terminal V=O bond of the monooxo surface VO4 spe-

cies [38].

In addition to the vanadate signals, intense Raman bands

at *255, *407, *457, *563, *740, *774, and

*837 cm-1 arising from the h-Al2O3 support are observed

[39]. As expected, the Raman bands from crystalline V2O5

(*995 cm-1) are not present in any of the catalysts. Only

the higher vanadium load AP2C material exhibited an

extremely weak (almost at the background noise level)

band at *900 cm-1 that might belong to the V–O–V

200 300 400 500 600 700 800

~214 nm~271 nm

F(R

INF)

Wavelength (nm)

(e)

(d)

(b)

(a)

(c)

Fig. 4 UV–vis DRS spectra of the supported vanadate on h-alumina

catalysts. (a) A01C, (b) AP1C, (c) AC1C, (d) A02C, and (e) AP2C.

Deconvolution lines are shown in grey

Table 5 Maxima of the UV–vis DRS absorption bands of supported

vanadate catalysts

Catalyst Band #1

kmax (nm)

Band #2

kmax (nm)

Band #3

kmax (nm)

Band #4

kmax (nm)

A01C 214 263 - -

A02C 219 266 - -

AP1C 212 269 - -

AP2C 215 270 - -

AC1C 212 267 - -

AA1C 212 264 - -

S01Ca 291 - - -

SPICa 286 - - -

AP1U 452 493 592 661

AP2U 423 476 577 647

a Besides the interpolated band, the materials exhibited flat absorp-

tion at k[ 500 nm

200 400 600 800 1000 1200

1008

900

AP2C

AP1C

457

407

563

740 77

4

983

837

255

A01C

Rel

ativ

e In

ten

sity

(a.

u.)

Raman Shift (cm-1)

AlAlAl

OOO

V

O

Fig. 5 In situ Raman spectra of supported vanadate on h-alumina

catalysts under dehydrated conditions: A01C, AP1C, and AP2C


123


stretch. However, assignment of this band in literature is

controversial. While originally this band was attributed

to the V–O–V stretching vibration [40], recent research

[41, 42] supports its assignment to the V–O-support group.

The 325 nm excitation line is located sufficiently away

from the ligand-to-vanadium charge transfer occurring at

222 nm to cause significant resonance enhancement of the

monovanadate V=O vibration that might skew the con-

clusions. It is reported [43] that even excitation at 244 nm

allows to observe the V–O–V Raman bands, albeit leading

to resonance enhancement of one out of three V=O bands.

The results of the Raman spectra coincide with UV–vis

DRS results and provide no evidence of vanadate oligo-

merization, which is not surprising for such low vanadate

loads. Therefore, presence of the phthalocyanine ligands

does not make polymerized oxovanadium, but rather

affects local structure of the deposited reaction sites. The

templating effect of the phthalocyanine ligands was found

to be much less significant at twice the vanadium load

(0.52 V/nm2), when initial active sites on the alumina

surface become saturated, and selectivity of vanadate

deposition decreases. Thus, neither the activation energy

nor the pre-exponential coefficient or selectivity for oxi-

dation of propane were significantly different for the A02C

(reference) and AP2C (phthalocyanine-templated catalyst,

Table 3). In contrast to the AP1C catalyst, the catalytic

activity and selectivity of AP2C catalyst exhibits lower

activity (conversion and TOF) and 100% selectivity to

CO2. The TPR-profiles of AP2C and AO2C (Fig. 3) show

that most of A02C reduction occurred at 653 �C and this

maximum shifted to 564 �C with AP2C. The maximum at

564 �C for AP2C is also much lower than for AP1C. In

conclusion, a small change of local structure created by

increasing the vanadium surface density dramatically

affected the kinetics values, catalytic activity, and reduc-

ibility of the catalyst. However, the UV–vis DRS and

Raman spectra showed that both A02C and AP2C have a

similar band (*270 nm) with A01C and AP1C and the

Raman spectrum of AP2C possesses similar bands with the

AP1C spectrum shown in Fig. 5. Based on the UV–vis

DRS and Raman spectra, we can conclude that the series of

phthalocyanine-templated catalysts possess only the iso-

lated tetrahedral oxovanadate species and these surface

structures are very similar to the reference samples. From a

separate experiment, we can infer that the difference in the

templating effect on the vanadate deposition for the

0.27–0.30 V/nm2 (catalysts A01C, AP1C) and 0.52 V/nm2

(catalysts A02C, AP2C) was not caused by the aggregation

of the deposited vanadyl phthalocyanine precursors

(materials AP1U and AP2U). UV–vis DRS-spectra of

AP1U and AP2U contain a very weak Q-band (*650 nm)

of the same intensity, which is not characteristic for the

phthalocyanine aggregation in solution (Table 5) [44].

