Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation

Journal of Catalysis xxx (xxxx) xxx

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Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation James W. Harris, Aditya Bhan ⇑ Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, USA

a r t i c l e

i n f o

Article history: Received 7 August 2019 Revised 22 September 2019 Accepted 25 September 2019 Available online xxxx Keywords: Ethylene epoxidation Silver Chlorine Kinetic modeling

a b s t r a c t High partial oxidation selectivity in ethylene oxidation over Ag catalysts is achieved in part via the presence of adsorbed chlorine promoters. Chlorine coverage during ethylene oxidation is maintained by co-feeding organic chlorides in ppm levels and alkane moderators in concentrations of ~0.5 mol%. The relative efficacy of organic chlorides in Cl deposition and of alkanes in Cl removal is typically estimated using empirical correlations. Apparent kinetics of Cl removal were measured by feeding ethyl chloride and propane during ethylene oxidation catalysis over a highly-promoted Ag/a-Al2O3 catalyst and quantifying the rate of chloropropane formation during steady state reaction. Propane oxychlorination forms 1- and 2-chloropropane in a ~1:2 ratio, as expected given the lower bond dissociation enthalpy of methylene than methyl CAH bonds in propane, and at total rates on the order of 10-4 mol C3H7Cl (mol Agsurf)-1 s
1. Concurrent ethylene oxidation rates of order 10-1 mol (mol Agsurf)-1 s
1 provide evidence that Cl moderation occurs in a distinct catalytic cycle over promoted Ag catalysts. Cl removal has a supralinear dependence on propane pressure (PC3H8) and an inverse dependence on ethyl chloride pressure (PC2H5Cl) at high PC2H5Cl/PC3H8, while Cl removal is positive order in pressures of both C2H5Cl and C3H8 at low PC2H5Cl/PC3H8, as expected for surfaces that are increasingly covered in Cl as PC2H5Cl/PC3H8 increases. Chloropropane formation rates were positive order in dioxygen pressure for all PC2H5Cl/PC3H8 ratios. Taken together, the kinetics of Cl removal require both rate determining deposition of Cl from an alkyl chloride and kinetically relevant removal of Cl by an alkane. We develop a kinetic model that describes these trends and allows for quantification of rate and equilibrium parameters, and provide evidence for the elementary steps enumerated by measuring a kinetic isotope effect and assessing the fate of alkyl fragments from chloroethane and chloropropane in gas-phase batch reactions. Rate and equilibrium constants regressed from this kinetic model are used to develop an isotherm for Cl coverage as a function of the ratio of organic chloride promoter to alkane moderator pressures. This isotherm quantitatively predicts Cl coverages measured in situ after steady state reactions with feedstreams containing both ethyl chlorideethane and ethyl chloride-propane mixtures. Measured reaction orders for chloropropane formation with respect to ethylene and carbon dioxide pressure reflect surface ethylene and carbon dioxide coverages that are lower for sites that form chloropropane than for sites that form ethylene oxide, suggesting that site ensembles for Cl deposition and removal vary from those involved in selective oxidation of ethylene. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Ethylene oxidation for ethylene oxide (EO) production is one of the largest volume chemical processes in the world, with annual production of ~27 million metric tons yr
1 [1]. Supported silver catalysts promoted with various cationic (e.g., Cs, Li, Mo, Re) and anionic (e.g., SO24 ) species are typically employed for production of EO at carbon selectivities of ~65% [2]. In addition to promoters included during catalyst synthesis, selective processes for EO production leverage co-fed promoters, typically alkyl chlorides in ⇑ Corresponding author.

parts per million concentrations, which lead to EO selectivity of ~90% [3,4]. Cl atoms bind strongly to Ag, requiring temperatures in excess of 800 K for thermal desorption [5]. Inclusion of alkyl chlorides in the feed thus requires co-processing alkanes, which remove Cl from the surface [2,6] and prevent formation of inactive bulk AgCl [7,8]. The kinetics of ethylene epoxidation vary with the relative pressures of alkyl chlorides and alkanes, observed in EO rates that
1 decrease from 13 * 10-6 to 8 * 10-6 mol g-1 cat s , EO selectivities that increase from ~81.7% to ~86.6%, apparent oxygen reaction orders that increase from 0.7 to 1.0, and apparent ethylene reaction orders that decrease from 0.5 to
0.3, as ethyl chloride pressure is increased from 0.8 to 3.3 Pa at constant 2.1 kPa ethane [9].

E-mail address: [email protected] (A. Bhan). https://doi.org/10.1016/j.jcat.2019.09.037 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

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Operation of high selectivity EO processes that employ co-fed alkyl chloride promoters and alkane moderators necessitates inquiry into the relative efficacy of various alkyl chlorides and alkanes in depositing and removing Cl, respectively. Historically, such descriptions have been based on empirical correlations derived from industrial experience [3,4], where relative increases in EO yield with increased alkyl chloride pressure, or decreased alkane pressure, at constant ethylene conversion were observed. These observations led researchers at Dow to report a weighted ratio of efficacy in Cl deposition from alkyl chlorides to efficacy in Cl removal from alkanes [3], termed ‘‘Z*”, where

Z ¼

cC2 H5 Cl ðppmv Þ þ cC 2 H3 Cl ðppmv Þ þ 2  c1;2
C2 H4 Cl2 ðppmv Þ þ 0:1  cCH3 Cl ðppmv Þ cC 2 H6 ðmol%Þ þ 0:01  cC 2 H4 ðmol%Þ þ 0:002  cCH4 ðmol%Þ

with ci as the concentration of species i given in the units listed in parentheses. A similar weighted expression was reported by researchers at Shell [4], termed ‘‘Q”, where

Q¼

ride further supported this hypothesis, as the C-Cl bond and C-H bonds in vinyl chloride are stronger than those in C2H5Cl [6]. In our recent work [10], we observed that hCl measured after steady-state ethylene oxidation catalysis with various organic chlorides (0.53 or 3.1 Pa Cl atoms fed from each, with 2.1 kPa C2H6, 513 K) decreased in the order 2-chloropropane>1,1-dichloroe thane>>1,2-dichloroethaneethyl chloridevinyl chloride, leading us to conclude that Cl deposition is a function of the enthalpy for heterolytic scission of CACl bonds (i.e., RCl ? R+ + Cl-). The chlorine coverage was not a strong function of the enthalpy for CAH bond scission, given the similar hCl measured after steady-state catalysis with each of the C2 chlorides whose CAH bond dissociation enthalpy (DHC-H, decrease in the order BDE)

1,2-DCE < C2H5Cl < C2H3Cl [10]. We have also noted that Cl deposition from ethyl chloride requires the presence of oxygen, resulting in 1.6 monolayers Cl deposited when the catalyst is exposed to

cC 2 H5 Cl ðmol%Þ þ cC 2 H3 Cl ðmol%Þ þ 2  c1;2
C 2 H4 Cl2 ðmol%Þ þ 1=3  cCH3 Cl ðmol%Þ 1000  cC3 H8 ðmol%Þ þ 60  ccyc
C3 H8 ðmol%Þ þ 85  cC 2 H6 ðmol%Þ þ cC 2 H4 ðmol%Þ þ 0:3  cCH4 ðmol%Þ

In subsequent work, we developed a technique to quantify Cl coverage (hCl, per surface Ag) in situ following steady-state ethylene epoxidation [2], in which the expression
0:33 hCl ¼ 12:3c0:45 C2H5Cl c C2H6

