Kinetic aspects and deactivation behaviour of chromia-based catalysts in hydrogen chloride oxidation

Abstract

The gas-phase HCl oxidation was studied over bulk and supported Cr₂O₃-based catalysts by means of kinetic experiments in a fixed-bed reactor at ambient pressure and variable temperature, inlet O₂/HCl ratio, and contact time. Cr₂O₃ exhibited high activity for Cl₂ production. X-ray diffraction of the used catalysts detected no phase change. Temperature-programmed reduction with hydrogen and X-ray photoelectron spectroscopy showed that the surface of the fresh catalyst contains chromium species in oxidation state (III) as well as higher oxidation states (V and VI) while that of the used sample features the reduction of Cr⁵⁺/Cr⁶⁺ and comprises a little amount of chlorine. Coupling the catalytic and characterisation data over Cr₂O₃ with activity tests over CrO₃, a redox cycle is proposed in which chromium species shift between Cr³⁺ and Cr⁵⁺ + Cr⁶⁺. The positive dependence of the HCl conversion on the inlet O₂ concentration suggests that catalyst re-oxidation is rate limiting. SiO₂ was identified as a better carrier for Cr₂O₃ than Al₂O₃ and TiO₂-anatase, as it favours the formation of Cr₂O₃ nanoparticles rather than (unstable) isolated chromate species. However, all the supported catalysts suffered from severe deactivation due to substantial chromium loss. The deactivation mechanism is assigned to the in situ formation of the highly volatile CrO₂Cl₂ from Cr⁶⁺ species and CrO₂(OH)₂ from both Cr³⁺ and Cr⁶⁺ species. The deactivation rate can be reduced, though not suppressed, by applying a high O₂ excess in the feed mixture, thus indicating that the deactivation route via the oxychloride might be predominant. The features observed represent critical reasons justifying the restricted industrial implementation of chromium-based catalysts for HCl oxidation.

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1. Introduction

The catalysed oxidation of HCl to Cl₂ has re-gained increasing interest as an energy-efficient route to recover chlorine from HCl-containing streams in the chemical industry, especially in polyurethane and polycarbonate production.¹ Since the introduction of the original CuCl₂/pumice catalyst by Deacon in 1868,² many copper-based systems have been reported in the literature for operation in fixed- and fluidised-bed reactors.^3–5 However, none of them were realised in a long-term commercial process as a consequence of the fast catalyst deactivation due to copper loss, operational problems such as particle coagulation, and severe corrosion issues in the plants. In 1980, Mitsui Chemicals established the MT-Chlor process using a Cr₂O₃/SiO₂ catalyst in a fluidised-bed reactor at 623–703 K.^6,7 The stability of the latter catalyst was improved with respect to the CuCl₂–KCl/SiO₂ catalyst of the Shell-Chlor process.³ This has been related to the fact that chromium(III) oxide operates via a redox cycle, without melting and without involving the chloride-oxide reaction cycle characteristic of copper-based catalysts.^8,9 Nonetheless, the implementation of Mitsui's process is limited to one medium-sized plant of 60 kton Cl₂ per year.⁸ In contrast, recently developed ruthenium-based catalysts are being installed in several large-scale Cl₂ production facilities.⁸ RuO₂-based materials exhibit a very high activity at low temperatures (550–650 K) and their lifetimes exceed several thousand hours.^8,10–12 Thus, these catalytic systems appear to satisfy a long-standing industrial need to count on development of complementary technologies for the manufacture of Cl₂ by electrolysis.

The practical interest in RuO₂-based catalysts has triggered investigations leading to in-depth knowledge on the catalyst structure, the chlorination behaviour, and the reaction mechanism and kinetics.^13–19 Keeping in mind that the ruthenium cost associated with a world-scale HCl oxidation plant equipped with a ruthenium-based material easily amounts to several million euros,¹ recent studies have attempted the identification of cheaper alternatives. The use of cuprous delafossite (CuAlO₂), displaying outstanding stability in comparison with conventional copper-based catalysts,²⁰ and the promising performance of CeO₂²¹ are representative examples. Astonishingly, the number of academic studies embracing HCl oxidation over chromium-based materials is limited,^22,23 which hampers a proper understanding of this interesting catalytic system. Short-term isothermal tests have shown that Cr₂O₃ is more active than other metal oxides such as CuO and CeO₂, being only surpassed by RuO₂.²³ One of the original patents by Mitsui⁷ states that fresh catalyst particles should be fed to the fluidised-bed reactor either continuously or intermittently to replenish the evaporated portion of the chromium while continuing the reaction. Therefore, chromium loss and the associated environmental concerns might be critical factors for the limited industrialisation.

Herein, we have conducted a systematic investigation of the gas-phase oxidation of HCl over Cr₂O₃-based catalysts in bulk and supported forms using a continuous-flow fixed-bed reactor operated at ambient pressure and variable temperature, feed O₂/HCl ratio, and contact time. The composition, structure, porosity, and chromium oxidation state of the catalysts prior to and after reaction were assessed in order to gain insights into the nature of the active species and the deactivation mechanism.

