Supported chromia catalysts for oxidation of organic compounds

Applied Catalysis B: Environmental 27 (2000) 73–85

Supported chromia catalysts for oxidation of organic compounds The state of chromia phase and catalytic performance C.M. Pradier a , F. Rodrigues a , P. Marcus a , M.V. Landau b,∗ , M.L. Kaliya b , A. Gutman b , M. Herskowitz b b

a Laboratoire de Physico-Chimie des Surfaces, ENSCP, 11 rue P.’ et M.Curie, 75005 Paris, France Chemical Engineering Department, Blechner Center for Industrial Catalysis and Process Development, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

Received 28 November 1999; received in revised form 2 February 2000; accepted 20 February 2000

Abstract A series of 13 bulk transition metals oxides frequently used as components of full oxidation catalysts was tested in air oxidation of n-butane and ethylacetate (EA). Co3 O4 , Cr2 O3 , CuO and MnO2 ,displayed the best activity, about one order of magnitude higher than the others. Chromia displayed the highest CO2 productivity. The activity and CO2 selectivity of Cr2 O3 catalyst deposited on mineral supports (SiO2 , Al2 O3 , MCM-41) depended strongly on the supports nature, texture and chromia loading. XPS, EDAX, XRD, FTIR and TPR–TPO measurements displayed two forms of Cr3+ -oxide on silica-supported catalysts: bulk ␣-Cr2 O3 nanocrystals of 10–20 nm and a monolayer of chromia surface species chemically bonded to the support: –Si–O–Cr=O. The chromia nanocrystals detected by XRD displayed higher activity and selectivity in complete EA oxidation due to higher redox mobility of chromium cations on their surface compared with grafted chromium silicate species. The optimized Cr2 O3 /SiO2 catalyst showed high efficiency in wet oxidation of amino-3-phosphonopropionic acid, comparable with the best catalysts reported in the literature. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Wet oxidation; Chromia catalysts; Silica; Complete oxidation

1. Introduction Catalytic complete combustion of organic compounds in gas and liquid (aqueous) phases studied extensively over the past two decades was reviewed recently [1–4]. The transition metal oxide catalysts were particularly investigated [4–8]. MnO2 , Cr2 O3 , Co3 O4 and CuO, were found to be the most active for complete oxidation. The ranking of transition metal oxides according to their activity in full oxidation of hydrocarbons depends on the substrate nature and gen∗

Corresponding author.

erally was obtained based on overall conversion rates [4–8]. Several processes take place on oxide catalysts at elevated temperatures: oxidative dehydrogenation to olefins [9,10] cracking [5,10] and partial oxidation to oxygenates and CO [11]. Therefore, the exact ranking of oxide catalysts for full oxidation should be done based on the relative contributions of all the conversion routes. Deposition of transition metal oxides on mineral supports affects the chemical state that can change the activity/selectivity patterns [4,8,12,13]. Cr2 O3 oxide was a highly efficient catalyst for full combustion. While the oxidation of hydrocarbons and CO was studied in details with bulk Cr2 O3 oxide

0926-3373/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 1 4 2 - 9

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of different morphological appearance [14], little is known about the relations between the chromia state and catalytic performance of supported chromia catalysts in full oxidation of hydrocarbons. Supporting chromia on silica or alumina yielded different structural characteristics [15–17]. On silica the structure is close to unsupported chromia as a result of weak chromia-support interaction, while on alumina, highly dispersed Cr–O species are anchored to the supports surface with Cr4+ and Cr6+ being the dominant oxidation states. This should strongly affect the activity and CO2 selectivity of supported chromia catalysts. The scope of the work was two-fold: • to rank the transition metals oxides frequently used as components of full oxidation catalysts taking in account all the possible conversion routes in n-butane oxidation as model reaction and • to investigate the effects of supports nature, metal loading and dispersion of chromia on activity and selectivity in gas-phase oxidation of n-butane and ethylacetate (EA) and to correlate the catalytic performance to the surface-chemical and structural characterization of a series of supported chromia catalysts. The performance of optimal supported chromia catalyst was also tested in wet oxidation of amino-3-phosphonopropionic acid (APPA) and compared with other wet oxidation catalysts. The catalysts efficiency in carbon combustion and mineralization of heteroatoms was measured.

