Selective Propylene Production via Propane Oxychlorination on Metal

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Selective Propylene Production via Propane Oxychlorination on Metal Phosphate Catalysts Guido Zichittella, Samuel Stähelin, Florian M Goedicke, and Javier Pérez-Ramírez ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Catalysis

Selective Propylene Production via Propane Oxychlorination on Metal Phosphate Catalysts Guido Zichittella, Samuel Stähelin, Florian M. Goedicke, and Javier Pérez-Ramírez* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland

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ABSTRACT This article investigates phosphates of transition metals (MPO, M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) as catalysts for the selective transformation of propane via oxychlorination chemistry to propylene, a pivotal chemical commodity. The oxychlorination activity of the catalysts decreases in the order: CuPO >> VPO > CoPO >> CrPO > FePO > MnPO ≈ NiPO ≈ TiPO, while the selectivity to propylene ranks as CrPO (95-98%) ≈ FePO (95-98%) > MnPO (67-85%) ≈ NiPO (65-85%) ≈ TiPO (66-75%) > CoPO (62-70%) > CuPO (33-45%) > VPO (30-42%) at 5-20% propane conversion. In-depth characterization using X-ray diffraction, Raman spectroscopy, and microscopy enables to assess the structural and morphological stability of the phosphate samples. The most selective catalyst, CrPO, whose performance, crystal structure, and morphology are demonstrated stable for over 60 h on stream, enables to reach propylene yields up to 50%, rivalling the best systems among any existing propane-to-propylene technology. Furthermore, the phosphate systems can be classified into four categories, depending on which products they favor, as propylene (CrPO, FePO), cracking products (MnPO, NiPO, TiPO, CoPO), chlorinated hydrocarbons (CuPO), or carbon oxides (VPO). Finally, a kinetic analysis of the oxychlorination, chlorination, and oxidation of propane as well as of propyl chloride dehydrochlorination and HCl oxidation over representative systems of each category rationalizes activity and selectivity patterns. In particular, the former is correlated to the material’s ability to evolve chlorine, while a high selectivity towards propylene is a consequence of fast dehydrochlorination kinetics in combination with a hindered ability to evolve molecular chlorine to the gas phase and a low propensity towards propane cracking and combustion. KEYWORDS: heterogeneous catalysis, metal phosphates, natural gas, propane oxychlorination, propylene, selectivity


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1. INTRODUCTION Vast natural gas reserves contains up to 10 mol.% of propane, offering the potential feedstock to manufacture propylene, a pivotal platform molecule for the production of polymers, chemicals, and pharmaceuticals.1-5 Today, this olefin is mainly manufactured via steam- and fluid-catalytic cracking of naphtha or ethane.6,7 Nevertheless, these technologies are highly energy-intensive, with steam cracking being estimated as the most energy-demanding process in the chemical industry.8 Moreover, these processes largely yield ethylene, resulting in a gap between propylene production and demand.9-13 It was predicted that the replacement of these technologies with “on-purpose”, one-step, and selective alkane oxidation processes would result in substantial economic benefits.14 Among various strategies proposed, including the catalytic dehydrogenation and oxidative dehydrogenation of propane,3,10-13 catalytic oxychlorination, which comprises the reaction of an alkane with hydrogen chloride (HCl) and O2, has been demonstrated as an attractive route for the transformation of propane into propylene.15,16 Intensive research efforts led to the discovery of efficient catalysts including iron phosphate (FePO4), europium oxychloride (EuOCl), and nickel oxide modified ceria (NiO-CeO2), reaching propylene yields of ≤ 15%, 40%, and 55% at high (80-98%) selectivity, respectively.15,16 In-depth kinetic analyses over these systems, as well as density functional theory (DFT) calculations over NiO-CeO2, provided strong insights on the mechanism of this reaction, which is believed to proceed via propyl chloride generation, followed by its dehydrochlorination into C3H6 over the catalyst surface.15-17 An analogous chlorine-induced surface confined mechanism has been recently demonstrated in ethane oxychlorination, EOC, over FePO4, which was shown to be pivotal for guaranteeing a high olefin selectivity.18 Despite these promising results, few systems have been disclosed to date as catalysts for propane oxychlorination, POC, thus


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generating great potential for new discoveries. This is most likely to be attributed to the much higher reactivity of propane and/or propane derivatives compared to ethane-derived compounds, which renders selectivity control a formidable challenge. Accordingly, selective catalysts for ethylene production via EOC, including titanium oxide (TiO2) and bulk CeO2, have been shown to generate significant amounts of carbon oxides (COx; selectivity 40% and 70%, respectively) when assessed in POC.15 A possible strategy to suppress over-oxidation pathways is to apply the concept of redox-site isolation in a quasi-inert matrix in order to better control the surface-active oxygen species, thus tuning the oxidizing potential of the catalyst. Metal phosphates represent a viable solution, since their crystal unit cells encapsulate metal oxide clusters (MOx) within a phosphate matrix, thereby altering the length, and thus the electronic properties, of M−O bonds compared to bulk metal oxides (Figure 1).19,20 In fact, these systems have been demonstrated as effective in several direct alkane oxidation routes, including oxidative dehydrogenation of light alkanes,10,21 oxidative coupling of methane,22,23 partial alkane oxidation to oxygenates,24,25 and methane oxybromination,26 and could thus open new opportunities for the development of selective catalysts for propylene production via oxychlorination chemistry. Herein, the phosphate of transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) are synthesized, characterized, and assessed in POC. Four categories are identified depending on the product preferentially generated, and a representative of each class is further evaluated in the oxidation of propane and of HCl as well as propyl chloride dehydrogenation, enabling to extract kinetic parameters that correlate to activity and selectivity patterns in POC. Detailed characterization of the samples elucidates on the structural and morphological stability of the catalysts upon exposure to the reaction environment. Finally, CrPO is found as the most selective system, reaching propylene yields that rivals the best catalysts reported to date.