3.1.2 Calixarene–Ammonium Vanadate Precursor

Next, we explored how pretreatment of the alumina surface

with macrocyclic octa-tert-butylcalix[8]arene 2 at the

density of 1 molecule/4 nm2 before the wet impregnation

with NH4VO3 (0.25 V/nm2) affects formation of the van-

adate active sites. As opposed to a pre-prepared vanadium

macrocyclic complex (vanadyl phthalocyanine), complex-

ation of the phenol groups of the physisorbed 2 with

vanadium is less likely to prevent clustering of the vana-

date species during calcination, but was expected to alter

the mode of vanadium interaction with the solid support.

We employed the calixarene–ammonium monovanadate

system as the precursor alternative to vanadyl phthalocy-

anine only for the h-Al2O3 solid support, whose basicity

strengthens interaction with acidic calixarenes. Comparing

to AP1C (vanadyl phthalocyanine), the activation energy

and pre-exponential coefficient of AC1C are much lower

and only CO2 was produced on both catalysts. Neither TPR

nor UV–vis DRS revealed significant changes in the

characteristics of AC1C versus the reference A01C.

In addition to macrocyclic octa-tert-butylcalix[8]arene 2

and vanadyl phthalocyanine 1 organic templates, we

investigated calixarene sulfonate (AA1C) effect on the

catalytic activity and kinetic parameters of oxidation.

Elemental analysis of the AA1C catalyst did not reveal any

significant amounts of residual carbon and sulfur (%C:

0.28, 0.23; %S: 0.06, 0.08). As expected, sodium

calix[8]areneoctasulfonate 3, which is able to bind to alu-

mina chemically due to the salt formation, showed higher

activation energy and pre-exponential coefficient values

than the physisorbed octa-tert-butylcalix[8]arene catalyst

(AC1C). However, these values were much lower than for

the phthalocyanine-templated catalyst (AP1C), and neither

CO nor propene was detected among the reaction products

at 450 �C. The UV–vis DRS spectra for this series of

materials were very similar, which suggests the presence of

exclusively isolated tetrahedral oxovanadate on the surface

(Table 5, spectra not shown for brevity). In summary,

activity of supported vanadate on h-alumina samples for

propane oxidation decreased in the following order:

A01C [ AP1C, AC1C, AA1C [ A02C, AP2C, while for-

mation of CO and propene was observed only for the

catalyst templated by the phthalocyanine ligand (AP1C).

3.2 TiO2-Supported Vanadate


Titania-supported vanadate catalysts were studied to com-

pare the templating effect with the Al2O3-series. Similarly,

the vanadyl phthalocyanine precursor was selected to

G. Sereda et al.

123


minimize oligomerization of monovanadates. Elemental

analysis of TP1C did not reveal any significant amounts of

residual carbon and nitrogen in the catalysts (%C: 0.01,

0.03; %N: -0.10, 0.02). Table 3 shows that the activation

energy for TP1C catalyst, prepared with the phthalocyanine

precursor, is close to the one of T01C (no template), but the

pre-exponential coefficient had nearly quadrupled. The

catalytic activity increased from 9 to 45% with introduction

of the phthalocyanine template. The lack of significant

template effect on the activation energy and increased

propane conversion and TOF for the TiO2-supported cat-

alysts was quite contrasting with its presence in the Al2O3-

series. Importantly, the selectivity trend for CO2, CO, and

propene was similar to the Al2O3 supported catalyst. Thus,

selectivity at 450 �C had shifted from 100% for the refer-

ence combustion catalyst (T01C) to 47% CO and 5%

propene (Table 3), which was similar to the Al2O3-series

trend. Therefore, shift of the reaction selectivity toward

formation of propene and CO can be clearly linked to the

templating effect of vanadyl phthalocyanine.