ð3Þ

with hCl in percent of one monolayer, cC2H5Cl as the concentration of ethyl chloride in ppmv, and cC2H6 as the concentration of ethane in mole percent, accurately captured hCl of 0–50% for C2H5Cl pressures between 0 and 3.3 Pa and ethane pressures from 1.1 to 74 kPa. The form of this power law expression predicts similar trends as would be expected from Z* or Q (Eqs. (1) or (2)); that Cl coverage increases with ethyl chloride pressure and decreases with ethane pressure. While trends in Cl coverage with alkyl chloride and alkane pressures have been inferred and the impact of Cl coverage on observed EO rates on promoted Ag/ a-Al2O3 catalysts has been quantified by us [2], the mechanisms for Cl deposition from alkyl chlorides and Cl removal by alkanes during EO catalysis over promoted Ag catalysts are not well understood. Inferences as to these mechanisms have been made via observation of the impact of identity of the alkyl chloride and alkane employed on the amount of Cl deposited. Monnier and colleagues [6] reported that 1.12 monolayers of Cl were deposited by 2-chlorobutane (1.06 Pa 2-chlorobutane, 453 K) over Ag/a-Al2O3 with 1250 ppm CsNO3 pre-deposited by observing the concentration of 2-chlorobutane during Cl deposition using a GC equipped with an electrolytic conductivity detector. In comparison, only 0.32 monolayers were deposited by ethyl chloride and 0.07 monolayers were deposited by vinyl chloride (1.06 Pa C2H5Cl or C2H3Cl, 453 K) [6]. Additionally, observation of vinyl chloride in the reactor effluent when 1,2-dichlorethane was used as the chlorinating agent suggested that Cl deposition occurred via dehydrochlorination (i.e., C2H4Cl2 ? C2H3Cl + HCl) [6]. The greater Cl deposition from ethyl chloride than vinyl chlo-

ð1Þ

ð2Þ

15.9 kPa O2 and 3.3 Pa C2H5Cl, while no Cl deposition was observed after exposure to 3.3 Pa C2H5Cl in balance He [2]. From these results, we postulated that Cl deposition involves heterolytic CACl scission step and participation of an O2-derived species [10]. Trends in efficacy in Cl removal with co-processed alkane identity have also been reported. The Cl coverage measured after steady-state EO catalysis with co-processed alkane (0.8 or 4.2 kPa C atoms of each alkane, 3.3 Pa C2H5Cl, 513 K) decreased in the order methane > ethane > isobutane > propane [10], in agreement with the trend in CAH bond dissociation enthalpies among these molecules: 439, 410, 400, and 395 kJ mol
1, respectively. Monnier et al. [6] observed higher concentrations of 2-chlorobutane in the effluent than ethyl chloride during steady state epoxidation of butadiene with a feed stream consisting of 0.5 Pa 1,2-dichloroethane, 9 kPa n-C4H10 and 53 kPa C2H6 (483 K), suggesting greater efficacy in Cl removal by butane than ethane, and supporting the authors’ conclusion that the rate of Cl removal increases with decreasing CAH bond strength. Removal of Cl by ethane in a gas-phase batch reactor immediately following steady state reaction required gas phase oxygen [2], consistent with previous proposals that Cl removal occurred via oxychlorination of alkanes [6,11]. Thus, we concluded that trends in Cl coverage with alkane moderator identity reflected trends in CAH bond strength and that Cl removal steps involve homolytic CAH bond scission by an O2-derived species [10]. In the present work, we extend the scope of these prior efforts to describe kinetic processes that govern Cl coverage over Ag catalysts by measuring kinetics of Cl deposition and removal during ethylene epoxidation catalysis. Industrial operation of EO catalysis typically involves co-processing ethane and ethyl chloride, however this feed stream does not allow for quantification of Cl removal rates as the reactant (ethyl chloride) has the same identity

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as the product of Cl removal by ethane. To circumvent this complication, we report kinetics of Cl removal by propane with ethyl chloride as the co-fed organic chloride promoter during steadystate ethylene oxidation catalysis. This feed stream composition results in formation of chloropropane, whose steady-state effluent concentration can be quantified separately from the steady-state effluent concentration of ethyl chloride. Rates are greater for 2chloropropane formation than 1-chloropropane formation, as expected given the weaker CAH BDE for methylene than methyl CAH bonds [12]. Cl removal has a supralinear dependence on propane pressure and an inverse dependence on ethyl chloride pressure at high PC2H5Cl/PC3H8, while Cl removal is positive order in pressures of both C2H5Cl and C3H8 at low PC2H5Cl/PC3H8, as expected for surfaces that are increasingly covered in Cl as PC2H5Cl/PC3H8 increases. Chloropropane formation rates are positive order in oxygen pressure, as expected for Cl deposition and removal routes that require oxygen [2]. We report a reaction mechanism that is consistent with experimental observations and develop a kinetic model to quantify relevant rate and equilibrium constants. We derive an isotherm for Cl coverage as a function of ethyl chloride to alkane ratio from this kinetic model and demonstrate that this isotherm nearly-quantitatively predicts measured Cl coverages for feed streams containing ethyl chloride, ethane, and propane. Measured inhibition of chloropropane formation rates with increasing carbon dioxide and ethylene pressures suggests different site ensembles may be involved in chloropropane formation than those required for selective ethylene oxidation.

2. Methods 2.1. Measurement of steady-state propane oxychlorination kinetics The catalyst used in this study is a highly promoted 35 wt% Ag/

a-Al2O3 catalyst, prepared as described by Shibata et al. [3]. The

catalyst surface area is 1.25 m2 g
1, and the concentrations of promoters were 628 ppm Cs, 440 ppm Re, 151 ppm SO24 , 115 ppm Mn, 42 ppm Na, and 33 ppm Li. The catalyst was sieved to 177–420 lm (40–80 mesh). We previously determined the density of surface Ag on this catalyst using N2O adsorption-decomposition in a gasphase batch reactor [9], while monitoring the consumption of N2O and evolution of N2 and assuming an O:Ag adsorption stoichiometry of 1:1 [13]. This resulted in an estimated dispersion of 0.47% [9], with dispersion defined as the ratio of surface Ag atoms to total Ag atoms. Kinetic measurements were conducted in a stainless steel tubular reactor (McMaster-Carr, ID 4.572 mm, OD 6.36 mm) held between two split-tube copper sleeves heated by cartridge heaters (Omega Engineering Inc., CIR-2100, 600 W), which were controlled by a Watlow temperature controller (96 series). Approximately 0.020 g catalyst was used for each experiment, and was held between two plugs of quartz wool (Perkin Elmer N6102354). A thermocouple (Omega Engineering, K-type) held in the center of the catalyst bed was used to maintain constant reaction temperature (513 K). Helium (Minneapolis Oxygen, 99.997%), ethylene (Matheson, 99.999%), dioxygen (Matheson, >99.997%), carbon dioxide (Matheson, 10% in He, gravimetric grade), propane (Matheson, 2.5% in He, gravimetric grade), ethane (Matheson, 10% in He, gravimetric grade), ethyl chloride (Matheson, 160 ppm in He, certified grade), and 2-chloropropane (Praxair, 80 ppm in He, certified standard grade) were supplied using mass flow controllers (MKS G-series). Reactions were typically conducted with a total flowrate of ~167 cm3(STP) min
1 (~500,000 cm3 h
1 cm-3 cat space velocity, SV) and total pressure 530 kPa. Experiments with C3D8 (SigmaAldrich, 99 atom% D) were performed at a total pressure of 108 kPa.