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2. EXPERIMENTAL 2.1. Catalyst Preparation. Commercial cobalt phosphate (CoPO) was calcined in static air. Vanadium phosphate (VPO) was prepared by (i) refluxing a suspension of V2O5 and H3PO4 with a molar P:V ratio of 1.2 in isobutanol and benzyl alcohol, and by (ii) filtering, drying in vacuum, and thermally activating in flowing N2 the resulting solid. Copper phosphate (CuPO), manganese phosphate (MnPO), and nickel phosphate (NiPO) were synthesized by (i) mixing an aqueous solution of CuCl2·2H2O, MnCl2·4H2O, Ni(NO3)2·6H2O with H3PO4 at a molar P:M (M = Cu, Mn, Ni) ratio of 2, by (ii) precipitating the solutions via dropwise addition of aqueous NH4OH, and (iii) by filtering, washing, drying in vacuum, and calcining in static air the obtained precipitate. Titanium phosphate (TiPO) was prepared by (i) reacting TiO2-rutile with H3PO4 in a molar P:Ti ratio of 3, and by (ii) drying and calcining in static air the resulting paste. Chromium phosphate (CrPO) was synthesized by (i) mixing stoichiometric amounts of CrCl3·6H2O and KH2PO4 with a solution of NaCH3COO·3H2O at a molar Cr:Na ratio of 0.9, and by (ii) filtering, washing, drying, and calcining in flowing air the obtained precipitate. Iron phosphate (FePO) was synthesized by (i) mixing an aqueous solution of Fe(NO3)3·9H2O and NH4H2PO4 in a molar P:Fe ratio of 1, followed by (ii) drying in vacuum and calcination in flowing air. All details concerning each step of the synthesis of every material are fully disclosed in the Supporting Information. The catalysts are referred to as MPO, where M denotes the specific metal, i.e., Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and PO represents any phosphate structure. It is important to clarify that the catalyst nomenclature (MPO) does not imply any phase composition, while it only represents the metal present in the material. 2.2. Catalyst Characterization. N2 sorption at 77 K was performed in order to characterize the textural properties of the catalysts, while their crystal phase composition and structural


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properties were assessed by powder X-ray diffraction (XRD) and Raman spectroscopy High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) were conducted to evaluate the morphological and structural stability of the phosphate samples upon exposure to the reaction environment Finally, X-ray photoelectron spectroscopy (XPS) measurements were performed on the CrPO in fresh form and after equilibration in propane oxychlorination at different times on stream to determine the stability and composition of the catalyst surface upon exposure to the reaction environment. All details concerning each technique used to characterize the catalysts are fully disclosed in the Supporting Information. 2.3. Catalyst

testing. Propane

oxychlorination

(C3H8 + HCl + O2),

propane

oxidation

(C3H8 + O2), propyl chloride dehydrochlorination (C3H7Cl), and hydrogen chloride oxidation (HCl + O2) were performed at ambient pressure in a continuous-flow fixed-bed reactor setup described elsewhere.15 Briefly, the gases: C3H8, HCl, O2, Ar, and He were fed using digital mass-flow controllers, and 1-C3H7Cl was dosed using a syringe pump with a water-cooled syringe coupled to a vaporizer. A quartz reactor was loaded with the catalyst, and placed in a homemade electrical oven. Prior to testing, the bed was heated in a He flow to the desired temperature (T = 400-953 K) and allowed to stabilize for at least 30 min before the reaction mixture was fed at desired space velocity (FT:Wcat = 6000 cm3 h-1 gcat-1) and feed composition (Table 1). All flow units correspond to standard temperature and pressure (STP) conditions, i.e., 273 K and 1 bar. All carbon-containing compounds and Ar were quantified on-line via a gas chromatograph coupled to a mass spectrometer. Quantification of Cl2 at the reactor outlet was performed by its absorption in an impinging bottle filled with 0.1 M KI solution followed by iodometric titration of the formed triiodide with 0.01 M sodium thiosulfate solution.


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The conversion of propane in oxychlorination and propane oxidation as well as the conversion of propyl chloride in dehydrochlorination, Xi (i: C3H8, C3H7Cl), the conversion of hydrogen chloride in hydrogen chloride oxidation, XHCl, the reaction rate expressed with respect to propane and based on the catalyst mass, rC3H8, or surface area, rSC3H8, the selectivity, Sj, and the yield, Yj, of product j (j: C3H6, C2H4, CH4, C3H7Cl, C3H5Cl, C2H5Cl, C2H3Cl, CH3Cl, CO, and CO2), and the error of the carbon mass balance, C, were calculated using eqs 1-7, respectively, in which niinlet and nioutlet are the molar flows of the reactant i at the reactor inlet and outlet, respectively, Scat is the specific surface area of the catalyst, njoutlet is the molar flow of product j at the reactor outlet, and NC,i and NC,j are the number of carbon atoms in the compound i and j, respectively. The error of the carbon mass balance was less than 5% in all experiments. The evaluation of the dimensionless moduli based on the criteria of Carberry, Mears, and Weisz–Prater confirmed that all the catalytic tests were performed in the absence of mass and heat transfer limitations.27,28

Xi 

niinlet  nioutlet  100, % niinlet

X HCl 

rC3H8 

rCS3H8 

2  nCloutlet 2 inlet nHCl

nCinlet  X C3H8 3 H8 100  Wcat

1 , molC3H8 h 1 g cat

nCinlet  X C3H8 3 H8 100  Wcat  Scat

Sj 

n

 100, %

2 , molC3H8 h 1 m cat

n outlet  N C, j j

inlet i

 nioutlet   N C, i

Yj 

Xi  S j 100

 100, %

,%

ACS Paragon Plus Environment

(1 ) (2)

(3)

( 4 ) ( 5 ) ( 6 ) ( 7 7 )