2. Experimental

2.1. Catalysts and characterisation techniques

Cr₂O₃ (Aldrich, nanopowder, 99%), SiO₂ (Fluka, Cab-O-Sil), γ-Al₂O₃ (Alfa Aesar), and TiO₂-anatase (Aldrich, nanopowder, 99.7%) were calcined in static air at 773 K (10 K min⁻¹) for 5 h prior to their use. CrO₃ (Aldrich, 99.99%) was used as received. Supported chromium catalysts with a nominal Cr loading of 14 wt% were prepared by incipient wetness of the dried carriers with an aqueous solution of Cr(NO₃)₃·9H₂O (Sigma-Aldrich, 99%), followed by drying at 338 K for 12 h, and static-air calcination at 773 K (10 K min⁻¹) for 5 h. The value of 14 wt% was selected on the basis of the optimal range (13–20 wt%) indicated in the patent literature.⁶

Powder X-ray diffraction (XRD) was measured on a PANanalytical X’Pert PRO-MPD diffractometer. Data were recorded in the 10–70° 2θ range with an angular step size of 0.017° and a counting time of 0.26 s per step. N₂ sorption was measured at 77 K in a Quantachrome Quadrasorb-SI gas adsorption analyser. The samples were degassed in vacuum at 473 K for 12 h prior to the measurement. Temperature-programmed reduction with hydrogen (H₂-TPR) was measured in a Thermo TPDRO 1100 unit equipped with a thermal conductivity detector. The samples were loaded in a quartz micro-reactor (11 mm id), pre-treated in He (20 cm³ STP min⁻¹) at 473 K for 30 min, and cooled to 323 K in He. The analysis was carried out in 5 vol% H₂/N₂ (20 cm³ STP min⁻¹), ramping the temperature from 323 to 1173 K at 10 K min⁻¹. The chromium content was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in a Horiba Jobin Yvon Ultima 2 instrument after dissolution of the samples in a HF/H₂SO₄ solution. X-ray photoelectron spectroscopy (XPS) was performed on a VG-Microtech Multilab 3000 spectrometer featuring a hemispheric electron analyser with 9 channeltrons (pass energy 50 eV) and non-monochromatised Mg Kα radiation at 1253.6 eV as the X-ray source. Samples were transferred to the spectrometer chamber under regular ambient exposure. The binding energy scale was referenced to the C 1s level of the carbon overlayer at 284.6 eV. The spectrum of fresh Cr₂O₃ was deconvoluted by applying fixed binding energy values for Cr³⁺ and Cr⁶⁺, retrieved from NIST,^24–26 and allowing the full width at half maximum to relax to achieve the best possible fit (R² = 0.996). The same procedure was followed for the decomposition of the spectrum of used Cr₂O₃ by initially applying a shift of 2 eV to the binding energies of Cr³⁺ and Cr⁶⁺, on the basis of the overall shift of the Cr 2p features with respect to those of the fresh sample. As the fit was not satisfactory (R² = 0.971), the binding energies were refined. The best result (R² = 0.991) was found for shifts equal to 2.6 and 1.9 eV for Cr³⁺ and Cr⁶⁺, respectively. The point of zero charge (PZC) of Al₂O₃ and SiO₂ was determined by measuring the zeta potential as a function of pH at different ionic strengths in a Zetasizer Nano ZS (Malvern Instruments). The carriers (0.1 wt%) were dispersed in aqueous solutions of pH 2.0 (Al₂O₃) or 11.2 (SiO₂). The suspensions were placed in a cell and titrated by adding 0.01–0.1 M NaOH (Al₂O₃) or 0.01–0.1 M HCl (SiO₂). The changes in zeta potential were recorded as a function of pH.

2.2. Catalytic tests

The gas-phase oxidation of hydrogen chloride was studied at ambient pressure in a setup described elsewhere,²⁷ which is equipped with (i) mass-flow controllers to feed HCl (Messer, purity 2.8, anhydrous), O₂ (Pan Gas, purity 5.0), and N₂ (Pan Gas, purity 5.0), (ii) a home-made electrically-heated oven hosting a 8 mm id quartz micro-reactor, and (iii) a Mettler Toledo G20 Compact Titrator for Cl₂ analysis. The catalysts (particle size = 0.4–0.6 mm) were loaded in the tubular reactor and pre-treated in N₂ at 688 K for 30 min. Thereafter, experiments at variable bed temperatures (T_bed = 560–680 K), inlet O₂/HCl ratios (0–7), and contact times (τ = 0.1–0.3 s) were carried out.

In the catalytic tests, the inlet HCl concentration was fixed at 10 vol%. The temperature dependence was measured using a catalyst weight (W) of 0.5 g (0.25 g for supported catalysts), a O₂/HCl ratio = 2, and a total volumetric flow (F_T) of 166 cm³ STP min⁻¹. The O₂/HCl dependence was measured by increasing the O₂ content in the inlet mixture from 5 to 70 vol% with N₂ as balance gas, applying T_bed = 688 K (678 K for supported catalysts), W = 0.5 g (0.25 g for supported catalysts), and F_T = 166 cm³ STP min⁻¹. Data were collected after 1 h on stream under each condition. The contact time dependence was studied at variable W = 0.5–1.5 g and F_T = 166–333 cm³ STP min⁻¹, at T_bed = 688 K and O₂/HCl = 2. In this case, measurements were made at 1, 2, and 3 h on stream and the data averaged. The space time, defined as the ratio of the catalyst weight and the inlet molar flow of HCl (W/ṅ⁰(HCl)), was in the range of 11.2–33.3 g h mol⁻¹. The Weisz–Prater criterion was fulfilled in all our catalytic tests, indicating the absence of intra-particle diffusion limitations. Stability tests (up to 40 h on stream) were conducted over the supported Cr₂O₃ catalysts at 678 K and O₂/HCl = 0.5–7. Used samples were collected for post-mortem characterisation after rapidly cooling down the reactor to room temperature in N₂ flow. The percentage of HCl conversion was determined as X_HCl = (2 × mole Cl₂ at the reactor outlet/mole HCl at the reactor inlet) × 100. The experimental error of the measurements is ±3%. Chromium losses in stability tests involving bulk Cr₂O₃ were determined using a guard bed of H-ZSM-5 (Zeolyst International, CBV 8014, 0.5 g, particle size = 0.4–0.6 mm) located in the cold zone of the reactor outlet (373 K).