3

The reactor effluent was sent to a gas chromatograph (GC, Agilent 7890) with separation performed using an HP-Plot Q column (30 m  320 lm  20 lm) followed by product quantification via a thermal conductivity detector connected in series to a flame ionization detector. Any reactor effluent that was not injected to the GC was passed through a scrubber containing 0.5 M sulfuric acid solution (BeanTown chemical) prior to venting to a fume hood. All gas lines upstream of the scrubber were heat traced and maintained at ~373 K. Product quantification was performed using the co-fed alkane, propane or ethane, as the internal standard. Under these conditions, ethylene conversion was <1% and dioxygen conversion <10%. The mass balance, defined as moles of carbon in the effluent per mol carbon fed, closed within ± 3%. Ethyl chloride conversions were always less than 20%, and typically less than 10%, and Cl atom balances, defined as moles of Cl in the effluent per mol Cl fed, always closed within ±8% and were typically within ±3%. Measurement of steady state kinetics over the promoted Ag catalyst employed here requires correction for slow evolution of the catalyst [14] and persistent deactivation with time on stream. To address these two issues, an initial break-in period of 18 h was allowed for each experiment at an initial condition considered thereafter as the reference condition (condref), followed by 12 h excursions to a first test condition (condtest,1), followed by 12 h periods condref between each subsequent test condition. Representative time-on-stream profiles for ethylene oxidation products and alkyl chlorides formed are shown in Figs. S.1 and S.2. Reaction rates were corrected for deactivation by assuming first order decay, resulting in an exponential decay in reaction rates with time on stream (e.g., as in Fig. S.3). First order deactivation constants were obtained by regression of reaction rates at the end of each 12 h reference period, and used to correct rates measured at the end of each 12 h condtest period using Equations S.1 and S.2. Decay constants for EO and H2O formation varied from 1.2–5.1 * 10-6 s
1 (average 2.8 * 10-6 s
1) and 1.4–4.9 * 10-6 s
1 (average 2.9 * 10-6 s
1), respectively, while decay constants for 1-chloropropane and 2-chloropropane formation varied from 0.91–3.75 * 10-6 s
1 (average 2.2 * 10-6 s
1) and 0.62–4.83 * 10-6 s
1 (average 2.5 * 10-6 s
1), respectively. 2.2. Gas-phase batch reactor studies Gas-phase batch reactor studies were performed using the same reactor as in the steady-state experiments described in Section 2.1. Reactant mixtures were fed via a bypass line and the concentration measured via GC at a total pressure of 130 kPa, while the reactor and catalyst housed inside (0.020 g) were heated to reaction temperature (~0.24 K s
1) in flowing He (1.67 cm3 s
1) at a total pressure of 130 kPa. Reactions began when two 2-way valves were switched such that the reactor was exposed to the reactant mixture in a closed system of volume ~1033 cm3. Gases were circulated using a recirculating pump (Senior Aerospace Metal Bellows MB-21) located between the outlet of the reactor and the GC sample loop. GC injections were taken every ~420 s to monitor the consumption of reactants and evolution of products over time. 2.3. Measurement of Cl coverages Cl coverages were measured in situ in a gas phase batch reactor following steady state catalysis using a previously reported procedure [2]. After an 18 h period of steady state reaction with the reactor configured as a downflow reactor, the feedstream to the reactor was set to 1.67 cm3 s
1 He with the reactor maintained at 513 K. After ~6 ks, the reactor was configured as a gas-phase batch reactor and exposed to ~13 kPa C2H6 and ~1.3 kPa CH4 in balance He.

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A recirculating pump (Senior Aerospace Metal Bellows MB-21) was used to circulate gases which were analyzed periodically by GC. After ~1.2 ks, dioxygen was dosed into the batch reactor using an electronically-actuated two-position six-port valve equipped with a 0.5 cm3 sample loop until the dioxygen pressure was ~7 kPa, as both ethane and oxygen are required in order to remove Cl from the catalyst at 513 K [2]. The total Cl content of the sample was calculated by determining the amount of ethyl chloride desorbed from these batch reactions, and then extrapolating to infinite Cl removal cycles using an exponential decay function determined previously [2]. 3. Results and discussion 3.1. Steady-state kinetics of Cl removal Rates of Cl removal were observed by feeding ethyl chloride and propane to moderate Cl coverage during ethene oxidation, resulting in formation of chloropropane when Cl is removed from the catalyst surface. Ethylene oxide rates were measurable across all experimental conditions tested, and ranged from 0.03 to 1.0 mol EO (mol Agsurf)-1 s
1 (<1% C2H4 conversion, 513 K), which is similar to the range of 0.014–0.15 s
1 (20% C2H4 conversion, 503–553 K) reported by Oyama [15]. To our knowledge, steady-state rates of chloropropane formation when propane and ethyl chloride are fed during EO catalysis have not been reported previously, but we estimate from the data reported by Monnier et al. [6] that the rate of Cl removal by n-butane to form 2-chlorobutane during butadiene epoxidation (9.1 kPa C4H6, 18.2 kPa O2, 0.2 Pa 2chlorobutane, balance n-C4H10, total pressure 101.3 kPa, 473 K) was ~7.4 * 10-5 mol 2-chlorobutane (mol Agsurf)-1 s
1, which is similar to the range of chloropropane formation rates reported here (~0.1–2.0 * 10-4 mol chloropropane (mol Agsurf)-1 s
1, 513 K). Propane has methylene CAH bonds with lower BDE (~395 kJ mol
1) than methyl CAH bonds in propane (~410 kJ mol
1) [12], resulting in predominant formation of 2-chloropropane relative to 1chloropropane. Varying ethyl chloride pressure from 2.65 to 18.6 Pa with all other pressures held constant results in similar variations in 2-chloropropane and 1-chloropropane formation rates such that the ratio of the two products remains constant near ~2.0 (Fig. 1a). However, when propane pressure was varied from

1.06 to 2.40 kPa, the ratio of 2-chloropropane: 1-chloropropane decreased monotonically from 2.25 to 1.5 (Fig. 1b). Assuming constant coverage of Cl at a given set of ethyl chloride and propane pressures, the mass balance on Cl is given as: 

dCl ¼ r deposition
r remov al ¼ 0 ds

ð4Þ

where rdeposition and rremoval are the rates of chlorine deposition and removal, respectively. The Cl atom balance closed with accuracy of analytical methods employed for all data points in this study suggesting there is no accumulation of Cl on the catalyst with time on stream. Assuming no net redeposition of Cl from the chloropropanes, which would result in decreasing rates of chloropropane formation with increasing space time (not experimentally observed, see Fig. S.4), the rate of chlorine deposition is the rate of ethyl chloride consumption:

rdeposition ¼ r ethylchloride

ð5Þ

and the rate of Cl removal is the sum of the rates of chloropropane formation:

rremov al ¼ r 1
chloropropane þ r 2
chloropropane

ð6Þ

A combination of Eqs. (4)–(6) results in rates of ethyl chloride consumption being equivalent to the sum of the formation rates of 1-chloropropane and 2-chloropropane:

rethylchloride ¼ r 1
chloropropane þ r 2
chloropropane

ð7Þ

As such, we will consider Cl removal kinetics by discussing trends in the sum of 1-chloropropane and 2-chloropropane formation rates as a function of various process parameters. The dependence of chloropropane formation rate on various feed pressures are reported in Figs. 2–5; these pressure dependences were noted to vary with the ratio of ethyl chloride to propane in the feed. For instance, with 5.3 Pa C2H5Cl in the feed, the rate of Cl removal depends on propane pressure as P0.74 C3H8, while with 18.6 Pa C2H5Cl in the feed, the rate of Cl removal varies with PC3H8 as P1.4 C3H8 (Fig. 2). With 1.6 kPa C3H8 in the feed, the Cl removal rate increases with PC2H5Cl from 2.5 to 8 Pa before becoming approximately constant from 8 to 20 Pa. With 0.53 kPa C3H8 in the feed, the Cl removal rate decreases monotonically with PC2H5Cl as ~P-0.52 C2H5Cl (Fig. 3). Similar results were obtained with feedstreams

Fig. 1. Variation in chloropropane formation rates with (a) ethyl chloride pressure at 1.59 kPa propane or (b) propane pressure at 15.9 Pa C2H5Cl. Reaction conditions for both (a) and (b) were 159 kPa C2H4, 15.9 kPa O2, 5.3 kPa CO2, balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Dashed lines are shown to guide the eye.

Please cite this article as: J. W. Harris and A. Bhan, Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.037

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Fig. 2. Variation in chloropropane formation rates with propane pressure at 5.3 Pa C2H5Cl (squares) or 18.6 Pa C2H5Cl (triangles). Reaction conditions were 159 kPa C2H4, 15.9 kPa O2, 5.3 kPa CO2, balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Dashed lines show power law regressions to experimental data.