Figure 6 shows the reduction profiles of titania sup-

ported vanadate catalysts. The TPR profile of T01C sample

is added as a reference. The profile of TP1C shows

reduction peaks at 422, 538, 567, and 708 �C. These values

are higher than for the reference T01C, which suggests

lower reducibility of TP1C versus the reference T01C.

The structural features of surface vanadate species on

titania were explored by UV-Raman spectroscopy. The

Raman spectrum of dehydrated vanadate/TiO2 samples is

presented in Fig. 7, which includes the spectra of T01C for

comparison. The weak intensity of the V=O stretching

band at 959 cm-1 is present for both samples. The strong

vibrations of the TiO2 support at lower values (\700 cm-1)

were easily distinguished by the strong Raman bands at

400, 520, 638, and 827 cm-1 in Fig. 7 [45]. Based on the

UV–vis and Raman spectra results, we can conclude that

the surface vanadate species on the titania consists mainly

of isolated surface monovanadate.

3.3 SiO2-Supported Vanadate


As for other phthalocyanine-templated catalysts, elemental

analysis of SP1C did not reveal any significant amounts of

residual carbon and nitrogen (%C: 0.06, 0.09; %N: -0.13,

0.00). The propane oxidation with supported vanadate on

SiO2 demonstrated that both the reference (S01C) and

phthalocyanine-templated materials (SP1C) catalyzed only

combustion of propane to CO2 (Table 3). However, the

activation energy and pre-exponential coefficient for SP1C

were higher than for the reference S01C, leading to a

nearly 10 fold increase of TOF and propane conversion.

The templating effect of both phthalocyanine ligands and

deposited calixarenes always leads to a significant increase

of the pre-exponential coefficient, compared to the refer-

ence non-templated samples A01C, T01C, and S01C.

Perhaps, the bulkiness of the templating agent prevented

deposition of vanadate deep inside the pores of the solid

support and, therefore, increased the accessibility of the

active centers to the reactants. In the TiO2 and SiO2 series,

this effect led to the increase of TOF and propane con-

version, while in the h-Al2O3 series, it was counteracted by

the significant increase of the activation energy. In this

respect, as shown in Fig. 8, the lower temperature reduc-

ibility of SP1C (major TPR peak at 582 �C) with H2,

200 300 400 500 600 700 800 900

567

708

422

512

538

386

687 752(a)

(b)

TC

D s

ign

al, a

.u.

Temperature, oC

Fig. 6 TPR profiles of the supported vanadate on titania catalysts:

(a) T01C and (b) TP1C

200 400 600 800 1000 1200

TiTiTi

OOO

V

959

827

319

638

520

TP1C

T01C

Rel

ativ

e In

ten

sity

(a.

u.)

Raman Shift (cm-1)

400 O

Fig. 7 In situ Raman spectra of supported vanadate on titania

catalysts under dehydrated conditions: TP1C and T01C


123


compared to S01C (major TPR peak at 597 �C) is not

surprising.

At similar surface density (\0.3 V/nm2), the alkane

TOF for SP1C was similar to AP1C, but lower than TP1C.

This result demonstrates that catalytic activity should be

related to support, as well as organic template.

Figure 9 and Table 5 show the UV–vis DRS spectra of

the reference (S01C) and phthalocyanine-templated mate-

rials (SP1C). All spectra show UV band near 287 nm,

which is characteristic for the presence of isolated tetra-

hedral oxovanadate species on the surface [22]. As opposed

to the alumina series, we did not observe highly reactive,

but inaccessible reaction centers for silica-supported van-

adate at the 0.22–0.25 V/nm2 surface coverage (S01C). In

order to achieve a similar effect for the silica-based

material, we have synthesized a mesoporous silica mate-

rial with incorporated vanadate at a very low loading

(0.057 V/nm2). This catalyst (V-MCM-48) has very high

activity for complete combustion of propane. Similarly to

the alumina-supported A01C catalyst, the large extent of the

propane conversion (*36%) did not allow us for the precise

measurement of the activation energy, for which we

obtained a rough estimate of *19 kJ/mol (Table 3). The

low activation energy along with low pre-exponential

constant, *3.2, points to the presence of highly reactive,

but low accessible centers on the mesoporous surface.