3. Results and discussion

3.1. Bulk Cr₂O₃

3.1.1. Catalytic activity. Cr₂O₃ displayed a remarkable and constant HCl conversion level (ca. 30%) in the course of a 4 h test at 688 K and O₂/HCl = 2 (Fig. 1a). Upon removal of O₂ from the feed, the HCl conversion was minimal and dropped to zero after 2 h on stream. The catalytic activity was immediately restored by switching the feed O₂/HCl ratio from 0 back to 2. The HCl conversion increased upon raising the feed O₂/HCl ratio (Fig. 1b), indicating that catalyst re-oxidation might be rate limiting, similarly to RuO₂, CeO₂, CuO, and CuAlO₂.^14,20,21,27 The formal reaction order on O₂, calculated using a power equation fitting, was found to be 0.2 (±0.02). As expected, higher contact times enhanced the HCl conversion (Fig. 1c). Nevertheless, the level reached at τ = 0.3 s was slightly lower than anticipated. Since the HCl conversion was not limited by thermodynamic constraints under any of the conditions applied in our catalytic testing (see equilibrium HCl conversion (X_eq) in Fig. 1c), this might be related to certain product inhibition at high Cl₂ production levels. From the Arrhenius plot (Fig. 1d), the apparent activation energy (E_a) was estimated to be 97 (±1.4) kJ mol⁻¹. This value is very close to those reported for high-temperature Deacon catalysts such as CeO₂ (90 kJ mol⁻¹) and CuAlO₂ (100 kJ mol⁻¹).^20,23

Fig. 1 HCl conversion versus (a) time-on-stream, (b) feed O₂/HCl ratio, and (c) contact time. (d) Arrhenius plot showing the logarithm of the rate of Cl₂ production as a function of the reciprocal temperature. Bulk Cr₂O₃ is represented by solid circles and Cr₂O₃/SiO₂ by open squares. The equilibrium HCl conversion is depicted as a dashed line (c). Conditions are detailed in Section 2.2.

3.1.2. Characterisation. Fresh and used Cr₂O₃ samples were characterised by bulk and surface techniques in order to assess possible alterations of the catalyst textural, structural, and compositional properties upon exposure to reaction conditions. The total surface area of Cr₂O₃ (10 m² g⁻¹) decreased to 8 m² g⁻¹ upon use, probably due to minor particle sintering. XRD analysis of Cr₂O₃ used in HCl + O₂ and HCl-only mixtures did not evidence changes in position and intensity of the chromium(III) oxide reflections or the appearance of chlorinated phases (Fig. 2). The stability of this material apparently resembles that of RuO₂.¹⁴ As it will be pointed out later, this does not hold true.

Fig. 2 XRD patterns of bulk Cr₂O₃: (a) fresh, (b) used in O₂/HCl = 2 at 688 K for 4 h, and (c) used in O₂/HCl = 0 at 688 K for 2 h. The pattern of Cr₂O₃ (JCPDS 70-3766) is shown by the vertical blue lines.

The H₂-TPR profile of fresh Cr₂O₃ displays two main reduction peaks (1 and 2, Fig. 3a). Due to variations in the experimental conditions applied in our analysis (2–16 times lower concentration of H₂ and 1.5 times slower ramping rate), these are observed at higher temperatures (500–650 and 650–850 K) with respect to literature data.^28,29 The less intense low-temperature feature seems to be composed of a peak and a shoulder (1 and 1′, visualised by deconvolution as blue curves), which are respectively attributed to the reduction of Cr⁶⁺ to Cr³⁺²⁸ and of Cr⁵⁺ to Cr³⁺. The latter assignment was substantiated by H₂-TPR and XRD data for CrO₃ after use in HCl oxidation (Section 3.1.3). As no reflections or bumps specific to Cr⁶⁺ or Cr⁵⁺ phases were detected by XRD, these species seem to be present at the near-surface level only. Peak 2 appears as the superimposition of a main component, related to the reduction of Cr³⁺ to Cr²⁺,^28,29 and two shoulders (2′ and 2′′, visualised by deconvolution as blue curves) that could refer to variable degrees of crystallisation of Cr₂O₃.²⁹ After use at O₂/HCl = 2 (b), peak 1′ became moderately more intense, whereas peak 1 almost vanished. These findings indicate that HCl oxidation is accompanied by an increase of Cr⁵⁺ species and depletion of Cr⁶⁺ species, which might occur either via volatilisation or reduction (vide infra). The signals were fully depleted after treatment in HCl-only (c). In the absence of O₂, HCl is able to consume all Cr⁶⁺ and Cr⁵⁺ species transforming them into Cr³⁺ species or, vice versa, Cr⁶⁺ and Cr⁵⁺ species oxidise HCl to Cl₂. Chlorine production was indeed observed under these conditions and eventually stopped when all chromium species in higher oxidation states had reacted (Fig. 1a). The subsequent exposure to a feed with O₂/HCl = 2 restored feature 1′ to a minor extent (d), demonstrating that gas-phase O₂ is able to in situ re-oxidise Cr³⁺ species. In profiles (b–d) peak 2 is shifted to lower temperatures, indicating that any treatment renders Cr₂O₃ somewhat more reducible.²⁹

Fig. 3 H₂-TPR profiles of bulk Cr₂O₃: (a) fresh, (b) used in O₂/HCl = 2 at 688 K for 4 h, (c) used in O₂/HCl = 0 at 688 K for 2 h, and (d) used in O₂/HCl = 0 at 688 K for 2 h and then in O₂/HCl = 2 at the same temperature for 2 h.