Fig. 3. Variation in chloropropane formation rates with ethyl chloride pressure at 1.6 kPa C3H8 (squares) or 0.53 kPa C3H8 (triangles). Reaction conditions were 159 kPa C2H4, 15.9 kPa O2, 5.3 kPa CO2, balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Dashed lines are shown to guide the eye.

containing 2-chloropropane and ethane, wherein the reaction order for C2H5Cl formation with respect to 2-chloropropane pressure (1.06–2.12 Pa) was –1.13 ± 0.18 with 53 kPa ethane in the feed (Fig. S.5), and the ethyl chloride reaction order with respect to ethane pressure (37.1–148.4 kPa) was ~1.6 with 1.59 Pa 2chloropropane in the feed (Fig. S.6). At constant ratios of ethyl chloride to propane pressure, rates depend on dioxygen pressure as either P0.31 O2 (5.3 Pa C2H5Cl and

5

Fig. 4. Variation in chloropropane formation rates with oxygen pressure at 2.65 kPa C3H8 and 5.3 Pa C2H5Cl (squares) or 1.06 kPa C3H8 and 18.55 Pa C2H5Cl (triangles). Reaction conditions were 159 kPa C2H4, 5.3 kPa CO2, balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Dashed lines show power law regressions to experimental data.

2.65 kPa C3H8) or P0.72 (18.55 Pa C2H5Cl and 1.06 kPa C3H8) O2 (Fig. 4). The increasing magnitude of the apparent reaction order for chloropropane formation with respect to dioxygen pressure with the increasing ratio of ethyl chloride to propane pressure is reminiscent of the increasing apparent reaction order for EO production with respect to dioxygen pressure (from 0.7 to 1.0) with increasing ethyl chloride pressure at constant ethane pressure reported previously [9]. The increase in the apparent dioxygen reaction order with ethyl chloride to propane ratio implies decreased coverages of surface species derived from dioxygen, consistent with decreased heats of adsorption for oxygen (by 4.6 kJ mol
1) with increasing Cl coverage (from 0 to 0.65) over Ag (1 1 1) [16], and with O* coverages that decreased to zero when Cl coverage exceeds 0.25–0.3 [5,17,18]. The dependence of Cl removal rate on ethylene and carbon dioxide pressures is reported in Fig. 5. Inhibition by ethylene increased with increasing ratio of ethyl chloride to propane pressure (Fig. 5a), reflected in apparent ethylene reaction orders of
0.5 (5.3 Pa C2H5Cl and 2.65 kPa C3H8) and
0.9 (21.2 Pa C2H5Cl and 0.53 kPa C3H8), suggesting higher heats of adsorption of ethylene on increasingly chlorided Ag surfaces. Similar findings were reported by Chen et al. [9], who observed apparent reaction orders for EO production with respect to ethylene pressure that decreased from 0.5 to
0.3 with increasing ethyl chloride pressure. Increased ethylene heats of adsorption (from 37 to 54 kJ mol
1) were observed on Ag single crystals with increasing Cl coverage (from 0 to 0.4) during ethylene temperature programmed desorption [5]. Ethylene does not remove Cl from Ag catalysts [2], and therefore inhibition of chloropropane formation by ethylene is not the result of an increasing rate of formation of ethyl chloride or vinyl chloride with increasing ethylene pressures. Inhibition of ethylene oxidation by CO2 has been observed previously [9,19–22], and the reaction order for CO2 inhibition of EO production was constant and 
0.5 with increasing ethyl chloride pressure [9]. Given the concurrent positive apparent reaction orders in propane, ethyl chloride, and oxygen, we presume kinetically relevant step(s) for chloropropane formation that depend on these species

Please cite this article as: J. W. Harris and A. Bhan, Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.037

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Fig. 5. Variation in chloropropane formation rates with (a) ethylene pressure at 2.65 kPa C3H8 and 5.3 Pa C2H5Cl (squares) or 0.53 kPa C3H8 and 21.2 Pa C2H5Cl (triangles) and (b) carbon dioxide pressure at 2.65 kPa C3H8 and 5.3 Pa C2H5Cl. Additional reaction conditions in (a) were 15.9 kPa O2, 5.3 kPa CO2, and in (b) were 159 kPa C2H4, 15.9 kPa O2. In both (a) and (b), the other reaction parameters were: balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Dashed lines show power law regressions to experimental data.

Scheme 1. Series of elementary steps for propane chlorination. Double arrows denote quasi-equilibrated reversible steps, and single arrows denote irreversible steps. Species marked with a subscript (g) are gas-phase species and those marked with an asterisk are adsorbed surface species.

or their derivatives. A plausible series of elementary steps is presented in Scheme 1. Oxygen adsorbs and dissociates in two quasi-equilibrated (QE) steps, steps 1 and 2. Ethyl chloride adsorbs in a QE step prior in step 3 to oxygen-assisted deposition of Cl in step 4. In step 5, propane reacts with adsorbed Cl and oxygen to form chloropropane and an adsorbed hydroxyl group. Chloropropane can desorb in a QE step in step 6, or re-deposit Cl to form C3H7/O* in step 7. We have previously shown that 2-C3H7Cl is more effective at Cl deposition than ethyl chloride [10], and thus redeposition of Cl from 2-C3H7Cl is likely, and presumably leads to decreasing ratios of 2-chloropropane to 1-chloropropane with increasing space time

(Fig. S.4). We presume C3H7/O* intermediates undergo further deprotonation to form C3H6/O* in step 11, which leads to propylene desorption in step 12. We discuss pathways for sequential reactions of C3H7/O* in Section 3.2. The reverse of step 8 results in association of adsorbed hydroxyls to produce water and O*. Step 9 is a quasi-equilibrated step for adsorption of CO2 at adsorbed O* to form CO2/O*, representing carbonate species observed via infrared spectroscopy during ethylene epoxidation [20]. Step 10 is a quasi-equilibrated adsorption step for ethylene on a bare silver site. We presume C2H5/O* undergoes further deprotonation to C2H4/O* (ethylene adsorbed at/on an oxygen adatom) in step 13, which can lead to desorption of C2H4 from an oxygen adatom in step 14, or reaction of C2H4/O* to form either EO, acetaldehyde, or combustion products, as we discuss in Section 3.2. We take step 5 as rate determining for chloropropane formation, and make a pseudo-steady-state-approximation to define the coverage of Cl*. We consider *, O*, CO2*, C2H4*, and Cl* as most abundant surface intermediates (MASIs) on the basis of observed inhibition by CO2 and C2H4, reduced rates with increasing Cl coverage inferred from Figs. 2–5, and our expectation that CO2 adsorbs at O* to form carbonates that have been reported in in situ IR spectra of Ag catalysts exposed to CO2 and O2 [21], suggesting some relevant coverage of O* is plausible. With these assumptions, the resultant rate expression is:

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r5 zK3 k4 K1 K2 PO2 PC2H5Cl ¼ 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ½L C2H5Cl 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ K1 K2 PO2 þ k4kK53PPC3H8

ð8Þ where r5 is the rate of step 5, z is a coordination number that reflects the number of adjacent sites, ki is the rate constant for step i, Ki is the equilibrium constant for step i, Pi is the pressure of species i, and [L] is the concentration of active sites (derivation shown in Section S.4). The reaction order with respect to ethyl chloride, nC2H5Cl is given by:

nC2 H5 Cl ¼

  r5 dln ½L

dlnðPC2H5Cl Þ

ð9Þ

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After the derivation shown in Section S.5, we find nC2H5Cl is equivalent to: nC 2 H5 Cl ¼ 1


C2H5Cl 2  k4kK53PPC3H8 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C2H5Cl 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ K1 K2 PO2 þ k4kK53PPC3H8

ð10Þ

From Eq. (10), it is clear that in the limit of low ratios of ethyl chloride pressure to propane pressure, 0 < nC2H5Cl < 1. The reaction order with respect to propane, nC3H8, is equal to:

nC 3 H8 ¼

C2H5Cl 2  k4kK53PPC3H8 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C2H5Cl 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ K1 K2 PO2 þ k4kK53PPC3H8