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C 

 N C, j ) niinlet  N C, i  (nioutlet  N C, i   n outlet j niinlet  N C, i

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 100, %

All details concerning the catalytic testing and analytics are fully disclosed in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Fresh Catalysts. N2-sorption, X-ray diffraction, and Raman spectroscopy were adopted to assess the textural and structural properties of the metal phosphate catalysts (Figure 2 and Tables 2 and S1). It is important to clarify that the catalyst nomenclature (MPO) does not imply any phase composition, while it only represents the metal present in the material. The specific surface areas of the metal phosphate samples were in the range of 316 m2 g-1, with the only exception of VPO that exhibited a higher surface area (30 m2 g-1). Excluding MnPO and CuPO, the XRD and Raman spectra of all other phosphate materials showed single phases (Figure 2 and Tables 2 and S1). The average crystallite size of the identified phases as determined by Scherrer equation were in the range of 31-56 nm (Table 2), indicating the high crystallinity of the phosphate samples, with the only exception of NiPO that showed crystalline domains within an amorphous matrix (Figure 2a). In particular, VPO, TiPO, and NiPO systems displayed the reflections of the corresponding metal pyrophosphates (Figure 2a and Table 2). Raman spectroscopy corroborated these observations, as it evidenced the characteristic bands of (VO)2P2O7 (925 and 1186 cm-1) and TiP2O7 (252, 1040, 1075, and 1163 cm-1),29-30 while over NiPO only one band at 480 cm-1 was observed (Figure 2b and Table S1), corroborating the lower crystallinity of this sample. The latter is characteristic of phosphate structures, since it is generally ascribed to symmetric bending modes ( 2) of O−P−O


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bonds in orthophosphates or to deformation modes () of PO3 groups in pyrophosphates,30-31 explaining its presence in the Raman spectra of virtually all metal phosphate samples investigated in this study (Figure 2b and Table S1). Similarly, the XRD patterns of CrPO, FePO, and CoPO displayed only reflections from the corresponding metal orthophosphates, which was corroborated by the detection of the typical Raman bands of -CrPO4 (305, 557, 945, 1082, and 1201 cm-1),32 FePO4 (272, 433, and 1010 cm-1),33 and Co3(PO4)2 (147, 335, 610, 964, and 1011 cm-1),34 respectively (Figure 2 and Tables 2 and S1). On the contrary, the XRD pattern of CuPO evidenced the reflections of Cu3P and non-stoichiometric Cu16O14.15 as well as those of Cu(OH)2 and Cu3(PO4)2, whose characteristic Raman bands at 495 and 958 cm-1 were observed, respectively.35-36 In addition, the spectra showed bands at 279 and 650 cm-1 that suggest the presence of CuO and Cu2(PO4)(OH), respectively (Figure 2 and Tables 2 and S1).35,36 Similarly, several phases were observed in the diffractogram of the MnPO sample, including MnP4, non-stoichiometric -Mn0.98O2, MnP4O11, and  '-Mn3(PO4)2. Accordingly, the Raman spectra evidenced bands at 932 and 954 cm-1, which are associated to symmetrical stretching ( 1) mode of the PO4 group.37 Additionally, Mn2P2O7 phase is likely present as well, as suggested by the detection of a band at 964 cm-1, which typically indicates the  1(PO3) mode of manganese pyrophosphate (Figure 2b and Table S1).37 3.2. Evaluation of Catalysts in Propane Oxychlorination. The activity and product distribution of the metal phosphate catalysts in propane oxychlorination, POC, was investigated in a wide range of temperatures and feed concentrations (Figure S1). The performance were evaluated over the materials that were equilibrated for over 5 h under the reaction environment in order to avoid any potential influence from catalyst restructuring (vide infra). The reaction rates expressed with respect to the mass of the catalyst (eq. 3) and measured at 723 K under


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kinetically-controlled conditions (conversion < 20%) reflected the activity differences between the metal phosphates (Figure 3a). In essence, the activity in POC decreased in the order CuPO >> VPO > CoPO >> CrPO > FePO > MnPO ≈ NiPO ≈ TiPO, which is comparable to the activity rankings observed for phosphate- and/or oxide-based material of these elements in congeneric HCl oxidation and/or methane oxyhalogenation.26,38-40 When comparing to the reaction rates based on the surface area of the catalyst (eq. 4), the relative order of activity between CoPO, CrPO, FePO, MnPO, NiPO, and TiPO was kept, while CuPO and VPO diverged and were classified as the second and fifth most active catalysts, respectively (Figure S2). The difference in rankings is likely attributed to the possible non-catalytic reaction pathways that can occur with in situ generated molecular chlorine, particularly over CuPO and VPO that are active HCl oxidation catalysts, which are not taken into account when considering a surface area-based reaction rate. Similar differences in rate-based comparisons were observed in methane oxyhalogenation, where gas phase pathways are of significant importance.40 The selectivity patterns obtained over the catalysts were compared at ca. 20% propane conversion (Figure 3b), which is representative of all conditions investigates in this study (Figure S1). The selectivity to C3H6 decreased in the order FePO (98%) ≈ CrPO (96%) > MnPO (67%) ≈ TiPO (66%) ≈ NiPO (65%) ≈ CoPO (62%) > CuPO (45%) > VPO (31%). Based on this criterion, the catalytic systems can be classified into four distinct categories. The first one is represented by CrPO and benchmark FePO, which led to the highest selectivity to C3H6, while MnPO, TiPO, NiPO, and CoPO represent the second one, which generated a significant fraction of cracking products, such as C2H5Cl, C2H3Cl, CH3Cl, and particularly C2H4 and CH4. The third and fourth categories are embodied by CuPO and VPO, which yielded substantial amounts of chloropropanes (C3H7Cl and C3H5Cl) as well as coke and carbon oxides, respectively