XPS was applied to further characterise the surface of the samples (Fig. 4). The band structure of the Cr 2p XPS spectrum for the fresh material (Fig. 4a), composed of the Cr 2p_3/2 and Cr 2p_1/2 peaks, is that characteristic of bulk Cr₂O₃.²⁸ Both features can be fitted by contributions of Cr³⁺, Cr⁵⁺, and Cr⁶⁺. Nevertheless, due to the equivocal distinction between the binding energies of Cr⁶⁺ and Cr⁵⁺ (Table 1),³⁰ a combined deconvolution line is shown for both. In order to derive information about the relative amounts of the different chromium species, the Cr 2p_3/2 signals were considered.³¹ Accordingly, Cr⁵⁺ + Cr⁶⁺ species were significantly more abundant than Cr³⁺ species in fresh Cr₂O₃ (Fig. 4a). Upon use in HCl oxidation, both Cr 2p_3/2 and Cr 2p_1/2 peaks were shifted by 2 eV to higher binding energy (Fig. 4b), pointing to the presence of chlorine. Surrounding atoms with high electronegativity are indeed reported to cause an increase in binding energy.³⁰ The noisy signal detected at 201.3 eV in the Cl 2p region of the XPS spectrum confirms that the surface of the used catalyst contains a little amount of chlorine (Fig. 4, inset). It is difficult to speculate on the type(s) of chlorine-containing species present. The signal of Cr³⁺ appears at 1.3 eV higher binding energy in the case of CrCl₃ with respect to Cr₂O₃ and larger shifts are observed when additional ligands such as en or NH₃ are present (Table 1). Shifts to higher binding energy are expected also in the case of substitution of O atoms bound to Cr⁵⁺ or Cr⁶⁺ by Cl. Thus, species likely present on the used sample could be surface (oxy/hydroxy)chlorides of chromium(III/V/VI), possibly solvated by water, but adsorbed hydrochloric acid and molecular chlorine cannot be excluded. As the individual shifts for Cr³⁺ and Cr⁵⁺ + Cr⁶⁺ are actually different and equal to 2.6 eV and 1.9 eV, respectively, chlorine might preferably bind to Cr³⁺ rather than Cr⁵⁺/Cr⁶⁺ ions, or Cr sites with different oxidation state could stabilize chlorine-containing compounds of different chemical nature. Finally, the Cr⁵⁺ + Cr⁶⁺ and Cr³⁺ contributions to the Cr 2p_3/2 feature became almost equivalent. The depletion of species in higher oxidation states is in line with the H₂-TPR results (Fig. 3, peaks 1 and 1′ in profiles a,b).

Fig. 4 Cr 2p core level XPS spectra of bulk Cr₂O₃: (a) fresh and (b) used in O₂/HCl = 2 at 688 K for 4 h. The inset shows the Cl 2p spectrum of the used sample.

Table 1 XPS data of chromium species

Cr species	Compound	Binding energy (eV)	Ref.
Cr^3+	Cr2O3	576.5	This study, 26
	CrCl3	577.8	25
	Cr(NH3)6·Cl3	578.5	25
	Cr(en)3·Cl3	578.3	25
	CrOx/ZrO2	577.0	31
Cr^5+	CrOx/ZrO2	579.0	31
Cr^6+	CrO3	578.9	This study, 27
	CrOx/ZrO2	580.0	31

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3.1.3. Evaluation of CrO₃. CrO₃ was studied in HCl oxidation as a reference sample. This material showed a HCl conversion level of 17% at 30 min on stream, which remained constant up to a reaction time of 2 h. Volatilisation of chromium species was observed in the first 15 min of the catalytic run, likely due to the low boiling point of CrO₃ (524 K). The diffractogram of the residual solid (Fig. 5a) revealed that the original CrO₃ phase fully transformed: Cr₂O₃ was the major phase, followed by Cr₂O₅ and CrO₂. The decomposition of CrO₃ into oxides with chromium in lower oxidation states is in agreement with literature data.³² Nevertheless, it is reported that Cr₂O₅ would be preferentially formed by heating CrO₃ in air at 573 K. The predominance of Cr₂O₃ in our case could originate from the different chemical environment and/or the higher temperature of the treatment. The H₂-TPR profile of used CrO₃ is characterised by the presence of an asymmetric reduction peak centred at 660 K, which can be deconvoluted into three components (Fig. 5b). In line with the phases detected by XRD (Fig. 5a), peak 1′ at 639 K is attributed to the reduction of Cr⁵⁺ to Cr³⁺, peak 3 at 654 K to the reduction of Cr⁴⁺ to Cr²⁺ or Cr³⁺, and peak 2 at 668 K to the reduction of Cr³⁺ to Cr²⁺.