Considering higher expected mobility of small H2 mole-

cules versus those of propane, the significant increase of

reducibility of V-MCM-48 (major TPR peak at 414 �C)

versus S01C (major TPR peak at 597 �C) was not surpris-

ing. Interestingly, the presence of organic templates affec-

ted not only distribution of vanadate on the solid support’s

surface, but the surface itself. Thus, hydroxyl-rich

molecules of calixarene derivatives significantly decreased

the surface area of the h-Al2O3-supported materials, per-

haps, due to increased cross-linking and dehydroxylation at

the step of calcination. This effect of the phthalocyanine

ligands was much weaker. As expected, among the catalysts

without templates, the most acidic alumina-supported cat-

alyst (A01C) has shows higher activity than its silica and

titania counterparts. Only vanadyl phthalocyanine tem-

plated catalysts supported by alumina and titania, produced

CO and propene, while the silica supported samples showed

100% selectivity toward combustion, which could not be

shifted by the templating effect. For the S01C catalyst, the

lower TOF comparing with its counterparts (A01C and

T01C) might be related to the lower rate constant (krds) for

the rate determining step. However, it is not clear whether

the lower krds value is related to a higher Ea or a lower pre-

exponential factor.

4 Conclusion

The templating effect of phthalocyanine ligands and cal-

ixarene molecules markedly affects properties of materials

200 300 400 500 600 700 800 900

331

(c)660414

582419

378

409

597

(a)

(b)

TC

D s

ign

al, a

.u.

Temperature, oC

Fig. 8 TPR profiles of the supported vanadate on silica and MCM-48

catalysts (a) S01C, (b) SP1C, and (c) V-MCM-48

200 300 400 500 600 700 800

F(R

INF)

(b)

(a)

Wavelength (nm)

~287 nm

Fig. 9 UV–vis DRS-spectra of the supported vanadate on silica and

MCM-48 catalysts: a S01C and b SP1C

G. Sereda et al.

123


prepared by deposition of vanadia on h-Al2O3, TiO2, and

SiO2 with the density about 0.25 V/nm2. In all experi-

ments, we observed increasing pre-exponential coefficient

toward oxidation of propane due to the increasing acces-

sibility of the introduced vanadate reaction centers. The

trends of all kinetic parameters and selectivity in the series

h-Al2O3, TiO2, and SiO2 are significantly influenced by

the templating effect. Using vanadyl phthalocyanine as

the vanadate precursor significantly shifts selectivity of

h-Al2O3, TiO2-supported propane combustion catalysts

toward CO and propene. This effect is sensitive to the

vanadium load. A calixarene derivative covalently attached

to the solid support affects the deposition of vanadate more

than its physisorbed counterpart. However, the templated

effect of the phthalocyanine ligands in the vanadyl phtha-

locyanine precursor was the strongest observed. Incorpo-

ration of the monovanadate centers into the mesoporous

silica framework led to an efficient combustion catalyst

with a very low activation energy. This material is a

promising target for the further research on the templated

vanadate catalysts.

Acknowledgments This work has been supported by the Director,

Office of Science, Office of Biological & Environmental Research,

Biological Systems Science Division, of the U.S. Department of

Energy under Contract No. DE-FG02-08ER64624, NSF-CHE-

0722632, NSF (EPSCoR Grants No. 0554609 and 0903804), and by

the State of South Dakota, University of South Dakota (Research

Excellence Development award, U.Discover Program), NSF NPURC

Grant 0532242 (support for Aubrey Jones and purchase of the ele-

mental analyzer). We thank Mr. Bruce Gray for performing elemental

analyses. Taejin Kim and Christopher L. Marshall acknowledge the

U.S. Department of Energy, Office of Basic Energy Science, under

Contract W-31-109-ENG-38 and DE-FG02-03-ER15457.

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Phthalocyanine- and Calixarene-Templating Effect on the Catalytic Performance of Solid Supported...

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