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(Figures 3b and S1). Further evaluation of the best performing CrPO under variable conditions showed that the high C3H6 selectivity (> 90%) can be preserved over a broad range of temperatures (673-773 K) and feed concentration of alkane (1.5-6 vol.%) and HCl (1.5-15 vol.%) (Figure 4a). In particular, a single-pass yield of C3H6 up to 50% at 95% selectivity could be obtained over CrPO, rivaling the best systems reported to date in POC and surpassing any other propylene production technology (Figure 4b).3,6,7,10,11,15,16 Furthermore, this catalyst was assessed in a long-term test at moderate propane conversion, evidencing its stable catalytic behavior for over 60 h in POC (Figure 5). 3.3. Characterization of the Equilibrated Catalysts. The X-ray diffractograms, Raman spectra, and N2-sorption analysis of the catalysts after 5 h equilibration in propane oxychlorination showed no changes in the crystal structure, average crystallite size, and textural properties in all phosphate samples, excluding CuPO and MnPO (Figure 2 and Tables 2 and S1), respectively. This indicates that the vast majority of the investigated metal phosphates are stable systems in this reaction from a crystal structure point of view, which is a great challenge for many catalysts in halogen chemistry.1 Reflections of only -Cu2P2O7 were observed in the XRD pattern of CuPO, which was corroborated by the detection of Raman bands at 358, 411 ( (PO3)) and 1050 cm-1 ( 1(PO3)), as well as at 544 cm-1 and at 1078 and 1145 cm-1, representing antisymmetrical bending ( 4) and stretching ( 3) modes of PO3 groups, respectively.41 On the other hand, the XRD pattern of MnPO after equilibration in POC resembled that of the fresh sample, since MnP4, MnP4O11, and  '-Mn3(PO4)2 phases were observed. Still, the reflections of -Mn0.98O2 disappeared, while those of Mn2P2O7 could be detected. Accordingly, the Raman spectra evidenced a strong increase in the intensity of the band at 964 cm-1, indicating the  1(PO3) mode of Mn2P2O7, as well as the appearance of a band at 11 ACS Paragon Plus Environment

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1028 cm-1, which represents the antisymmetrical stretching of manganese orthophosphate ( 3(PO4)).37 Despite the structural changes and the loss of surface area, the morphology of MnPO and CuPO was not significantly altered after POC, as assessed by scanning electron microscopy (Figure S3). Similar results were observed for VPO (Figure S3), which also showed a decrease in surface area (Table 2), and it was taken as representative of all other phosphate samples that remained structurally unchanged after POC. The SEM micrographs as well as the XRD patterns and Raman spectra of CrPO after 5 and 62 h equilibration in POC were essentially identical to those of the sample in fresh form (Figures 2 and 6), demonstrating its morphological and structural stability in this reaction. This was corroborated by high-resolution transmission electron microscopy, evidencing the preservation of the interlattice distances of 2.5 and 3.5 Å (Figures 6 and S4), which correspond to the main -CrPO4(112) and -CrPO4(111) planes, respectively, as also observed by XRD. Furthermore, the inspection of the HRTEM micrographs revealed a slight decrease in the irregularity of the CrPO crystals after the reaction. X-ray photoelectron spectroscopy (XPS) was conducted to probe the surface of the CrPO catalyst prior to and after equilibration in POC. The analysis of the Cr 2p, P 2p, and O 1s core level XPS spectra showed no significant alternations between the CrPO in fresh form or after 5 and 62 h equilibration in POC (Figures 7a and S5), indicating the stability of the catalyst surface in the reaction. This was corroborated by surface atomic concentrations that revealed constant Cr:P ratios (ca. 0.85 at.%:at.%) in the fresh CrPO and after 62 h in POC (Table S2). Additionally, this surface phosphorous enrichment might explain the very low Cr6+ content and the absence of any other coordination states of chromium observed by XPS, compared to the typical Cr 2p core level XPS spectra of Cr2O3,42 as well as the low oxidizing potential of this catalyst. Finally, chlorine was observed over the catalyst surface


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only after equilibration in POC (ca. 0.3 at.%), as indicated by the analysis of the Cl 2p core level XPS spectra and surface atomic concentrations (Figure 7b and Table S2). Despite being relatively low, the detection of chlorine on the CrPO surface could indicate its role in the reaction network of POC. Recent mechanistic studies in ethane oxychlorination, EOC, over FePO demonstrated the pivotal role of surface chlorine in driving the reaction towards the olefin.18 Additionally, ex-situ XPS study after EOC of FePO, which has similar surface area to CrPO, showed a chlorine content of ca. 0.3 at.%, which is directly comparable to that observed over CrPO after POC.18 3.4. Rationalization of the Catalytic Performance in Propane Oxychlorination. Previous kinetic analysis in POC over several catalysts, including FePO as well as cerium oxide- and europium-based systems, have provided strong insights on the mechanism of this reaction, which is believed to mainly proceed via catalytic generation of C3H7Cl and its subsequent dehydrochlorination into C3H6,15-17 as schematized in Figure 8a. Although the direct catalytic formation of C3H6 from propane cannot be excluded, this route is generally considered negligible in oxyhalogenation chemistry, as corroborated by the tests in the absence of HCl (vide supra Figure 4a) and in agreement with previous studies.15-18 Nevertheless, C3H7Cl could also be formed by the non-catalytic reaction of propane with Cl2, which can be generated over the catalyst surface by the reaction of HCl and O2 (Figure 8a), as observed over several alkane oxyhalogenation systems.40,43 If Cl2 is produced during POC, it will react in the gas phase with propane, since the onset of this non-catalytic reaction is ca. 100 K lower in temperature than that of POC (Figure S6). This route, however, is not desired as it promotes unselective pathways, including polyhalogenation and coking as well as cracking and combustion (Figure 8a), as demonstrated in ethane oxyhalogenation over FePO,18 and observed in propane oxybromination