Fig. 5 (a) XRD patterns of bulk CrO₃ prior to and after use in O₂/HCl = 2 at 688 K for 2 h. The pattern of CrO₃ (JCPDS 32-0285) is shown by the vertical blue lines, while other phases are indicated by symbols: (○) Cr₂O₃, JCPDS 70-3766, (◇) Cr₂O₅, JCPDS 07-0247, and (□) CrO₂, JCPDS 84-1821. (b) H₂-TPR profile of used CrO₃.

3.1.4. Redox cycle. Herein we describe the main mechanistic fingerprints of HCl oxidation over bulk chromia. The reaction is proposed to occur over the surface of Cr₂O₃ according to a simplified redox cycle: Cr³⁺ terminal species are oxidised by gas-phase O₂ to Cr⁵⁺ and Cr⁶⁺ species, which react with HCl generating Cl₂ and H₂O, thus reconverting into Cr³⁺ species. The suggested catalytic cycle is in line with literature studies correlating the oxidation activity of chromium with its ability to reversibly shift between different oxidation states,^33,34 and supports some mechanistic aspects gathered for HCl oxidation on (Cr–Cu–Mn)/γ-Al₂O₃ and Cr₂O₃/γ-Al₂O₃ catalysts under methane oxychlorination conditions.²² The key features of the mechanism are herein substantiated and further discussed based on our characterisation and catalytic data.

XPS and H₂-TPR analyses indicated the presence of Cr⁶⁺ and Cr⁵⁺ species in fresh Cr₂O₃. Production of Cl₂, although in small amounts, in the absence of gas-phase O₂ over this catalyst was observed up to 2 h on stream (Fig. 1a). After this time no HCl was further converted and H₂-TPR analysis of the used sample evidenced the complete depletion of Cr⁶⁺ and Cr⁵⁺ species (Fig. 3c). Furthermore, CrO₃ was active for HCl oxidation. These lines of evidence prove that Cr³⁺ species are per se not able to oxidise HCl, while species in higher oxidation states are. The fact that CrO₃ reaches a lower HCl conversion level than Cr₂O₃, in spite of the exclusive presence of Cr⁶⁺ species, is likely due to its rapid volatilisation and decomposition. The instability of chromium(V) and (VI) compounds both at the calcination and reaction temperature, the absence of diffraction lines or bumps specific to bulk phases with chromium in higher oxidation state, and the detection of Cr⁵⁺ and Cr⁶⁺ species by XPS suggest that the latter are exclusively present at the surface of Cr₂O₃. Thus, HCl oxidation seems to occur at the catalyst surface. Oxidised chromium species can be generated during reaction in the presence of gas-phase O₂. This is supported by (i) the disappearance of the reduction peak of Cr⁶⁺/Cr⁵⁺ species in the H₂-TPR profile of the sample exposed to HCl-only (Fig. 3c) along with (ii) the immediate increase in HCl conversion to its original level (ca. 30%) when feeding a mixture with O₂/HCl = 2 after the treatment at O₂/HCl = 0 (Fig. 1a) and the re-appearance of the reduction peak of Cr⁵⁺ species in the H₂-TPR profile of the sample after this test (Fig. 3d). The literature reports that surface chromyl species (Cr=O) can form via dissociative adsorption of O₂ on coordinatively-unsaturated Cr³⁺ cations, i.e. transforming Cr³⁺ into Cr⁵⁺ and completing its coordination shell.^35–37 The generation of chromyl terminations with Cr in (VI) oxidation state is not supported by any experimental evidence and may require higher temperatures and O₂ pressures than in our study^38,39 but cannot be ruled out. Thus, it is suggested that the Cr₂O₃ surface active for HCl oxidation to Cl₂ contains terminal Cr = O species of Cr⁵⁺ and Cr⁶⁺. With regards to the kinetics of the reaction steps, we expect the activation of gas-phase O₂ by Cr³⁺ ions to produce oxidised surface chromium species to be rate limiting, as the HCl conversion was found to positively depend on the feed O₂ concentration (Fig. 1b).

3.2. Supported Cr₂O₃

3.2.1. Catalytic activity. The alumina-, silica-, and titania-supported Cr₂O₃ catalysts were preliminarily screened for HCl oxidation in short-term tests. The HCl conversion values obtained at 15 min and 3 h on stream were in the range of 14–30% (Table 2). Cr₂O₃/Al₂O₃ and Cr₂O₃/SiO₂ showed the highest and most stable activity. Cr₂O₃/TiO₂ resulted initially quite active, but the HCl conversion dropped to ca. 14% at the end of the test. While the other catalysts were apparently stable in the evaluated time frame, significant volatilisation of the active phase occurred for Cr₂O₃/Al₂O₃ immediately after exposure to the reaction mixture. As no activity deterioration was observed for the latter, we assumed that, in view of the relatively high chromium loading, enough active phase was left on the support to account for a constant Cl₂ production for the duration of the run. Based on the pronounced structural instability of Cr₂O₃/Al₂O₃ and the considerable deactivation suffered by Cr₂O₃/TiO₂, further kinetic studies were performed on the silica-supported catalyst.