ð11Þ resulting in a positive order dependence on propane pressure. In the limit of high ratios of ethyl chloride pressure to propane pressure, a supralinear dependence of chloropropane rate on propane pressure would be predicted (1 < nC3H8 < 2), as well as inhibition by ethyl chloride (nC2H5Cl < 0). Based on Eq. (8), the chloropropane formation rate divided by the ethyl chloride pressure at constant dioxygen, carbon dioxide, and ethylene pressures should be a single valued function of u, where:

u¼

PC2H5Cl  104 PC3H8

ð12Þ

with PC2H5Cl and PC3H8 are reported in kPa. As shown in Fig. 6, the C3H7Cl rate is a single valued function of u for both 15.9 and 37.1 kPa O2. If step 5 is rate determining, and the enthalpy of homolytic C-H bond cleavage determines efficacy in moderation of Cl coverage [10], then stabilizing the ground state for this elementary step relative to the transition state by replacing propane with propane-d8 in the feed should result in reduced rates of Cl removal by propane. To test this, C3H7Cl formation rates were measured at ambient pressure (given the low gas cylinder pressure for isotopically enriched compounds) and C3H8 in the feed stream was replaced with C3D8. For u = 175, switching from C3H8 to C3D8 (Fig. 7, Segment I to Segment II) resulted in complete elimination of measurable C3D7Cl formation rates. With u = 20, increased formation rates of C3H7Cl allowed for quantification of C3D7Cl after C3H8 was

7

replaced with C3D8 in the feed, and quantification of an H/D kinetic isotope effect (KIE) of 3.8 (Fig. 7, Segments IV and V). While not quantifiable at u = 175, these results suggest C-H scission in C3H8 is kinetically relevant for u = 20–175. The expected C/H KIE for a transition state involving a C-H stretch (mC-H = 3000 cm
1), at 513 K is ~2.95 (derivation in Section S.6). However, given that k5 appears in the term for Cl* in the denominator of Eq. (8), the maximum possible H/D KIE for a step involving CAH cleavage could vary between 0–9 depending on the relative coverages of the MASI species. As such, the measured value of 3.8 at u = 20 likely reflects an H/D KIE of ~3.0 for k5, that leads to increased coverage of Cl* on a surface with non-negligible coverages of *, O*, CO2*, and C2H4*. The combination of (1) positive reaction orders for propyl chloride formation with respect to ethyl chloride, oxygen, and propane pressures at low u, (2) inhibition by ethyl chloride and supralinear dependence of propyl chloride formation on propane pressure at high u, and (3) observation of an H/D KIE at low and high u, support the description of this reaction via the series of steps defined in Scheme 1. At low coverages of O*, this series of elementary steps and the rate expression (Eq. (8)) can accommodate positive dependence of the reaction rate on dioxygen pressure (Fig. 5), although the maximum dioxygen reaction order possible would be 0.5 (if the surface was devoid of O* and CO2/O*). At high chlorine coverage, the measured reaction order in dioxygen was ~0.7, suggesting unforeseen impacts of dioxygen on Cl deposition or removal, or a change in rate determining step at high u. If oxygen adsorption and dissociation was rate determining, and Cl coverage was still dependent on irreversible deposition and Cl removal steps, the reaction rate would be increasingly sensitive to oxygen pressure. Next, we report observations from a series of batch reactions in a gradientless gas-phase reactor in order to probe the fate of alkyl fragments resultant from deposition of Cl from alkyl chlorides.

3.2. Gas-phase batch reactor studies for elucidation of sequential reaction pathways To probe the fate of the C2H5/O* species included in Step 4 of Scheme 1, and to determine the pathways for secondary reactions of C3H7/O* after redeposition of Cl from C3H7Cl, we performed

Fig. 6. Variation in chloropropane formation rates per ethyl chloride pressure with u at (a) 15.9 kPa O2 and (b) 37.1 kPa O2. Reaction conditions were 159 kPa C2H4, 5.3 kPa CO2, 0.53–5.3 kPa C3H8, 2.65–26.5 Pa C2H5Cl, balance He; T, 513 K; P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Solid lines reflect the results of regression of Eq. (8) to the experimental data using Athena Visual Studio.

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Fig. 7. Chloropropane formation rates with time on stream. Segments I and III: 0.21 kPa C3H8, 3.7 Pa C2H5Cl; Segment II: 0.21 kPa C3D8, 3.7 Pa C2H5Cl; Segments IV and VI: 0.53 kPa C3H8, 1.05 Pa C2H5Cl; Segment V: 0.53 kPa C3H8, 1.05 Pa C2H5Cl. Additional reaction conditions in all segments were 31.5 kPa C2H4, 3.15 kPa O2, 1.05 kPa CO2, balance He; T, 513 K; Total P, 105 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Vertical dashed lines denote times at which feed stream composition was varied.

sition from C2H5Cl results in formation of C2H4, which can then further react to either EO or CO2 and H2O. The selectivities for C3 products are shown in Fig. 9a. C3 selectivity was defined based on Eq. (13),

molesC in i Si ¼ P ð molC in C3 productsÞ þ molC in CO2

ð13Þ

where Si is the selectivity of 2-chloropropane, 1-chloropropane, propylene, acrolein, or CO2. CO2 is included in the calculation of C3 selectivity, as the majority of the CO2 formed presumably results from propane combustion, and not ethyl chloride combustion, given the 33 higher initial concentration of propane than ethyl chloride. Propylene forms immediately as an apparent primary product, and oxidation of propylene leads to formation of acrolein (Fig. 9a). Chlorination of propane results in formation of 2-chloropropane and 1-chloropropane. The amount of 2-chloropropane relative to 1-chloropropane decreases rapidly at propane or ethyl chloride conversions < 10% (Fig. 9). Cl selectivities, shown in Fig. 9b, were calculated as:

Si ¼ Fig. 8. Amounts of ethyl chloride (diamonds), ethylene (circles) and ethylene oxide (squares) as a function of time in a gas-phase batch reactor. Initial reaction conditions were 3.9 kPa O2, 0.13 kPa C3H8, 3.9 Pa C2H5Cl, 0.065 kPa CH4, balance He; T, 513 K; Total P, 130 kPa; mass catalyst, 0.020 g.

moles Cl in i moles C2 H5 Cl converted

ð14Þ

with the moles of Cl deposited (Cl*) calculated based on the Cl mass balance in Eq. (15), 

Cl ¼ ðinitial moles C2 H5 ClÞ
ðmoles Cl in gas phase speciesÞ ð15Þ

reactions in a gas-phase batch reactor, which allows for facile observation of sequential reaction pathways [23]. Fig. 8 shows the C2 products observed after initially charging the reactor with ethyl chloride, propane, and oxygen. Ethyl chloride is consumed in deposition of Cl and the resultant C2 fragments react to either form C2H4, C2H4O (EO) (Fig. 8), or CO2 and H2O. With extended reaction times (>50 ks) as ethyl chloride conversions approach 100%, ethylene and EO consumption results in decreasing amounts of these C2 products. These results provide evidence that Cl depo-

where the moles in gas phase species includes moles in ethyl chloride, 1-chloropropane, and 2-chloropropane. Based on our observations demonstrating significantly higher Cl coverage after steady state reactions with 2-chloropropane than ethyl chloride [10], and given the similar CACl bond strength in ethyl chloride and 1chloropropane, we interpret the trend in the Cl selectivities of the chloropropanes (Fig. 9) as rapid formation of 2-chloropropane followed by redeposition of Cl from 2-chloropropane that is more facile than redeposition of Cl by 1-chloropropane. Decreasing ratios

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Fig. 9. (a) Selectivity of propane conversion to propylene (diamonds), 2-chloropropane (triangles), 1-chloropropane (circles), acrolein (filled squares), and carbon dioxide (open squares) as a function of propane conversion in a gas-phase batch reactor (b) Cl selectivity to 2-chloropropane (triangles), 1-chloropropane (circles), and adsorbed Cl (open squares) as a function of ethyl chloride conversion. Initial reaction conditions were 3.9 kPa O2, 0.13 kPa C3H8, 3.9 Pa C2H5Cl, 0.065 kPa CH4, balance He; T, 513 K; Total P, 130 kPa; mass catalyst, 0.020 g. Lines are shown to guide the eye.