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over europium oxybromide (EuOBr).17 Still, regardless of the pathway to generate C3H7Cl, the catalyst surface needs to evolve chlorine in the form of C3H7Cl and/or Cl2 for the reaction to proceed. Consequently, one system representative of each category, i.e., CrPO, MnPO, CuPO, VPO, was evaluated in the oxidation of HCl to Cl2 in order to assess the catalyst ability to evolve chlorine (Figure 8b). The activity in this reaction was ranked in the order CuPO > VPO > CrPO > MnPO. Furthermore, the temperature at which 10% of HCl is converted in HCl oxidation, T10(HCl), was taken as a quantitative measure of the propensity of the catalyst to evolve chlorine, which is high if the value of this extracted parameter is low. This temperature was found to correlate linearly with the rate of propane consumption in POC at 723 K (Figure 9), indicating that the evolution of chlorine from the catalyst surface is a pivotal aspect in the mechanism of POC. These findings are in agreement with other studies showing that halogen evolution is a critical part of the reaction network of alkane oxychlorination reactions.18,40,43 Interestingly, no significant correlation was found between the rate of propane consumption in POC at 723 K based on the catalyst surface area and the parameter T10(HCl), as shown in Figure S7. This is in agreement with the previous observations on the divergence between the weight-based and surface area-based reaction rates (Figure 3a), since the latter does not take into account the potential gas phase pathways of alkane activation, likely occurring over VPO and particularly CuPO, which are common in halogen chemistry.18,40 To rationalize the selectivity patterns observed in POC, the representative catalysts of each category, i.e., CrPO, MnPO, CrPO, VPO, were investigated in the dehydrochlorination of C3H7Cl (Figures 8c and S8) as well as in the oxidation of C3H8 (Figures 8d and S9). The former reaction shows the ability of the catalytic systems to produce C3H6 as soon as C3H7Cl is present in the reaction medium. The reactivity of the systems was ranked in the order CrPO >>


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MnPO ≈ VPO > CuPO. Strikingly, this rank is opposite to that observed in HCl oxidation, which indicates that the ability to evolve chlorine anti-correlates with a high propyl chloride dehydrochlorination kinetics. This is in agreement with previous findings in EOC over FePO4(102), which showed that a highly chlorinated surface is beneficial for olefin generation via alkyl chloride dehydrochlorination.18 Similarly to HCl oxidation, the temperature at which 50% C3H7Cl was converted, T50(C3H7Cl), was taken as a quantitative measure of the catalyst propensity to generate C3H6 via C3H7Cl dehydrochlorination, which is high if the value of this extracted parameter is low (Figure 8c). On the other hand, C3H8 oxidation was used to probe the tendency of the phosphate systems towards alkane cracking in an oxidative environment. In particular, the product distribution in this reaction was compared at ca. 15% propane conversion (Figure 8d), which showed that MnPO was the only catalyst that produced cracking products (C2H4 and CH4), while CrPO and particularly VPO and CuPO mainly resulted into carbon oxides (COx) formation. Therefore, the selectivity towards the sum of C2H4 and CH4 in propane oxidation, Scracking, was taken as a relative measure of the catalyst propensity towards propane cracking under oxidative conditions. Interestingly, the activity in propane oxidation over these catalysts was found to be up to four times lower than that in POC, particularly over CuPO, MnPO, and CrPO (Figures S1 and S9). These differences could be attributed to possible surface structural rearrangements of the metal phosphates upon halogen adsorption, which can allow easier alkane activation when spectator halogen species are present, as recently demonstrated in EOC over FePO4(102).18 Still, despite some similarities between ethane and propane, a detailed mechanistic study, comprising advanced operando techniques as well as DFT calculations, is necessary to shed light on the reaction network of POC, which has a higher level of complexity compared to that of EOC, and it will be the focus of a future study.


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Another important factor that can affect the product distribution is the chlorine evolution ability of the catalysts, T10(HCl), as previously anticipated. A recent mechanistic study over a FePO4(102) surface in EOC has shown that evolution of chlorine in the form of Cl2 requires considerably more energy compared to that of alkyl chloride.18 Consequently, since the generation of Cl2 favors polyhalogenation and coking, a good catalyst for propylene production should inhibit the evolution of Cl2 and only favor that of propyl chloride, thus having a comparatively high value for T10(HCl). All these parameters, i.e., T10(HCl), T50(C3H7Cl), Scracking, extracted for CrPO, MnPO, CuPO, and VPO were summarized in a radar plot, as shown in Figure 10. Additionally, the selectivity towards COx determined during POC at ca. 20% propane conversion, SCOx, was used as a measure of the combustion propensity of the catalysts investigated. This kinetic parameter was extracted directly from POC in order to take into account the oxidizing potential of materials under HCl-containing conditions, which is known to vary drastically if halogens are present in the reaction environment.40 Finally, the selectivity to C3H6 obtained in POC at ca. 20% propane conversion was also chosen as axis of the radar plot to more clearly show the differences between the catalysts, even if it is not an independent variable. Therefore, all the extracted kinetic parameters define an area for each phosphate system within the radar plot, which is directly proportional to the catalyst ability to selectively generate propylene from propane via oxychlorination chemistry (Figure 10). Accordingly, CrPO presented the largest area, and its high selectivity towards C3H6 in POC could be rationalized by its low propensity towards cracking and combustion, its moderate ability to evolve chlorine, and its strong ability to dehydrochlorinate C3H7Cl into C3H6. Despite MnPO showed the lowest chlorine evolution performance and a combustion tendency comparable to CrPO, its high propensity towards propane cracking and medium C3H7Cl dehydrochlorination properties limited


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C3H6 selectivity in POC. Finally, the low C3H6 selectivity attained during POC over CuPO and VPO can be rationalized by their similarly hindered dehydrochlorination ability as well as by their respective high tendency to generate Cl2, thus explaining the observed coke formation, and strong propensity towards combustion.