Table 2 Characterisation and catalytic data of supported Cr₂O₃ catalysts

Catalyst	State	X HCl (%)		S BET (m^2 g^–1)	Cr content (wt%)	Cr loss^c (wt%)	Crystallite size^e (nm)
		15 min	3 h
a Conditions: O2/HCl = 2, Tbed = 678 K, P = 1 bar, W/ṅ^0(HCl) = 5.6 g h mol^−1. b Total surface area of the supports in brackets. c Determined after reaction for 3 h under the conditions in ‘a’. d Chromium content/loss after 40 h on stream in brackets. e Determined by application of the Scherrer equation to the XRD data. f Chromium content/loss at O2/HCl = 4, Tbed = 678 K, P = 1 bar, W/ṅ^0(HCl) = 5.6 g h mol^−1, and t = 10 h.
Cr2O3/SiO2	Fresh	28	26	149 (189)^b	13.4	—	27
	Used	—	—	149	12.8 (9.7)^d	4.5 (27.6)^d	30
	Used	—	—	—	12.6^f	6.3^f	—
Cr2O3/Al2O3	Fresh	28	30	162 (191)	12.7	—	8
	Used	—	—	148	8.5	32.5	10
	Used	—	—	—	9.2^f	27.4^f	—
Cr2O3/TiO2	Fresh	21	14	34 (52)	13.3	—	14
	Used	—	—	32	12.8	3.7	16

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Fig. 6a shows the dependence of the rate of Cl₂ production per gram of catalyst on the feed O₂/HCl ratio over Cr₂O₃/SiO₂. Data for bulk Cr₂O₃ have been included for comparative purposes. Cr₂O₃/SiO₂ appears ca. 2 times more active than bulk Cr₂O₃ at O₂/HCl = 7. If the rate is calculated per mol of chromium, the activity of Cr₂O₃/SiO₂ exceeds that of bulk Cr₂O₃ by one order of magnitude at the same O₂/HCl ratio. The rate increased upon raising the relative O₂ content in the feed mixture and the formal reaction order on O₂ for Cr₂O₃/SiO₂ is 0.3 (±0.02). This dependence is thus stronger for the supported catalyst than for bulk Cr₂O₃ (reaction order = 0.2, Fig. 1b and 6a). Catalytic testing of Cr₂O₃/SiO₂ at temperatures between 560 and 680 K at O₂/HCl = 2 (Fig. 1d) enabled the estimation of E_a, which is 96 (±1.5) kJ mol⁻¹. The value is practically identical to that obtained for unsupported Cr₂O₃ (97 kJ mol⁻¹), suggesting that the kinetics of HCl oxidation is not affected by deposition of the active phase on this carrier.

Fig. 6 (a) Rate of Cl₂ production per gram of catalyst versus feed O₂/HCl ratio. (b) Stability test of Cr₂O₃/SiO₂ and Cr₂O₃/Al₂O₃ showing the fraction of initial activity versus time-on-stream. (c) Deactivation constant and chromium loss in stability tests versus feed O₂/HCl ratio. Conditions are detailed in Section 2.2.

The long-term performance of Cr₂O₃/SiO₂ was tested in a 40-h run at O₂/HCl = 2. Fig. 6b shows the fraction of initial activity (F = X_t/X₀) as a function of time for this catalyst, where X_t and X₀ represent the HCl conversion at time t and 0 h, respectively. X₀ was obtained by extrapolation of the linear trend line of the X_HClversus time-on-stream plot. F consistently declined throughout the test, indicating strong and constant deactivation. Aiming at determining experimental conditions that would minimise this phenomenon, Cr₂O₃/SiO₂ as well as Cr₂O₃/Al₂O₃ were tested at different feed O₂/HCl ratios (between 0.5–7) for 10 h on stream. The deactivation constant, γ (h⁻¹), was derived in each case from the slope of the trend lines (F = 1–γt) attained by plotting F versus t, as depicted in Fig. 6b. The resulting correlation between γ and the O₂/HCl ratio is shown in Fig. 6c. For Cr₂O₃/SiO₂, the value of γ linearly decreased raising the O₂/HCl ratio up to 4, but a further increase in the O₂/HCl ratio did not lead to better stabilisation. Consequently, O₂/HCl = 4 was identified as the optimum feed ratio for this catalyst. For Cr₂O₃/Al₂O₃, γ was ca. 2 times higher than for Cr₂O₃/SiO₂, indicating much stronger deactivation (Fig. 6c). Very high O₂/HCl ratios, which would lie out of the ranges of economic operation conditions, would likely be required to curtail the activity loss. The origin of the observed deactivation is tackled in Section 3.3.

3.2.2. Characterisation. The chromium content, as determined by ICP-OES, was about 13 wt% for all the catalysts, thus only slightly lower than the nominal value of 14 wt% (Table 2). The XRD patterns of the materials exhibit the characteristic reflections of chromium(III) oxide and of the corresponding carriers (Fig. 7). The peaks of Cr₂O₃ on Al₂O₃ are remarkably less intense than in the other cases. Applying the Scherrer equation to the (110) reflection, a much smaller particle size (Table 2) was estimated for Cr₂O₃/Al₂O₃ (8 nm) than for Cr₂O₃/TiO₂ (14 nm) and Cr₂O₃/SiO₂ (27 nm), suggesting that the nature of the support strongly influences the degree of dispersion and crystallinity of the active phase. XRD analysis of the used samples revealed no phase alteration (i.e. by chlorination), but a slight increase in crystallinity for all catalysts (Table 2), which indicates minor sintering. The original S_BET of the pure carriers dropped rather significantly upon chromium incorporation. Pore blockage by the CrO_x phase is the most probable reason, due to the relatively high chromium loading (Table 2). S_BET of used Cr₂O₃/SiO₂ and Cr₂O₃/TiO₂ was practically unaltered, while it slightly decreased in the case of Cr₂O₃/Al₂O₃.