2. Experimental 2.1. Catalysts The V, Nb, Ti, Mo, W, Zr, Hf, Dy and Mn oxides were purchased from Aldrich. Their surface areas measured by BET (N2 ) are defined in parentheses as m2 /g: V2 O5 (1.0); Nb2 O5 (1.4); TiO2 (anatase) (10); TiO2 (rutile) (8); MoO3 (1); WO3 (3); ZrO2 (4), HfO2 (1.5); Dy2 O3 (8.5) and MnO2 (3.6). The bulk Cr2 O3 (8.0) was obtained by decomposition of chromium(III) acetate hydroxide Cr3 (CH3 CO2 )7 (OH)2 (Aldrich) at 550◦ C, CuO(8.2) and Co3 O4 (10.1) by decomposition of corresponding nitrates (Aldrich) at the same temperature. The phase purity of the oxides was tested by XRD. The texture characteristics of mineral sup-

ports used for chromia deposition (SiO2 -1022 and SiO2 -1030, PQ Corp.; ␥-Al2 O3 , Engelhard Al-4191; ␪-Al2 O3 , Engelhard Q311-09 and Si-MCM-41, Al-MCM-41 (4.6% Al2 O3 ) prepared according to procedures described in [18]) are listed in Table 2. Deposition of chromia on all the supports (extrudates or flakes 1.3–2 mm diameter) precalcined at 550◦ C was done by incipient wetness from aqueous solution of chromium(III) acetate hydroxide (Aldrich). The supports were evacuated followed by drying in air at ambient temperature for 18 h then at 120◦ C for 3 h and calcination at 550◦ C for 2 h. The chromia loading in the catalysts was regulated by varying the concentration of chromium salt in impregnation solution. The composite (Mn–Ce–O [19], Co–Bi–O [20], Ce–Zr–Mn–O [21]) and supported (Cu–Mn–La/Al–Zn–O [22]) wet oxidation catalysts were prepared according to procedures described in the literature. Their compositions are listed in Table 7. 2.2. Catalysts characterization The chemical composition of catalysts — contents of corresponding oxides (wt.%, average of five measurements at different points of the solid) was measured by EDAX method (JEM-35 microscope, JEOL Co., link system ANB-1000, Si–Li detector). Surface areas, pore volumes and pore size distributions were obtained from N2 adsorption–desorption isotherms using the conventional BET and BJH methods. The calcined samples were outgassed under vacuum at 250◦ C. Isotherms were measured at liquid nitrogen temperature with a NOVA-1000 (Quantachrome, Version 5.01) instrument. Infrared spectra were recorded by Nicolet Impact 410 FTIR spectrometer in KBr pellets (0.005 g sample and 0.1 g KBr), scan number 36, resolution 2 cm−1 and analyzed by OMNIC software. Wideangle XRD patterns were collected on a Philips difractometer PW 1050/70 (Cu K␣ radiation) with graphite monochromator. The data were recorded with the 0.02◦ step size, 2 s at every step. The peak positions and the instrumental peak broadening (β) were determined using silicon powder XRD spacing (NIST standard reference material 640b). The crystal domains size was determined using the Scherrer equation: l= Kλ/[(B2 −β 2 )0.5 cos(2θ /2)], where K=1.000; λ= 0.154 nm; B is the peak broadening at 2θ =54.7–54.9◦ .

C.M. Pradier et al. / Applied Catalysis B: Environmental 27 (2000) 73–85

The content of Cr2 O3 crystalline phase in silicasupported catalysts was calculated based on the integral intensities of (1 1 6) peak (2θ=54.86◦ ) compared with a calibration curve recorded with a series of mechanical mixtures 5–30 wt.% (bulk ␣-Cr2 O3 / SiO2 -1030). A VG ESCA lab, Mk II spectrometer, using the Mg K␣ X-ray source (1253.6 eV) and operating at constant pass energy (20 eV) was used for the XPS analyses. Powder of supported chromia deposited on SiO2 was pressed on a gold-plated grid to a thin layer. This ‘impregnated’ grid was deposited on an Al-coated sample holder. The pressed powder was then admitted to the UHV chamber and maintained at 350 K overnight to eliminate OH contamination. Some of the samples were treated in hydrogen or oxygen at 700 K in a preparation chamber prior to surface analysis. A well-defined procedure was used to process the spectra, satellite and background subtraction of peak synthesis. Reference spectra recorded with bulk chromia [23] were considered for binding energy comparison. The problems in analyzing insulating samples were first the calibration of the binding energies and then, the nonuniform charging of the (grid+powder) system. The first one was resolved by taking the binding energy of the 4f7/2 core level of gold while the second was tentatively overcome by choosing the C 1s level of carbon. This intrinsic reference was taken at 285.0 eV. Intensities of all spectra were measured after background subtraction. The thermoprogrammed reduction (TPR) and thermoprogrammed oxidation (TPO) experiments were carried out in AMI-100 Catalyst Characterization System (Zeton-Altamira) equipped with TCD detector. 0.3 g of catalyst was loaded and treated in 10 vol.% H2 –Ar, 25 cm3 /min (TPR) or 5 vol.% O2 –He, 25 cm3 /min (TPO) increasing the temperature from ambient to 550 (TPR) or 650◦ C (TPO) at 5◦ C/min. The amount of hydrogen or oxygen consumed by catalyst sample in a given temperature range (␮M/g) was calculated by integration of corresponding TCD signal intensities taking in account calibrated values obtained in separate experiments. 2.3. Catalytic tests Oxidative conversion of n-butane was studied at atmospheric pressure, 550◦ C and O2 /n-butane molar