of 2-chloropropane to 1-chloropropane were also observed during steady-state reactions with increasing space time (Fig. S.4). When 2-chloropropane concentrations are significantly reduced from redeposition of Cl (at ~20% ethyl chloride conversion, Fig. 9b), redeposition of Cl from 1-chloropropane occurs, resulting in reduced C3 and Cl selectivities of 1-chloropropane with increasing propane or ethyl chloride conversion (Fig. 9). As C2H5Cl conversion approaches 100% and 1-chloropropane and 2-chloropropane Cl selectivity approach zero, the majority of the Cl initially present as C2H5Cl is adsorbed on the catalyst, resulting in near 100% Cl selectivity to Cl* (Fig. 9b). The propylene formed during the C2H5Cl-C3H8-O2 batch reaction can be formed either as a consequence of re-deposition of Cl from 1-C3H7Cl and 2-C3H7Cl or via oxidative dehydrogenation of propane. Deposition of Cl from C2H5Cl resulted in formation of C2H4 (Fig. 8), so we presume deposition of Cl from C3H7Cl could

form C3H6 in an analogous dehydrodechlorination reaction. To further probe the resultant products from Cl deposition from C3H7Cl, we performed a batch reaction starting with 1.3 Pa 2-C3H7Cl, 12.74 kPa C2H6, and 3.9 kPa O2 and the resultant concentration profiles are shown in Fig. 10. With this feedstream, 2-chloropropane was completely consumed (Fig. 10a), resulting in formation of propylene from dehydrochlorination of 2-chloropropane, and propylene was oxidized to acrolein, resulting in increasing C selectivity to acrolein with time (Fig. 10b). The C selectivity shown in Fig. 10b does not include CO2, as the majority of CO2 and H2O formed in this experiment were presumed to result from combustion of C2 species, however it is likely that both propylene and acrolein were further converted to total oxidation products. The rate of formation of ethyl chloride was approximately equal to that of 2-chloropropane consumption (Fig. 10c), as expected for coupled irreversible steps in which the

Fig. 10. (a) Conversion of 2-chloropropane (circles), ethane (diamonds), and oxygen (squares) as a function of time in a gas-phase batch reactor (b) C selectivity to propylene (diamonds) and acrolein (circles) (c) amounts of 2-chloropropane (circles) and ethyl chloride (squares) as a function of time. Dashed lines in (b) included to guide the eye and in (c) represent linear regression of the first four data points in each data series. Initial reaction conditions were 3.9 kPa O2, 12.74 kPa C2H6, 1.3 Pa C3H7Cl, 0.065 kPa CH4, balance He; T, 513 K; Total P, 130 kPa; mass catalyst, 0.020 g.

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rate of deposition of Cl from an alkyl chloride (e.g., 2chloropropane) is equivalent to the rate of removal of Cl by an alkane (e.g., ethane), as described by Eqs. (4)–(7). Formation of propylene provides further evidence that Cl deposition occurs by dehydrochlorination of alkyl chlorides, as previously posited by Monnier et al. [6]. This process results in deposition of a Cl atom, formation of an olefin, and presumably formation of a hydroxyl formed by H abstraction by an adsorbed oxygen adatom. While the fate of ethylene formed via dehydrochlorination of ethyl chloride during EO catalysis is difficult to determine due to the presence of ethylene as a reactant, the batch reaction studies reported here demonstrate that ethylene is formed after Cl deposition from ethyl chloride, and ethylene can then participate in either selective oxidation to EO or complete oxidation to CO2 and H2O. Similarly, a molecule of C3H6 forms when Cl is deposited from C3H7Cl, and this C3H6 may further react to form either acrolein or total oxidation products. We have previously demonstrated [2] that C2H4 does not remove Cl from Ag by (1) exposing a Clcovered Ag catalyst to C2H4 and dioxygen in a gas-phase batch reactor, which resulted in no production of Cl-containing species, and by (2) varying the steady state C2H4 pressure (106–212 kPa) and measuring invariant Cl coverage after reaction. Given these observations, we conclude that once Cl is deposited from C2H5Cl, forming Cl*, (O)H*, and C2H4, the Cl* formed can only be removed by reaction with an alkane in presence of oxygen (e.g., step 5 in Scheme 1).

Ki is the equilibrium constant for adsorption of i from the gas phase, and Pi is the gas phase pressure of species i. Based on the elementary steps in Scheme 1 and the assumptions resulting in Eq. (8), the coverage of Cl in this system is equivalent to:

hCl ¼

k4 K3 PC2H5Cl k P

5 C3H8 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C2H5Cl 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ K1 K2 PO2 þ k4kK53PPC3H8

ð18Þ

Therefore, a Langmuir-type isotherm for Cl coverage (at constant PCO2, PO2, and PC2H4) is expected if Pi in Eq. (16) is replaced with u, as shown in Eq. (19):

hCl ¼

keff ;Cl u a þ keff ;Cl u

with keff,

Cl

equivalent to

ð19Þ k4 K3 10
4 . k5

From the values reported in

Table 1, it is possible to predict coverage of each species as a function of u for all experimental conditions tested here (Fig. 11). This analysis results in Cl coverages that range from 6 to 89% as u increases from 5 to 500. The chloropropane rate normalized by the propane pressure is given by Eq. (20):

r5 PC3H8

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PC2H5Cl z K1 K2 PO2 k4 KP3C3H8 ½L ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k4 K3 PC2H5Cl 2 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ K1 K2 PO2 þ k5 PC3H8 ð20Þ

3.3. Prediction of surface Cl coverage using kinetic model results

If we take the denominator terms other than that for Cl* as a constant, a (at constant PO2, PCO2, and PC2H4), and then replace

After measurement of steady-state propyl chloride formation kinetics as a function of relevant gas-phase pressures and confirmation of the choice of rate determining step by measurement of an H/D KIE, we endeavored to quantify the rate and equilibrium constants in Eq. (8) in order to predict species coverages across the range of process conditions considered here. The values of the resultant regression are reported in Table 1. In some cases, determination of individual parameters was not possible as they are coupled in Eq. (8) (e.g., K1K2, K3k4). Regression of these parameters describes nearly all experimental measurements within ~30% uncertainty (Fig. S.7), therefore this set of rate and equilibrium constants nearly quantitatively captures the observed trends in chloropropane formation rate (within experimental uncertainty). The coverage of an individual surface species as a function of its gas-phase pressure in a Langmuir-Hinshelwood formalism is equivalent to

PC2H5Cl PC3H8

hi ¼

K i Pi

r5 PC3H8

with u, Eq. (20) becomes:

¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi z K1 K2 PO2 keff ;Cl u½L   a þ keff ;Cl u 2

ð21Þ

The form of Eq. (21) results in chloropropane rates per propane pressure that increase at low values of u before decreasing with further increases in u, as shown in Fig. 12.

ð16Þ

a þ K i Pi

where hi is the fractional coverage of species i and a is defined as:

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

a ¼ 1 þ K9 PCO2 K1 K2 PO2 þ K10 PC2H4 þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K1 K2 PO2

ð17Þ

Table 1 Rate and equilibrium parameters in the rate expression for chloropropanes formation (Eq. (8)) determined by regression of the experimental data in Athena Visual Studio. Parameter

Valuea

Units

K1K2 K3k4 k5 K9 K10

0.11 ± 0.43 9.0 * 10-4 ± 2.9 * 10-3 1.3 * 10-3 ± 5.1 * 10-3 0.095 ± 0.17 9.7 * 10-3 ± 2.4 * 10-2

kPa
1
1 mol kPa
1 Ag-1 surf s
1 mol kPa
1 Ag-1 surf s kPa
1 kPa
1

a Uncertainties represent 95% confidence intervals generated by Athena Visual Studio. Large uncertainties result from minor contributions of adsorbed oxygen, ethylene, and carbon dioxide to the quality of the regression. If K1K2, K9, and K10 are kept constant, the relative uncertainties in K3k4 and K5 are both ~15%.