Fig. 7 XRD patterns of the supported Cr₂O₃ samples in fresh and used (in O₂/HCl = 2 at 678 K for 3 h) forms. Reflections of Cr₂O₃ (JCPDS 70-3766) are indicated by (○). Unmarked reflections belong to the carriers.

The H₂-TPR profiles of fresh and used supported catalysts (Fig. 8) are characterised by two peaks due to chromium reduction, while the supports were irreducible under the applied conditions. The position of these features is substantially similar to the case of bulk Cr₂O₃. Thus, peak 1 is again attributed to the reduction of Cr⁶⁺ and/or Cr⁵⁺ to Cr³⁺ and peak 2 to the reduction of Cr³⁺ to Cr²⁺. Nevertheless, the variable degree of dispersion of the active phase, its particle size, and extent of interaction with the supports determined evident shifts of the peaks maxima.³² Still, in all of the cases their relative intensity reveals that a larger amount of chromium is present in higher oxidation states in the fresh catalysts, contrarily to the bulk oxide. This finding, combined with the greater exposed surface, likely explains the higher activity of supported with respect to bulk Cr₂O₃ (Fig. 6a). The intensities of peak 1 for the various catalysts are clearly different. For instance, Cr₂O₃/Al₂O₃ contains ca. 18 times more Cr⁵⁺/Cr⁶⁺ species than Cr₂O₃/SiO₂. Hence, the properties of supports also determine in which chemical form chromium is stabilised on their surface. The anchoring process of chromium on metal oxides has been described as an acid–base type reaction, in which the weaker acid H₂O is replaced by the stronger acid H₂CrO₄ (or H₂Cr₂O₇).⁴⁰ Typically, silica is more acidic than alumina. Consistently, the PZC of the SiO₂ and Al₂O₃ employed in this study were measured as 3.4 and 8.0, respectively. Thus, mono-chromates are readily fixed onto the alumina surface in a well-dispersed manner, but a combination of mono- and poly-chromates would rather form on silica. In the latter case, the interaction with the surface hydroxyl groups of the support is insufficient to break the poly-chromate clusters into mono-chromates. In addition, silica usually offers fewer OH sites than alumina. This determines that a lower number of chromates can actually graft, while a larger fraction is only weakly attached.⁴¹ As a result, Cr⁶⁺ species are less dispersed and calcination leads to bigger Cr₂O₃ aggregates.⁴⁰ XRD is most likely blind with respect to chromate species in view of their very small size. The prevalence of supported chromium species in either high or low oxidation state is corroborated by the appearance of the catalysts (Fig. 9). The dark orange colour of fresh Cr₂O₃/Al₂O₃, more similar to CrO₃, suggests that it is rich in Cr⁶⁺ species, whereas the pale olive tone of Cr₂O₃/SiO₂ resembles that of Cr₂O₃, pointing to the dominance of Cr³⁺ species in this catalyst. Considering the H₂-TPR profiles of the used samples, peak 1 remarkably decreased in intensity, suggesting that Cr⁶⁺ species either volatilised or reduced to Cr³⁺ during reaction, as observed for bulk Cr₂O₃ (Fig. 3, profiles a, b). The peak was almost depleted for Cr₂O₃/Al₂O₃. The compositional change can be visualised by a clear alteration of the colour from dark orange to pale green (Fig. 9). Additionally, a coloured condensate was collected downstream the reactor in the first few minutes of the test (vide supra). Although this catalyst is expected to be more active than Cr₂O₃/SiO₂ in view of the higher amount of Cr⁵⁺/Cr⁶⁺ species present, its instability likely caused substantial deactivation already before the first activity measurement. For Cr₂O₃/SiO₂ the intensity of the H₂-TPR peak 1 was about halved and the material assumed a somewhat darker green shade.

Fig. 8 H₂-TPR profiles of the supported Cr₂O₃ samples in fresh and used (in O₂/HCl = 2 at 678 K for 3 h) forms.

Fig. 9 Pictures of the representative catalysts prior to and after reaction.

3.3. Chromium loss and catalyst stability

Stability constitutes one essential parameter in catalyst development and is particularly critical in the case of HCl oxidation. Due to the health hazards associated with chromium, it is relevant to discuss the deactivation mechanism and, thus, if chromium is lost upon use and to which extent and in which form this occurs.

Bulk Cr₂O₃ exhibited a stable activity in the short-time screening and no bulk chloride phases were detected by XRD in the used sample. Nevertheless, analysis of the zeolite material used as a guard bed during the test indicated a minor but measurable chromium loss (ca. 0.05 wt% during 5 h on stream). The performance of supported Cr₂O₃ catalysts has been later shown to strongly deteriorate already in a short time frame (10–40 h). Elemental analysis of the used supported catalysts confirmed chromium depletion in all cases (Table 2). Cr₂O₃/Al₂O₃ suffered from the highest chromium loss, ca. 32% after 3 h on stream. For SiO₂- and TiO₂-supported catalysts, the decrease in metal content was only ca. 4% upon the same treatment, but a similar value (28%) was attained for Cr₂O₃/SiO₂ after 40 h on stream. Fig. 6c indicates that the variation of the deactivation constant and of the chromium loss at different O₂/HCl ratios agrees well for Cr₂O₃/SiO₂ and Cr₂O₃/Al₂O₃. This proves that the prime origin of activity deterioration of Cr₂O₃-based catalysts is the volatilisation of the active phase. The fact that no bulk chloride was detected after reaction suggests that the latter occurs via a different mechanism compared to copper-based catalysts,²⁰ or that the instability of the chlorine-containing chromium phases possibly formed is much higher.