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ratio=1.0 in a tubular titanium reactor equipped with on-line GC as described elsewhere [10]. 0.5–5.0 g catalyst pellets 0.4–0.8 mm diluted with quartz pellets at 1:5 ratio was loaded between two quartz layers. The butane WHSV was regulated over a wide range (0.3–68.0 h−1 ) to keep its conversion at about 15%. The O2 /n-C4 ratio of 1.0 was selected in order to keep the catalysts zone in the reactor isothermal. At higher O2 /n-C4 ratios with most efficient catalysts, the temperature could not be stabilized at high conversions because of intensive heat evolution. No internal or external mass transfer limitations were detected. Changing the pellet diameter between 0.4 and 1.0 mm and increasing the linear velocity of gases with the most active Co-oxide catalyst by a factor of 3 at constant WHSV did not affect the n-butane conversion. Selectivity to olefins, CO and CO2 , that was greater than 90%, was calculated based on the GC analysis. Other products were a result of n-butane cracking (CH4 , C2 H6 , C3 H8 ). Only trace of oxygenates and hydrogen was detected. Oxidation of EA was studied at atmospheric pressure, 250◦ C, O2 /EA molar ratio=10 and EA WHSV= 1 h−1 in a stainless steel 21 mm i.d.×35 cm long reactor. The air flow rate to the reactor was controlled by Brooks mass controller. EA was fed to the reactor by a syringe pump (Razel Inc.). 1–3 g of catalyst pellets diluted with quartz pellets at 1:5 ratio was loaded between the two quartz layers. Three independently controlled heating zones kept the axial temperature gradient in the catalyst layer to less than 5◦ C. The product was analyzed on two GC: Chrompack CP 9001 (FID detector, WCOT capillary column with CP-sil-5-CB static phase) for analysis of unconverted EA and products of its partial oxidation to ethane and acetaldehyde and Gow-Mac 580 (TCD detector HaySepQ 80/100 packed column) for analysis of the CO2 concentration. The wet oxidation of APPA in aqueous solution (9 g/l, total organic carbon (TOC)=2570 ppm, N= 750 ppm, P=1660 ppm) was carried out in a 3 l autoclave, manufactured by Autoclave Engineers. The reactor was charged with 1.0 l of liquid feed and 10 g of powdered catalyst. It was heated under stirring (1200 rpm) to 200◦ C. Then the oxygen pressure was adjusted to 1.0 MPa. The liquid was treated for 1.5 h and cooled quickly to room temperature. The liquid

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was analyzed for APPA content (HPLC, Shimadzu SPD-10A instrument, UV–VIS detector, wavelength 205 nm, Aminex HPX-87H column). Total organic carbon was measured with TOC-5000A instrument (Shimadzu Co.), total phosphorus content by ICP AES, instrument Optima-3000, Perkin-Elmer, total nitrogen was measured by Kieldahl method. PO4 −3 , NO3 − and NO2 − content was measured by ion chromatography, Dionex instrument, Ionpac AS4ASC column (250 mm×4 mm), eluent 1.8 mM Na2 CO3 / 1.7 mM NaHCO3 .

3. Results 3.1. n-C4 oxidation with selected transition metals oxides Four oxides: Cr2 O3 , Co3 O4 , CuO and MnO2 , in a decreasing order, displayed the highest activities, as shown in Table 1, in agreement with data for paraffins oxidation reported in [5]. The specific activity of the four oxides and V2 O5 , based on surface area, was similar and 1–2 orders of magnitude higher than the other oxides. Co3 O4 produced little olefins, but high level of CO. V2 O5 produced significantly less CO. Cr2 O3 and MnO2 produce no CO and considerable CO2 .

Table 1 Performance of selected transition metal oxides in n-butane oxidation Catalyst

V2 O5 Nb2 O5 TiO2 (anatase) TiO2 (rutile) MoO3 WO3 ZrO2 HfO2 Dy2 O3 Cr2 O3 Co3 O4 CuO MnO2

WHSV (h−1 ) (15% conversion)