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4. CONCLUSIONS We performed a comprehensive assessment of phosphates of transition metals (MPO, M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) as catalysts for propane oxychlorination, revealing stark differences in the reactivity and particularly in the product distribution as a function of the metal. The oxychlorination activity of the catalysts decreased in the order: CuPO >> VPO > CoPO >> CrPO > FePO > MnPO ≈ NiPO ≈ TiPO, while the selectivity to propylene ranked as CrPO (95-98%) ≈ FePO (95-98%) > MnPO (67-85%) ≈ NiPO (65-85%) ≈ TiPO (66-75%) > CoPO (62-70%) > CuPO (33-45%) > VPO (30-42%) at 5-20% propane conversion. Accordingly, four categories of catalysts have been identified based on the preferred generated product, such as propylene (CrPO), cracking products (MnPO, NiPO, TiPO, CoPO), chlorinated hydrocarbons (CuPO), or carbon oxides (VPO). In particular, CrPO, whose performance was demonstrated stable for over 60 h on stream, was identified as the most selective system for POC, reaching up to 50% propylene yield, thus rivalling the best catalysts reported to date. Its high stability was corroborated by in-depth characterization by means of X-ray diffraction, Raman and X-ray photoelectron spectroscopies as well as scanning and high-resolution transmission electron microscopies, which showed no alternations in the structure and morphology after the long-term run. Finally, selected systems representing each of the identified catalyst categories were assessed in the oxidation of propane and of HCl as well as in propyl chloride dehydrochlorination. This analysis revealed that the phosphate’s reactivity is a function of the materials’ ability to evolve chlorine from its surface, while a high selectivity towards propylene is a consequence of fast dehydrochlorination kinetics in combination with a hindered ability to generate molecular chlorine in the gas phase and a low propensity towards propane cracking and combustion.


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ASSOCIATED CONTENT Supporting Information. Supplementary information associated with this article, containing additional catalytic and characterization data, can be found in the online version. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Guido Zichittella: 0000-0002-7062-8720 Javier Pérez-Ramírez: 0000-0002-5805-7355 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by ETH research grant (ETH-04 16-1). The authors acknowledge Dr. Antonio José Martín Fernández and Evgeniya Vorobyeva from ETH Zurich for microscopy analyses, the Scientific Center for Optical and Electron Microscopy (ScopeM) and Prof. Ralph Spolenak of ETH Zurich for granting access to the microscopy and Raman spectroscopy facilities, respectively, Dr. Roland Hauert from Empa, Dübendorf, for X-ray photoelectron spectroscopy analyses, and Dr. Vladimir Paunović from ETH Zurich for the valuable discussions.


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(8) Ren, T.; Patel, M.; Blok, K., Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes. Energy 2006, 31, 425-451. (9) Gärtner, C. A.; van Veen, A. C.; Lercher, J. A., Oxidative Dehydrogenation of Ethane: Common Principles and Mechanistic Aspects. ChemCatChem 2013, 5, 3196-3217. (10) Cavani, F.; Ballarini, N.; Cericola, A., Oxidative Dehydrogenation of Ethane and Propane: How Far from Commercial Implementation? Catal. Today 2007, 127, 113-131. (11) Carrero, C. A.; Schloegl, R.; Wachs, I. E.; Schomaecker, R., Critical Literature Review of the Kinetics for the Oxidative Dehydrogenation of Propane over Well-Defined Supported Vanadium Oxide Catalysts. ACS Catal. 2014, 4, 3357-3380. (12) Grant, J. T.; Carrero, C. A.; Goeltl, F.; Venegas, J.; Mueller, P.; Burt, S. P.; Specht, S. E.; McDermott, W. P.; Chieregato, A.; Hermans, I., Selective Oxidative Dehydrogenation of Propane to Propene using Boron Nitride Catalysts. Science 2016, 354, 1570-1573. (13) López Nieto, J. M.; Solsona, B.; Concepción, P.; Ivars, F.; Dejoz, A.; Vázquez, M. I., Reaction Products and Pathways in the Selective Oxidation of C2–C4 Alkanes on MoVTeNb Mixed Oxide Catalysts. Catal. Today 2010, 157, 291-296. (14) Brazdil, J. F., Strategies for the Selective Catalytic Oxidation of Alkanes. Top. Catal. 2006, 38, 289-294. (15) Zichittella, G.; Aellen, N.; Paunović, V.; Amrute, A. P.; Pérez-Ramírez, J., Olefins from Natural Gas via Oxychlorination. Angew. Chem. Int. Ed. 2017, 56, 13670-13674. (16) Xie, Q.; Zhang, H.; Kang, J.; Cheng, J.; Zhang, Q.; Wang, Y., Oxidative Dehydrogenation of Propane to Propylene in the Presence of HCl Catalyzed by CeO2 and NiO-Modified CeO2 Nanocrystals. ACS Catal. 2018, 8, 4902-4916.


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Modified by Niobium Doping for Mild Oxidation of n-Butane to Maleic Anhydride. J. Catal. 2002, 208, 238-246. (26) Paunović, V.; Zichittella, G.; Moser, M.; Amrute, A. P.; Pérez-Ramírez, J., Catalyst Design for Natural-Gas Upgrading through Oxybromination Chemistry. Nat. Chem. 2016, 8, 803809. (27) Carberry, J. J., Physico-Chemical Aspects of Mass and Heat Transfer in Heterogeneous Catalysis. In Catalysis Science and Technology; Springer-Verlag: Berlin, 1987; Vol. 8, pp 131-171. (28) Mears, D., Diagnostic Criteria for Heat Transport Limitations in Fixed Bed Reactors. J. Catal. 1971, 20, 127-131. (29) Xue, Z.-Y.; Schrader, G. L., In Situ Laser Raman Spectroscopy Studies of VPO Catalyst Transformations. J. Phys. Chem., B 1999, 103, 9459-9467. (30) Loridant, S.; Marcu, I. C.; Bergeret, G.; Millet, J. M. M., TiP2O7 Catalysts Characterised by In Situ Raman Spectroscopy during the Oxidative Dehydrogenation of n-Butane. Phys. Chem. Chem. Phys. 2003, 5, 4384. (31) Ramakrishnaiah, R.; Rehman, G. U.; Basavarajappa, S.; Al Khuraif, A. A.; Durgesh, B. H.; Khan, A. S.; Rehman, I. U., Applications of Raman Spectroscopy in Dentistry: Analysis of Tooth Structure. Appl. Spectrosc. Rev. 2014, 50, 332-350. (32) Šantić, A.; Moguš-Milanković, A.; Furić, K.; Bermanec, V.; Kim, C. W.; Day, D. E., Structural Properties of Cr2O3–Fe2O3–P2O5 glasses, Part I. J. Non-Cryst. Solids 2007, 353, 1070-1077. (33) Zhang, L.; Brow, R. K., A Raman Study of Iron-Phosphate Crystalline Compounds and Glasses. J. Am. Ceram. Soc. 2011, 94, 3123-3130.