According to the literature, Cr₂O₃ (Cr³⁺) as well as surface chromyl species (Cr⁵⁺) produced during reaction should be stable at the reaction temperature,³⁷ while Cr⁶⁺ species, namely, CrO₃, CrO₂(OH)₂, and CrO₂Cl₂ are very labile.^38,42–44 The contribution of CrO₃ to the chromium loss is considered negligible in view of the reasons cited in Section 3.1.4 and its reactivity (herein explained). Therefore, the observed metal depletion is suggested to be mainly due to CrO₂(OH)₂ and/or CrO₂Cl₂. CrO₂(OH)₂ is generated by the action of O₂ and H₂O above 673 K from Cr^3+38,44 and of H₂O only from Cr⁶⁺ species between 408 and 458 K⁴⁵ and, therefore, possibly formed also under our experimental conditions (eqn (1) and (2)). CrO₂Cl₂ can be produced as a gaseous species by the reaction of HCl with surface chromates already at 403 K (eqn (3)).^42,43 This compound can either leave the solid or, upon interaction with surface hydroxyl groups, generate surface Cr(VI) monochlorides, which can decompose into chromates liberating Cl₂ in the presence of O₂.⁴² Hence, a higher feed O₂ concentration should reduce the chromium loss related to the formation of CrO₂Cl₂. A significantly slower deactivation rate and lower metal loss were in fact observed when the reaction was performed in O₂ excess (Fig. 6c), although the effect levelled off at a feed O₂/HCl ratio above 4. Consequently, formation of CrO₂Cl₂ might be regarded as the dominant deactivation path. The higher chromium depletion for Cr₂O₃/Al₂O₃ can be rationalised in view of the presence of a larger initial amount of oxidised chromium species, compared to Cr₂O₃/SiO₂ and Cr₂O₃/TiO₂ (Section 3.2.2, Fig. 8), which can directly react with HCl or product H₂O resulting in both CrO₂Cl₂ and CrO₂(OH)₂ (eqn (1) and (3)).⁴²

½Cr₂O₃ + ¾O₂ + H₂O ↔ CrO₂(OH)₂ (1)

CrO₃ + H₂O ↔ CrO₂(OH)₂ (2)

CrO₃ + 2HCl ↔ CrO₂Cl₂ + H₂O (3)

The deactivation pathway of chromium-based catalysts offers a novel facet with regards to the activity loss of Deacon catalysts. In fact, it is neither based on bulk chlorination, as in the case of copper-based systems,¹ nor on sintering, as for the ruthenium-based system,¹² but rather proceeds through the formation of species (containing and/or non-containing chlorine) which are highly unstable under reaction conditions.

Taking into account the enormous activity drop, chromium loss, and, possibly, operational issues (mechanical instability of the catalyst shapes), chromium-based systems, in a fixed-bed configuration, prove not suitable for industrial application to HCl oxidation. Indeed, as mentioned in the Introduction, Mitsui suggested the use of a fluidised-bed reactor, the regular replenishment of the vaporised catalyst portion by a fresh load,⁷ and the addition of (allegedly) stabilizing compounds of potassium, copper, and lanthanum to the Cr₂O₃/SiO₂ catalyst.⁴⁶ Nevertheless, off gas and waste liquid streams have to be carefully treated to comply with the allowable limit of Cr(VI) (0.1 ppm).

4. Conclusions

Fundamental aspects of HCl oxidation to Cl₂ on bulk and supported Cr₂O₃ catalysts, covering performance, stability, and mechanism, have been investigated. Bulk Cr₂O₃ resulted remarkably active and apparently stable. Kinetic experiments of this oxide as well as CrO₃ in a fixed-bed reactor coupled to various physico-chemical characterisations provided explicit evidence for the active species and enabled us to derive a catalytic redox cycle for HCl oxidation, in which chromium reversibly cycles between the (III) and higher (V and VI) oxidation states. Oxidation of Cr₂O₃ by gas-phase O₂ to generate surface chromyl species appeared as rate determining, owing to the positive dependence of the HCl conversion on the inlet oxygen content. The chemical, textural, and structural properties of the carriers (silica, alumina, and titania) greatly influenced the particle size and distribution as well as the dominant oxidation state of supported chromium. Cr₂O₃/SiO₂, mainly featuring bigger particles and chromium in (III) oxidation state, exhibited superior performances. The kinetics of the partial O₂ pressure and the dependence on the reaction temperature found for this catalyst were similar to those of bulk Cr₂O₃. Nevertheless, all supported catalysts underwent substantial chromium loss, ultimately resulting in dramatic activity deterioration. Among the various labile Cr⁶⁺ species possibly responsible for the metal depletion, CrO₂Cl₂ seems to play a predominant role, as suggested by the significant decrease in the deactivation rate at high inlet O₂/HCl ratios. The short lifetime and the major chromium loss likely represent important reasons for the limited industrial success of chromium-based catalysts in HCl oxidation.

Acknowledgements

We thank Bayer MaterialScience for granting permission to publish these results.

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