6.7 0.8 0.9 0.5 0.4 0.3 2.7 2.2 7.9 58.6 68.0 48.0 24.3

Selectivity (wt.%) CO

CO2

Olefins

52.5 10.4 51.0 17.2 13.6 45.2 37.2 52.4 21.0 0.0 82.6 14.3 0.0

40.4 34.6 34.0 34.5 27.3 50.2 33.8 18.7 30.0 70.0 11.8 35.7 68.1

3 52 12 45 53 4 25 26 47 23 2 45 25

3.2. Supports and loading effects on Cr2 O3 performance in EA oxidation Six chromia catalysts deposited on different supports with similar chromia loading (10.5–13.9 wt.%) were tested in EA oxidation. The results are listed in Table 2. Silica-based catalysts displayed higher activity compared with chromia supported on alumina. Cr2 O3 /SiO2 -1022 yielded mostly CO2 while all others yielded ethane and acetaldehyde. Decreasing the surface area and acidity of alumina, also improved the EA full oxidation. Increasing the chromia loading on the SiO2 -1030 support increased the CO2 selectivity of the catalyst (Table 3). It was tentatively concluded that the support nature and chromia loading affect the chemical state or structure of chromium oxide at the catalyst surface thus changing the selectivity in EA oxidation. 3.3. XPS data characterization of Cr2 O3 /SiO2 Fig. 1 shows the core level of Si 2p3/2 , O 1s, C 1s and Cr 2p3/2 regions recorded with the 29 wt.% Cr2 O3 /SiO2 -1030 catalyst. The Si 2p3/2 line has a FWHM of 2 eV. It is centered at 108 eV. That is the Si 2p binding energy expected for SiO2 at 103.4 eV, showing a remaining charge shift in spite of the gold-platted grid support. The shift due to charging was evaluated from the difference between the observed BE of the Si 2p peak and its expected value at 103.4 eV [24]. The other peaks were affected by the same charge effect and consequently similarly shifted. The O 1s line of the catalyst could be fitted with two contributions. The most important was recorded at 532.8 eV with a shoulder at 532 eV (BE values have been corrected by the identified Si shift). These two peaks, O(I) and O(II), are attributed to oxygen from the bulk and from surface oxygen and remaining OH groups that could interact with chromia, respectively. The Cr 2p3/2 region consists of two peaks, one at 577 eV and the other at 580 eV (BE not corrected). The XPS analysis of the other silica-based chromia catalysts showed similar spectra in the same Si 2p3/2 , O 1s, C 1s and Cr 2p3/2 regions, but their BE and intensity ratios were substantially affected by silica type, chromia loading and catalysts pretreatment (Table 4). No substantial differences in the electrons

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Table 2 Effect of support nature on the efficiency of supported Cr2 O3 in EA oxidation Support

␥-Al2 O3 ␪-Al2 O3 SiO2 -1022 SiO2 -1030 Si-MCM-41 Al-MCM-41

Supports properties Surface area (m2 /g)

Pore volume (cm3 /g)

Pore diameter (Å)

260 113 180 300 1200 1300

0.82 0.55 1.40 1.00 0.92 0.97

126 190 250 130 30 30

BE of silica and oxygen atoms in catalysts with different silica supports and chromia loading were observed. Two contributions in the Cr 2p3/2 region, at very similar positions (576.7–577.1 and 579.0–581.1 eV), are found with all the samples except Cr2 O3 /SiO2 -1022, where only one peak at 579.0 eV was recorded. The ratio of the Cr(I)/Cr(II) peaks intensities, representing the relative contributions of the two chromia states, strongly depend on silica support, chromia loading and catalyst pretreatment varying over a wide range from 0 to 3.20 (Table 4). Increasing the chromia content in SiO2 -1030 from 8 to 11.3 wt.% caused a substantial increase of surface Cr/Si ratio indicating the higher dispersion of chromia phase. Further increase of chromia loading on this support to 20.1 wt.% and then to 29.0 wt.% yielded a decrease and stabilization of surface Cr/Si ratio due to decrease of chromia phase dispersion probably as a result of larger chromia crystals formation. For similar values of Cr2 O3 content (∼11.5 wt.%) the dispersion of chromia phase on SiO2 -1022 support (Cr/Si=0.020) is much lower compared with SiO2 -1030 (Cr/Si=0.062). The highest

Table 3 Effect of Cr2 O3 loading on the performance of Cr2 O3 /SiO2 -1030 catalyst in EA oxidation Cr2 O3 loading (wt.%)

EA conversion (%)

CO2 selectivity (mol%)

8.0 11.3 20.1 29.0

58 75 84 100

0 0 10 100

Cr2 O3 loading (wt.%)

13.9 12.8 11.9 11.3 10.5 11.2

Catalysts performance EA conversion (%)

CO2 selectivity (wt.%)