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(34) Casadio, F.; Bezúr, A.; Fiedler, I.; Muir, K.; Trad, T.; Maccagnola, S., Pablo Picasso to Jasper Johns: a Raman Study of Cobalt-based Synthetic Inorganic Pigments. J. Raman Spectrosc. 2012, 43, 1761-1771. (35) Frost, R. L.; Williams, P. A.; Martens, W.; Kloprogge, J. T.; Leverett, P., Raman Spectroscopy of the Basic Copper Phosphate Minerals Cornetite, Libethenite, Pseudomalachite, Reichenbachite and Ludjibaite. J. Raman Spectrosc. 2002, 33, 260-263. (36) Deng, Y.; Handoko, A. D.; Du, Y.; Xi, S.; Yeo, B. S., In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu(III) Oxides as Catalytically Active Species. ACS Catal. 2016, 6, 24732481. (37) Koleva, V.; Stefov, V.; Cahil, A.; Najdoski, M.; Šoptrajanov, B.; Engelen, B.; Lutz, H. D., Infrared and Raman Studies of Manganese Dihydrogen Phosphate Dihydrate, Mn(H2PO4)2·2H2O. I: Region of the Vibrations of the Phosphate Ions and External Modes of the Water Molecules. J. Mol. Struct. 2009, 917, 117-124. (38) Moser, M.; Rodríguez-García, L.; Amrute, A. P.; Pérez-Ramírez, J., Catalytic Bromine Recovery: An Enabling Technology for Emerging Alkane Functionalization Processes. ChemCatChem 2013, 5, 3520-3523. (39) Amrute, A. P.; Mondelli, C.; Hevia, M. A. G.; Pérez-Ramírez, J., Mechanism–Performance Relationships of Metal Oxides in Catalyzed HCl Oxidation. ACS Catal. 2011, 1, 583-590. (40) Zichittella, G.; Paunović, V.; Amrute, A. P.; Pérez-Ramírez, J., Catalytic Oxychlorination versus Oxybromination for Methane Functionalization. ACS Catal. 2017, 7, 1805-1817. (41) Pogorzelec-Glaser, K.; Pietraszko, A.; Hilczer, B.; Połomska, M., Structure and Phase Transitions in Cu2P2O7. Phase Transit. 2006, 79, 535-544.


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(42) Biesinger, M. C.; Brown, C.; Mycroft, J. R.; Davidson, R. D.; McIntyre, N. S., X-ray Photoelectron Spectroscopy Studies of Chromium Compounds. Surf. Interface Anal. 2004, 36, 1550-1563. (43) Paunović, V.; Zichittella, G.; Hemberger, P.; Bodi, A.; Pérez-Ramírez, J., Selective Methane

Functionalization

via

Oxyhalogenation

over

Supported

Noble

Metal

Nanoparticles. ACS Catal. 2019, 9, 1710-1725.


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Table 1. Reactions and feed compositions studied in this work.a Concentration / mol.%

Reaction

C3H8

HCl

Cl2

C3H7Cl

O2

Arb

Hec

1.5-6

1.5-15

-

-

3

4.5

71.5-85

Propane chlorination

6

-

3

-

-

4.5

86.5

Propyl chloride dehydrochlorination

-

-

-

1

-

4.5

94.5

Propane oxidation

6

-

-

-

3

4.5

86.5

HCl oxidation

-

6

-

-

3

4.5

86.5

Propane oxychlorination

Other conditions: T = 400-953 K, FT:Wcat = 6000 cm3 h-1 gcat-1, P = 1 bar. b Internal standard. c Carrier gas. a


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Table 2. Crystalline phases, average crystallite size, and porous properties of the metal phosphate catalysts in fresh form and after propane oxychlorination, POC. Catalyst

Phasesa / -

dXRDc / nm

SBETd / m2 g-1

fresh

POC

fresh

POC

fresh

POC

(VO)2P2O7

30

20

56

16

7

Co3(PO4)2

33 56, 45, 36, 24, -, 55

33

CoPO

(VO)2P2O7 Cu3(PO4)2, Cu3P, Cu16O14.15, Cu(OH)2, CuOb, Cu2(PO4)(OH)b Co3(PO4)2

55

5

3

NiPO

-Ni2P2O7

-Ni2P2O7

56

56

7

6

TiPO MnPO

TiP2O7

55 56, 42, 55, 56 31

3

MnP4,  '-Mn3(PO4)2, MnP4O11, Mn2P2O7 FePO4

55 34, 42, 55, 40, 31

3 12

7

FePO

TiP2O7 MnP4,  '-Mn3(PO4)2, MnP4O11, -Mn0.98O2, Mn2P2O7b FePO4

5

5

CrPO

-CrPO4

-CrPO4

42

42

5

4

VPO CuPO

a

-Cu2P2O7

As determined by XRD and Raman spectroscopy. b Assigned by Raman spectroscopy only. c Scherrer equation. d BET model.