Fig. 11. Coverage of Cl (green squares), ethylene (blue circles), empty sites (red triangles), oxygen atoms (black diamonds), and CO2/O* (white squares) as a function of u for all experimental conditions tested. Reaction conditions ranged from 106 to 212 kPa C2H4, 10.6–44.3 kPa O2, 3.7–21.2 kPa CO2, 0.53–5.3 kPa C3H8, 2.65–26.5 Pa C2H5Cl, balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g.

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Fig. 12. Variation in chloropropane formation rates per propane pressure with (a) u and (b) hCl. Reaction conditions were 159 kPa C2H4, 15.9 kPa O2, 5.3 kPa CO2, 0.53–5.3 kPa C3H8, 2.65–26.5 Pa C2H5Cl, balance He; T, 513 K; P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. Solid lines reflect the results of regression of Eq. (8) to the experimental data using Athena Visual Studio.

The maximum rate per propane pressure predicted from the kinetic model occurs when hCl is equal to 0.5 (Fig. 12b), which is consistent with the experimentally observed trend in chloropropane rate per propane pressure. As the rate of chloropropane formation is controlled by the rate of step 5, the rate per propane pressure is maximized when the product hO * hCl is maximized. Thus, the maximum rate per propane pressure would be expected when hO = hCl = 0.5. However, the coverage of O* is always less than 0.5 for the feedstreams considered here (Fig. 11), as oxygen competes with CO2 and ethylene for coverage of empty sites and the pressure of dioxygen in the feed is maintained below 37.1 kPa in order to avoid formation of explosive mixtures with ethylene. The operating conditions studied span from Ag surfaces devoid of Cl to surfaces nearly covered in Cl (6–89% Cl coverage), and the kinetic model developed here captures chloropropane rates across this Cl coverage regime as well as the observed maximum in chloropropane formation rate at hCl = 0.5. The functional form of Eq. (19), in which Cl coverage depends on u, is consistent with the empirical expression we determined previously for Cl coverage as a function of ethane and ethyl chloride pressures (Eq. (3)) [2]. Equipped with the functional form of the Cl coverage isotherm given by Eq. (18), which resulted from the kinetic studies performed here, we regressed Eq. (22) to the Cl coverages measured after steady-state EO catalysis by Harris and Bhan [2] with ethane and ethyl chloride include in the feed stream:

hCl ¼

C2H5Cl P1 P2PPC2H6

1þ

P2 PC2H5Cl PC2H6

ð22Þ

where P1 and P2 are regressed parameters and the units for hCl, PC2H5Cl and PC2H6 are %, Pa, and kPa, respectively. With regressed values of 50.17% for P1 and 1.98 for P2, this regression accurately captures the measured hCl reported previously. If we regressed the Cl coverage data without P1, which would assume a maximum coverage of 100%, the fit was less accurate at both low and high Cl coverages (Fig. 13, red dashed line). Inclusion of P1 resulted in more accurate prediction of the data from Harris and Bhan (Fig. 13, red circles and thin red line) [2].

Fig. 13. Experimentally measured Cl coverages for C2H6-C2H5Cl feedstreams (red circles), for C2H6-C3H8-C2H5Cl feedstreams (blue triangles), and for C2H5Cl-C3H8 containing feedstreams (black squares), predicted Cl coverage for C2H5Cl-C2H6 containing feedstreams without parameter P1 (red dashed line) and with parameter P1 (solid red line), and predicted Cl coverage for C2H5Cl-C3H8 containing feedstreams with the numerator of Eq. (18) multiplied by 50.17 (regressed value of P1, Eq. (22), solid black line). Steady-state reaction conditions were 0.27–3.33 Pa C2H5Cl, 1.06–74.2 kPa C2H6 (C2H6-C2H5Cl experiments); 3.33 Pa C2H5Cl, 2.12 kPa C2H6, 0.27 or 1.4 kPa C3H8 (C2H6-C3H8-C2H5Cl experiments); 3.33–18.6 Pa C2H5Cl, 1.06–1.6 kPa C3H8 (C3H8-C2H6 experiments); with constant 159 kPa C2H4, 15.9 kPa O2, 5.3 kPa CO2. In all experiments, the remaining experimental conditions were: balance He; T, 513 K; Total P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g. The red circles and white square represent data reported by Harris and Bhan in Journal of Catalysis [2] while the blue triangles represent data reported by Harris et al. in Journal of Catalysis [10].

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Exposure of Ag(1 1 1) to either 4 [24] or 6 [17] Langmuirs (10-6 Torr s = 1 L) Cl2 at 300 K resulted in formation of an ordered 10x10 structure observed by low energy electron diffraction (LEED), which corresponded to a Cl coverage of 0.49 [17,24] consistent with the saturation Cl coverage of 0.52 monolayers on Ag(1 1 1) reported by Campbell [16]. We note however that coverages as high as ~10 monolayers were observed with exposure of Ag (1 1 1) to ~1600 L Cl2 at 300 K [25]. Rovida et al. [26,27] observed a well-ordered 3x3 Cl structure when Cl was deposited by 1,2dichloroethane (1.2 * 105 L, 423 K [27]), corresponding to a Cl coverage of 6.2 * 1014 atoms cm
2, or 0.45 monolayers (assuming 1.38*1015 atoms cm
2 in Ag(1 1 1) [16]). The change in Ag work function after saturation of the surface by Cl2 (4 L, 300 K) over Ag (1 1 1) was 1.8 eV, nearly double the value reported by Rovida et al. [27] (0.9 eV) for saturation of Ag(1 1 1) with 1,2dichloroethane, suggesting Cl deposition by organic chlorides is less efficacious than Cl deposition by Cl2. Given the similar efficacy in Cl deposition of 1,2-dichloroethane and ethyl chloride observed previously [10], we conclude from these studies that ethyl chloride is less effective at depositing Cl than is Cl2, and that a maximum surface coverage of Cl of 0.5 monolayers when Cl is deposited from ethyl chloride and moderated by the presence of a co-fed alkane is consistent with literature precedents. We previously measured Cl coverage after steady-state EO catalysis with C2H5Cl and C3H8 (Fig. 13, open squares) [2] and we report additional measurements of Cl coverage as filled squares in Fig. 13. This Cl coverage is similar to the predicted value from Eq. (18), using the rate and equilibrium constants reported in Table 1, if Eq. (18) is multiplied by the maximum coverage of 50.17% (Fig. 13, thick solid line). That the predictions from Eqs. (18) and (22) reported in Fig. 13 capture Cl coverages measured after steady-state catalysis with both C2H6-C2H5Cl and C3H8C2H5Cl containing feedstreams demonstrates the ubiquity of both the maximum Cl coverage (50.17%) for conditions relevant for selective EO formation over the promoted Ag catalyst tested here, and of equations of the form of Eqs. (18) and (22) for prediction of Cl coverages during EO catalysis. Using the regression to the C2H6-C2H5Cl data and the parameters determined from regression of Eq. (8), we re-cast Eq. (22) in terms of parameters from Eq. (8), define 50.17 as hCl, max and denote if the parameter refers to a feedstream containing ethane or propane:

hCl;C2H6 ¼

hCl;max akk4 K 3 PC2H5Cl PC2H6

ð23Þ

5;ethane

1 þ akk4 K 3 PC2H5Cl PC2H6 5;ethane

where a is the value of terms in the denominator of Eq. (18) other than the term reflecting Cl*. Using the known value of k4K3 from Table 1, and parameter P2 from Eq. (22), we can solve for k5,ethane as

k5;ethane ¼

k4 K 3 aP2

DDGz5 ¼ DGz5;propane
DGz5;ethane

ð26Þ

where DGà5,i is the free energy difference from the reactants to the transition state when i is the alkane moderator employed. The ratio of rate constants is related to DDGà5 by