30 67 96 75 60 68

0 8 90 0 0 0

dispersion of chromia phase (Cr/Si=0.10 at 10.5 wt.% Cr2 O3 ) was observed on Si-MCM-41 support. This could be attributed to highest surface area of the support. 3.4. FTIR data The FTIR spectra of silica supports (Si-MCM-41, PQ-1030 and PQ-1020) calcined at 550◦ C were compared in the 3700–3800 cm−1 region with the spectra of corresponding calcined Cr2 O3 /SiO2 catalysts at similar chromia loading 10.5–11.9 wt.% (Fig. 2). Introduction of chromia caused a strong decrease (>80%) of the bands intensities at 3730–3750 cm−1 in the IR spectra of Si-MCM-41 support. Substantial reduction of the intensities of these bands (70%) after chromia deposition was detected in IR spectra of SiO2 -1030 support. In case of SiO2 -1022 the intensities of these bands remained almost unchanged after chromia deposition. 3.5. XRD data The ␣-Cr2 O3 crystals having domain diameter >20 Å should give a characteristic X-ray diffractogram with well-defined most intensive peaks corresponding to the planes with h k l (1 0 4), (1 1 0), (1 1 6), (0 1 2), (0 2 4) and (3 0 0) in the decreasing intensity sequence (JCPDS-ICDD 6-504). The XRD patterns of Cr2 O3 /SiO2 samples and standard mechanical mixtures (bulk ␣-Cr2 O3 /SiO2 -1030) with known compositions were obtained at the same conditions. The superposition of (0 1 2), (1 0 4) and (1 1 0) peaks of Cr2 O3 phase with 75, 100 and 95%

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Fig. 1. XPS spectra of the Si 2p, O 1s and Cr 2p3/2 of supported 29 wt.% Cr2 O3 /SiO2 -1030 catalyst.

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Table 4 Summarized XPS data measured for Cr2 O3 /SiO2 catalysts Silica support

PQ-1022 Si-MCM-41 PQ-1030 PQ-1030 PQ-1030 PQ-1030 PQ-1030 PQ-1030 a b

Cr2 O3 loading (wt.%)

11.9 10.5 8.0 8.0a 11.3 20.1 29.0 29.0b

Binding energy (eV)

Intensity ratio

Si 2p3/2

O 1s

Cr 2p3/2 (I)

Cr 2p3/2 (II)

Crtotal /Si

Cr(I)/Cr(II)

108.6 107.1 108.1 108.7 107.7 108.1 108.1 108.1

538.0 536.5 537.4 538.0 536.9 537.4 537.5 537.4

– 576.9 577.1 576.8 576.9 576.9 576.5 576.7

579.0 579.0 581.0 581.1 579.1 580.2 580.5 580.6

0.020 0.100 0.029 0.021 0.092 0.062 0.063 0.067

0 2.90 1.38 1.08 3.20 1.60 0.72 0.85

After reduction in H2 at 500◦ C for 1 h. After oxidation in O2 at 500◦ C for 0.5 h.

relative intensities and amorphous halo of silica did not allow to use these peaks for calculation of chromia concentrations. In addition the orientation mode of Cr2 O3 crystals at the surface of silica in supported catalysts was different compared with bulk Cr2 O3 . It affected the relative intensities of (1 0 4) and (1 1 0) peaks (Fig. 3). Therefore, the quantities of crystalline Cr2 O3 phase in Cr2 O3 /SiO2 catalysts were calculated from the relative integral intensities of XRD peak corresponding to (1 1 6) plane (90% relative intensity) that was not affected by silica phase and orientation effects. These data are compared in Table 5 with the total Cr2 O3 contents measured by EDAX. Lower values measured by XRD compared with EDAX were a result of part of chromia being in form of small X-ray amorphous species. Based on these differences the % crystallinity of chromia phase in Cr2 O3 /SiO2 samples were calculated and listed in Table 5. 3.6. TPR–TPO characterization

Fig. 2. IR spectra of silica supports and Cr2 O3 /SiO2 catalysts: (1) 10.5 wt.% Cr2 O3 /Si-MCM-41; (2) Si-MCM-41; (3) 11.9 wt.% Cr2 O3 /SiO2 -1030; (4) SiO2 -1030; (5) 11.3 wt.% Cr2 O3 /SiO2 -1022 and (6) SiO2 -1022.

The TPR spectra recorded with catalysts 11.9% Cr2 O3 /SiO2 -1022 and two catalysts supported on SiO2 -1030 with Cr2 O3 loading of 11.3 and 29.0 wt.% are shown in Fig. 4. Two peaks centered at 275 and 440◦ C with about equal intensities displayed a catalyst supported on SiO2 -1022 with lower surface area. In case of chromia supported on SiO2 -1030 at low metal loading most of H2 was consumed at high temperature (peak centered at 440◦ C). Increasing the chromia loading to 29 wt.% yielded a substantial change of TPR spectra so that most of the hydrogen

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Fig. 3. XRD patterns of Cr2 O3 /SiO2 composites: (1) 11.3 wt.% Cr2 O3 /SiO2 -1030; (2) 11.9 wt.% Cr2 O3 /SiO2 -1022 and (3) mechanical mixture (10 wt.% bulk Cr2 O3 /SiO2 -1030).

was consumed at lower temperature (peak centered at 275◦ C). TPO spectra recorded with reduced samples after TPR runs demonstrated a trend close to that observed in TPR experiments. Changing from SiO2 -1030 to SiO2 -1022 at low metal loading and increasing the

chromia loading on the SiO2 -1030 support (Fig. 5) substantially increased the amount of oxygen consumed at lower temperature of 250◦ C. The quantities of hydrogen and oxygen consumed by supported Cr2 O3 /SiO2 catalysts in TPR and TPO experiments are listed in Table 5. The oxygen