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Figure 1. Examples of unit cells of different crystal structures corresponding to metal oxide (top), metal pyrophosphate (middle), and metal orthophosphate (bottom), where the metal, M, has an oxidation state of +2. Color code: M (gray), O (red), P (orange).


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Figure 2. a) X-ray diffraction patterns and b) Raman spectra of the metal phosphate catalysts in fresh form and after propane oxychlorination, POC, which are labeled using the color code given 29 ACS Paragon Plus Environment

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in the bottom of panel a). Reference diffraction patterns are shown as vertical lines below the measured diffractograms or filled symbols above them and are identified with their ICDD-PDF numbers.

Equilibration

conditions:

C3H8:HCl:O2:Ar:He = 6:6:3:4.5:80.5,

FT:Wcat = 6000 cm3 h-1 gcat-1, P = 1 bar, and tos = 5 or 62 h.


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Figure 3. a) Rate of propane consumption and b) product selectivity in propane oxychlorination over the metal phosphate catalysts. In panel a), the rates are determined at 723 K, while in panel b), the product selectivities are determined at ca. 20% propane conversion. In panel b), C3-Cl and cracking products refer to the sum of chloropropanes (C3H7Cl, C3H5Cl) and of any observed cracking products (C2H4, CH4, C2H5Cl, C2H3Cl, and CH3Cl). Reaction conditions: C3H8:HCl:O2:Ar:He = 6:6:3:4.5:80.5, FT:Wcat = 6000 cm3 h-1 gcat-1, T = 723-813 K, P = 1 bar.


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Figure 4. a) Selectivity to propylene as a function of propane conversion in propane oxychlorination over CrPO under various reaction conditions. As shown in the legend in panel a), the shape of the symbols specify the reaction temperature while their interior and color provides

information

on

the

feed

C3H8:HCl:O2:Ar:He = 1.5-6:0-15:3:4.5:71.5-86.5,

composition.

Reaction

FT:Wcat = 6000 cm3 h-1 gcat-1,

conditions: P = 1 bar.

b) Selected catalytic data extracted from panel a) are compared with the state-of-the-art performance (colored areas) achieved in propane oxychlorination (POC),15-16 oxidative dehydrogenation (ODH)10-11 and catalytic dehydrogenation (CPD)3 of propane, as well as fluid catalytic cracking (FCC)7 and steam cracking (SC)6 of ethane and naphtha. These areas are meant to serve as a relative comparison among different technologies, since they do not take into account differences in temperature, pressure, space velocity, and ultimately olefin space time yield. The dotted gray lines in panels a) and b) denote the yield of C3H6, YC3H6. All results are presented in molar basis.


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Figure 5. Stability

test

of

CrPO

in

propane

oxychlorination.

Reaction

conditions:

C3H8:HCl:O2:Ar:He = 6:6:3:4.5:80.5, FT:Wcat = 6000 cm3 h-1 gcat-1, T = 773 K, P = 1 bar.


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Figure 6. Scanning electron microscopy (SEM) and (left panel) and high-resolution transmission electron microscopy (HRTEM; right panel) images of CrPO in fresh from and after propane oxychlorination, POC, according to the equilibration conditions reported in the caption of Figure 5. The insets on the top right corner of the right panels magnify the corresponding HRTEM micrographs, highlighting the interlattice distance that corresponds to the -CrPO4(112) plane.


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Figure 7. a) Cr 2p and b) Cl 2p core-level X-ray photoelectron spectra of CrPO in fresh form and after propane oxychlorination, POC. Equilibration conditions are reported in the caption of Figure 5.


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Figure 8. a) Simplified reaction network of propane oxychlorination, POC, which comprises of catalytic (solid arrows) and non-catalytic (dotted arrows) pathways. The former ones involve the surface-catalyzed

propyl

chloride

generation

(black

arrow)

and

its

consequent

dehydrochlorination to C3H6 (light blue arrow), propane cracking (blue arrow) and combustion (red arrow) as well as the oxidation of HCl to Cl2 (green arrow). The latter can react with propane in the gas phase during POC. In order to rationalize the response of the metal phosphates based on this reaction network, the samples have been investigated in the oxidation of b) HCl to Cl2 and of d) propane as well as in the c) dehydrochlorination of propyl chloride. From this analysis, kinetic parameters, i.e., T10(HCl), Scracking, T50(C3H7Cl), were extracted to measure the ability of the catalysts to evolve chlorine and to produce cracking products, as well as to generate C3H6 from C3H7Cl, respectively, during POC. Reaction conditions:


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ACS Catalysis

b) HCl:O2:Ar:He = 6:3:4.5:86.5;

c) C3H7Cl:Ar:He = 1:4.5:94.5;

d) C3H8:O2:Ar:He = 6:3:4.5:86.5, XC3H8 = 15%; b-d) FT = 6000 cm3 h-1 gcat-1, P = 1 bar.


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Figure 9. Rate of propane consumption in oxychlorination as a function of the ability of the catalyst to evolve chlorine during oxychlorination (T10(HCl)). The latter parameter is extracted from the HCl oxidation experiments, as shown in Figure 8b. Reaction conditions are reported in the caption of Figure 3.


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ACS Catalysis

Figure 10. Rationalization of the selectivity patterns in propane oxychlorination, POC, on representative catalysts based on kinetic parameters extracted from POC, i.e., SC3H6, SCOx, as well as propyl chloride dehydrochlorination, i.e., T50(C3H7Cl), and oxidation of HCl, i.e., T10(HCl), and of propane, i.e., Scracking. The obtained areas are directly proportional to the catalyst ability to produce propylene via oxychlorination chemistry. This enables to derive four categories of catalysts, which favors the generation of propylene (e.g., a) CrPO), cracking products (e.g., b) MnPO), chlorinated hydrocarbons (e.g., c) CuPO) or carbon oxides (e.g., d) VPO). 39 ACS Paragon Plus Environment

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Table of Contents Graphic


Selective Propylene Production via Propane Oxychlorination on Metal

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