 z
DDG 5 k5;propane ¼ e RT k5;ethane

ð27Þ

where R is the ideal gas constant and T is the reaction temperature. For Cl removal occurring at 513 K, a ratio in rate constants of 13 reflects a DDGà5 of
10.9 kJ mol
1. Assuming similar entropies of reaction when C2H5Cl forms from C2H6 compared to when C3H7Cl forms from C3H8, this difference in free energies could be ascribed to a difference in the enthalpies of reaction for Cl removal via either ethane or propane. Based on the lower Cl coverages measured after steady state reaction with alkanes of decreasing C-H bond dissociation enthalpy [10] and observation of an H/D KIE when the C3H8 in the feed was replaced with C3D8 (Fig. 7), the enthalpy of reaction for Cl removal by alkanes is likely related to the C-H bond dissociation enthalpy of the alkane. The BDE for methylene bonds in propane is 395 kJ mol
1, while that for methyl bonds in ethane is 410 kJ mol
1. Thus, the 15 kJ mol
1 difference in C-H BDE between propane and ethane is comparable and slightly more negative than DDGà5. These values are likely within error of one another, though slightly muted dependence on C-H BDE could result from the fact that some Cl removal by C3H8 occurs via cleavage of methyl bonds in propane, which have the same BDE as methyl bonds in ethane and results in formation of 1-chloropropane (Fig. 1). Additional explanations for the slight difference in these two values would be higher entropies of reaction for Cl removal by propane than by ethane, or incomplete C-H bond scission at the transition state. Incomplete C-H bond scission is also consistent with the observed KIE, which was less than the maximum value of 8.7 (Section S.6). The ratio k5,propane/ k5,ethane is consistent with the difference in C-H BDE, formation of 1-chloropropane, and the observed KIE, and provides a quantitative comparison of kinetic parameters that describes the phenomenological observation that alkanes with lower C-H BDE are more effective at removing Cl than those with higher C-H BDE.

ð24Þ 3.4. Implications on, and comparisons with, EO catalysis


1 which results in a value of k5,ethane of 1.0 * 10-5 mol kPa
1 Ag-1 surf s. Cl coverages reported in Harris et al. [10] using ethyl chloride as a Cl source and mixtures of ethane and propane as the alkane moderators should therefore be captured by the expression:

hCl;C2H6andC3H8 ¼

feedstream (Fig. 13, triangles). The agreement between all three sets of experimentally measured Cl coverages and the functional form of the Cl isotherm predicated by the kinetic data in Section 3.1, with parameters either regressed to experimental Cl coverage data (hCl, max, k5,ethane) or determined directly from the kinetic modeling (K3k4, k5,propane, a) suggests this isotherm form, and therefore the kinetics of Cl deposition and removal, is common for moderation of Cl by either ethane or propane. The ratio k5,propane/k5,ethane is equal to ~13, and reflects a lower free energy for Cl removal by propane than for Cl removal by ethane. We can define the difference in free energies as DDGà5, where

hCl;max aðk 1 þ aðk

k4 K 3 PC2H5Cl

5;ethane PC2H6 þk5;propane PC3H8 Þ

k4 K 3 PC2H5Cl

ð25Þ

5;ethane PC2H6 þk5;propane PC3H8 Þ

Using the expression in Eq. (25), where a, k4K3, and k5,propane were determined by regression to steady-state chloropropane formation data (values reported in Table 1), and hCl, max and k5,ethane were determined from regression of parameters P1 and P2 in Eq. (22), accurately predicts the Cl coverages measured after steadystate EO catalysis with both ethane and propane included in the

Kinetics for ethylene oxidation over the catalyst used in this study were reported previously, with apparent reaction orders for EO formation that were 0.7–1.0 in dioxygen, 0.5 to
0.3 in C2H4, and
0.5 in CO2, as C2H5Cl pressure increased from 0.8 to 3.3 Pa with constant 2.1 kPa C2H6 (513 K) [9]. The C2H4 reaction orders for chloropropane formation are reported in Fig. 5a, and were
0.5 for u = 20 and
0.9 for u = 400. The apparent C2H4 reaction orders for EO formation during these experiments are reported in Fig. 14, and were ~0.2 for u = 20 and ~-0.2 for u = 400. From Eq. (8), one can perform a log-derivative analysis [28] to determine what coverage of C2H4 is reflected in the apparent C2H4 reaction orders for chloropropane formation at u = 20 and u = 400, which are hC2H4 equivalent to 0.25 and 0.45, respectively

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Fig. 14. Variation in EO (circles) and H2O (squares) formation rates with ethylene pressure at (a) u = 20 and (b) u = 400 and with (c) carbon dioxide pressure at u = 20. Reaction conditions in (a) and (b) were 106–212 kPa C2H4, 5.3 kPa CO2, 2.65 or 0.53 kPa C3H8, 5.3 or 21.2 Pa C2H5Cl. In (c), the reaction conditions were 159 kPa C2H4, 3.7– 21.2 kPa CO2, 2.65 kPa C3H8, 5.3 Pa C2H5Cl. In (a-c), the remaining reaction conditions were 15.9 kPa O2, balance He; T, 513 K; P, 530 kPa; SV, 500,000 h
1; mass catalyst, 0.020 g.

Table 2 Apparent reaction orders and resultant predicted coverages of CO2 and C2H4 derived species based on EO formation rates and chloropropane formation rates. Gas phase species

C2H4 CO2

u

20 400 20

Apparent reaction order

Resultant species

EO

C3H7Cl

EO

C3H7Cl

EO

Species coverage C3H7Cl

0.20
0.15
0.47


0.5
0.9
0.22

C2H4/O* C2H4/O* CO2/O*

C2H4* C2H4* CO2/O*

0.40 0.60 0.23

0.25 0.45 0.11

(derivation in Section S.8). A similar analysis, assuming formation of a common intermediate is rate determining [9] and EO formation requires two sites, results in values of hC2H4 at u = 20 and u = 400 for sites that produce EO equivalent to 0.4 and 0.6, respectively. When the same analyses are performed to rationalize the measured CO2 reaction orders in terms of surfaces coverages, considering the CO2 reaction order for chloropropane formation was
0.22 (Fig. 5b), and that for EO formation was
0.47 (Fig. 14c), results in hCO2 of 0.11 for chloropropane formation and hCO2 of 0.23 for EO formation. The measured apparent reaction orders and resultant species coverages are reported in Table 2. From the species coverages reported in Table 2, it is possible that the sites for the two reactions are distinct, as for neither ethylene, at different values of u, nor for CO2, are the estimated intermediate coverages the same. Details of the site geometries required for the two reactions are not apparent from the data reported here and are topics of on-going study in our research effort.

chloropropane formation rates with respect to ethylene pressures suggest ethylene heats of adsorption vary with Cl coverage, while oxygen reaction orders that exceed 0.5 suggest oxygen adsorption and/or dissociation become increasingly kinetically relevant at high Cl coverages. Langmuir-type isotherms for Cl coverage require ratios of organic chloride promoter and alkane moderator appear in both the numerator and denominator of fractions that determine Cl coverage, and these isotherms accurately predict Cl coverages measured following steady state catalysis with feed streams containing ethane-ethyl chloride, propane-ethyl chloride, and (ethane + propane)-ethyl chloride as Cl moderators and promoters. Differences in predicted coverages of ethylene and carbon dioxide on site ensembles that form chloropropane compared to site ensembles that form ethylene oxide suggest the site requirements for these two reactions may be distinct. These findings allow for mechanism-based, quantitative predictions of Cl coverages during steady state EO catalysis as a function of process conditions.

4. Conclusions

Acknowledgements

Kinetic measurements of Cl deposition and removal during EO catalysis demonstrate the concurrence of two irreversible steps, one which deposits Cl from organic chlorides and one in which gas phase alkanes remove adsorbed Cl. A primary kinetic isotope effect for CAH bond cleavage in propane demonstrates the kinetic relevance of this step, and order of magnitude higher rate constants for Cl removal with propane than ethane relate the kinetic measurements to trends in Cl coverage with alkane identity reported previously. Parameter estimation demonstrates that the proposed mechanism can accurately capture the observed trends in the data. Increasingly negative apparent reaction orders for

The authors acknowledge Mr. Krishna Iyer for assistance with parameter estimation in Athena and data collection and Mr. Jacob Miller and Mr. Brandon Foley for helpful technical discussions. The authors acknowledge generous funding from Dow through the University Partnership Initiative.

Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.09.037.

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Please cite this article as: J. W. Harris and A. Bhan, Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.037