Table 5 Crystallinity of Cr2 O3 phase in Cr2 O3 /SiO2 catalysts according to XRD Silica support

Total Cr2 O3 content (EDAX) (wt.%)

Crystalline Cr2 O3 content (XRD) (wt.%)

Cr2 O3 phase crystallinity (%)

Average Cr2 O3 domain diameter (Å)

PQ-1022 PQ-1030 PQ-1030 PQ-1030 Si-MCM-41

11.9 11.3 20.1 29.0 10.5

9.6 5.7 11.6 20.0 4.0

81 50 58 69 38

155 110 120 130 200

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Fig. 4. TPR-spectra of Cr2 O3 /SiO2 catalysts of different chromia content loaded at different silica.

consumption by reduced samples was by about three to six times less compared with hydrogen consumption in TPR experiments. 3.7. Wet oxidation of APPA The results of the comparative tests of the best wet oxidation catalysts reported in the literature [19–22] and the most efficient catalyst prepared in this study (29 wt.% Cr2 O3 /SiO2 -1030) in wet oxidation of APPA are listed in Table 6. All the catalysts yielded high, almost full conversion of the APPA substrate at selected conditions, but the carbon conversion differs substantially being less than 80%. The

mineralization of phosphorus was close to 100% with all the catalysts while the nitrogen mineralization extent was very low. Nitrogen was converted mainly to ammonia and low amines that increased pH value of the solution from 5.1 to 6.5–7.0. The most efficient catalyst, both in carbon conversion and phosphorus mineralization, was the multioxide composition supported on magnesia–alumina composite carrier developed by Levec et al. [22]. Other catalysts displayed comparable or even lower efficiency compared with silica-supported chromia catalyst, especially in decarbonization and mineralization of APPA (Table 7).

4. Discussion

Fig. 5. TPO-spectra of Cr2 O3 /SiO2 -1030 catalysts with different chromia content.

The metal oxides tested in butane oxidation are most frequently used as oxidation catalysts or their components [3,4,9,11]. In agreement with previous data [4–8], Cr2 O3 , Co3 O4 , CuO and MnO2 were found to be the most active oxides. Chromia displays the highest CO2 productivity and should be considered as a leading component in efficient wet oxidation combined with the proper support and promoter. The total Cr 2p level intensity does not increase linearly with chromium loading. This suggests that a higher loading is accompanied by a lower metal dispersion (clustering effect). Such behavior has reported by Cimino et al. [25], where the chromium loading varied between 1 and 9 wt.%. XPS analysis shows two different Cr species or chemical states on the catalyst

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Table 6 Amounts of hydrogen and oxygen consumed by Cr2 O3 /SiO2 catalysts in TPR–TPO experiments Catalyst sample (wt.%)

11.9Cr2 O3 /SiO2 -1022 11.3Cr2 O3 /SiO2 -1030 29.0Cr2 O3 /SiO2 -1030

Amount of H2 consumed at reduction stage (␮M/g)

Amount of O2 consumed at oxidation stage (␮M/g)

275◦ C

440◦ C

250◦ C

420–570◦ C

140 80 240

254 330 141

36 28 45

16 20 17

surface. The simplest interpretation would be to assign the lowest BE peak — Cr(I), to Cr3+ and the highest BE contribution to Cr6+ . Coexistence of Cr2 O3 and chromates deposited on SiO2 has often been observed [26]. These BE values are close, though a few tenths of eV higher to the binding energies is commonly accepted [27]. An increase of the Cr(I)/Cr(II), that is of the Cr3+ /Cr6+ ratio when the % Cr increases (Table 4), is in agreement with the loss of dispersion mentioned above. As a matter of fact, other authors have shown that Cr6+ tends to be dispersed over the whole surface, whereas Cr3+ forms crystallites larger than 100 Å in diameter, greater than the escape depth of the photoelectrons [25]. When the total chromium content increases, part of the Cr3+ in the crystallites is not detected by XPS, the depth sensitivity being below 100 Å. The abnormal behavior of the 8.0 wt.% Cr2 O3 /SiO2 -1030 catalyst can also be explained if one considers the Crtotal /Si ratio, equal to 0.03, which indicates a poorer dispersion than on the 11 wt.% Cr2 O3 /SiO2 -1030 catalyst. A low Cr(I)/Cr(II) ratio is then expected though the amount of chromium is low. However, at constant dispersion, several authors report an opposite result i.e. the Cr3+ /Cr6+ ratio

increases with the overall chromium loading [28]. This interpretation assumes that there is no charge effect on chromium though chromium oxide is deposited on a charged silica support. Moreover, the existence of chromates requires the presence of an anion for instance potassium to make K2 Cr2 O4 ; this is unlikely considering the preparation mode of our catalysts. Another hypothesis relies on the coexistence of Cr4+ in CrO2 and Cr3+ in Cr2 O3 . CrO2 is also known to display a quasi-metallic behavior [29], it could consequently be affected by a lower charge effect than Cr2 O3 , an insulating oxide. In that case, after subtracting the charge effect calculated according to the shift on Si 2p, the binding energy of Cr4+ would equal 576.9 eV and the one of Cr3+ , would be comprised between 574.8 and 576.3 eV depending on the sample. These values are not in agreement with the well established BE values for Cr4+ and Cr3+ , respectively. Cr4+ does not obey the common rule of the binding energy increase with the level of oxidation and, known to be lower than the one of Cr3+ [25,29]. Moreover, CrO2 is generally hardly detectable in SiO2 supported catalysts.

Table 7 Catalytic wet oxidation of APPA with 29% Cr2 O3 /SiO2 -1030 and the best CWO catalysts known from the literaturea Catalyst composition (wt.%)

29.0Cr2 O3 /SiO2 -1030 0.8MnO2 /9.6ZrO2 /CeO2 [21] 33.0Bi2 O3 /Co3 O4 [20] 6.2MnO2 /CeO2 [19] 1.6MnO2 /4.2CuO/44.1ZnO/ 2.3La2 O3 /Al2 O3 [22] a

Conversion (%)

Phosphorus concentration (ppm)

APPA

Total carbon

Total

In PO4

94 95 100 96 95

65 51 53 64 77

1630 1640 1650 1650 1650

1597 1607 1567 1551 1633

Nitrogen concentration (ppm) 3−

Total

In NO3 − and NO2 −

500 740 730 625 520

9 – – 31 12

The mass balance for phosphorus was 95–97% while for nitrogen it was 87–91%. A part of the nitrogen was converted to NOx .

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its BE (Table 4) –Si–O–Cr ==O + 21 H2 → –Si–OH + Cr ==O Note that the binding energy of Cr in the grafted species decreased by 0.3 eV. This limited shift suggests a partial reduction of the charge on Cr cations in the grafted surface species. Calcination can lead to partial redispersion of small chromia crystals (Cr(II)) by reaction with free silanols at the supports surface 2(–Si–OH) + O==Cr–O–Cr ==O → 2(–Si–O–Cr ==O) + H2 O

Fig. 6. Two states of chromium at the surface of silica support.

An additional possibility assumes two types of Cr2 O3 species, affected by different charge effects. Cr3+ would be either contained in Cr2 O3 crystallites or in a continuous film on the support (as shown in Fig. 6): Cr(I) surface grafted species (–Si–O–Cr=O) and Cr(II) bulk particles (O=Cr–O–Cr=O) The former, Cr(I) constituting the surface film in contact with the gold grid, would be affected by a small or even a zero charge effect. The latter Cr(II), would be highly charged. When the Cr2 O3 content increases, the crystallites grow. Chromia dispersion decreases with increasing the share of bulk chromia crystals that explains the decrease of the Cr(I)/Cr(II) ratio (Table 4). Lowering the surface area of silica support from Si-MCM-41 to SiO2 -1022 also decreases the dispersion of chromia phase affecting the Cr(I)/Cr(II) ratio and increasing the chromia loading (Table 4). The XPS results obtained after reduction/oxidation treatments of Cr2 O3 /SiO2 catalysts are consistent with the hypothesis assuming that the local structure is determining factor for the charge effect and consequently the observed change in the Cr(I)/Cr(II) ratio. The possible explanation for these data is breaking of Si–O–Cr bonds in Cr(I) state with formation of reduced chromia species decreasing the concentration of Cr(I) and

increasing the relative contribution of Cr(I) state. These assumptions correlate good well with the changes in Cr(I)/Cr(II) ratios and surface Cr/Si ratios representing the dispersion of chromia phase after reduction/oxidation treatments listed in Table 4. It is known that raising the temperature enhances surface mobility and surface rearrangement when heating high surface area Cr2 O3 samples under O2 at 400◦ C [30]. These authors considered possible oxygen migration to form new oxide islands of lower coordination. This type of structural modifications could occur and lead to variations in the dispersion. They also detected heterogeneous character of the surface of a high surface-area Cr2 O3 which could also justify the existence of two well separated electronic levels. Further confirmation was obtained by characterization of Cr2 O3 /SiO2 catalysts with FTIR and XRD methods. It is well established that free surface hydroxyl groups in calcined silica-gels are characterized by stretching bands 3740–3750 cm−1 in IR spectra reflecting the basic valence vibrations [31]. Interaction of these hydroxyls with chromia, that forms grafted chromia species –Si–O–Cr=O, should remove these bands from IR-spectra or decrease their intensity observed in Fig. 2. The most intensive interaction of surface silanols with Cr2 O3 was detected in the case of Si-MCM-41 (about full disappearance of 3730–3750 cm−1 bands), derived from the decreasing band intensity after chromia deposition. A strong but less intensive interaction of surface silanols with chromia was observed for SiO2 -1030, while in case of SiO2 -1022 no visible interaction was observed. These data are in agreement with dispersion of chromia reflected by surface Cr/Si

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ratios estimated with these catalysts by XPS. They are also consistent with our attribution of the two detected chromium states made according to the hypothesis that strong chromia–silica interaction yields the highest Cr(I)/Cr(II) ratios measured by XPS, while in absence of such interaction the catalyst does not contain the Cr(I) form at all (Cr2 O3 /SiO2 -1022). The two chromia states mentioned above presume the existence of Cr2 O3 either in crystalline (Cr(II)) or in amorphous (Cr(I)) forms, the later as isolated –Si–O–Cr=O species. The XRD crystallinity of Cr2 O3 was much higher in 11.9 wt.% Cr2 O3 /SiO2 -1022 catalyst (81%) compared with 11.3 wt.% Cr2 O3 /SiO2 -1030 (50%) in agreement with FTIR data, while the lowest crystallinity (38%) was detected in catalyst based on Si-MCM-41 support. Moreover, increasing the chromia content in Cr2 O3 /SiO2 -1030 catalyst increased the % crystallinity of Cr2 O3 phase, so that the absolute content of crystalline phase defined as Cr(II) increased substantially. These data confirm the two chromia states detected in Cr2 O3 /SiO2 catalysts by XPS. The lack of amorphous Cr(I) state in SiO2 -1022 based catalyst according to XPS and about 20% of amorphous phase detected in this catalyst by XRD (Table 5) could be explained by low sensitivity of XRD method to small Cr2 O3 crystals (<20 Å) in this catalyst (their contribution to the integral of the (1 1 6) peak was negligible). All catalysts contained relatively large Cr2 O3 crystallites, 110–200 Å in diameter, corresponding to the diameter of pores in amorphous silica supports PQ-1022 and PQ-1030. In case of Si-MCM-41 with pore diameter of about 30 Å, the amorphous chromia is probably located inside the pores while the large crystals (200 Å) are at the outer surface of silica particles. Cr2 O3 in supported catalysts exists in two forms — crystalline and amorphous (grafted chromia species) — based on the catalysts characterization data obtained by XPS, FTIR and XRD. The content and the relative contribution of these two forms could be regulated by the nature of support, its texture, chemical properties and chromia loading. Comparing the catalytic performance of catalysts samples (Tables 2 and 3) with the appearance of these two chromia morphologies measured by physical methods (Tables 4 and 5) indicates that crystalline Cr2 O3 defined as Cr(II) displays a much higher catalytic activity than Cr(I) in amorphous form, towards deep oxidation of organic

compounds.Formation of surface chromium silicates or aluminates is likely to decrease the surface chromia oxygen mobility and consequently reduces the rate of complete oxidation catalytic cycle. This assumption was confirmed by the data of TPR–TPO experiments (Figs. 4 and 5, Table 6). The two chromia states defined in Fig. 6 have different reducibility and after reduction, different ability to oxidation. The form of chromia that could be reduced at lower temperature of 275◦ C and reoxidized at 250◦ C — temperatures similar to that used in EA catalytic oxidation test, could be attributed to chromia cations not bonded to silica and located at the surface of ␣-Cr2 O3 nanoparticles. The contribution of chromium cations with poor reducibility (∼440◦ C) and oxidation ability (∼420–570◦ C) strongly increases at low chromia loading or on SiO2 support with higher surface area (where the relative contribution of grafted chromia species surface, namely chromium silicate, is high). Those cations cannot participate in redox catalytic cycle at testing temperature of 250◦ C. As a result, the catalytic activity of such samples in EA full oxidation is low and could be enhanced by reducing the chromia-support interaction. In this respect silica, that has a lower reactivity with chromia, compared with alumina, is a preferable support for complete oxidation chromia-based catalysts especially at high metal loading. The use of more inert alumina support with lower surface area, like ␪-Al2 O3 also favors the formation of crystalline Cr2 O3 and higher CO2 production rate (Table 3). As an alternative approach implementation of unsupported chromia catalysts with high surface area prepared by advanced sol–gel technology and/or using of self assembling surfactants aggregates as templates is a viable tool for further improvement of transition metal oxide catalysts for full oxidation of hydrocarbons. The APPA is a suitable model compound for testing the different functions quality of wet oxidation catalysts and their efficiency in carbon combustion and mineralization. The efficiency of both these functions of silica-supported chromium catalyst are shown to be close to the best polyoxide heterogeneous catalysts developed for wet oxidation. Further optimization of the state of Cr2 O3 phase in supported chromia catalyst and introduction of promoters affecting the surface oxygen mobility is a viable reasonable option for improvement of complete oxidation catalysts.

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