Analysis of coke formed during zeolite-catalyzed supercritical dodecane cracking: Effect of supercritical water

Journal Pre-proof Analysis of Coke Formed during Zeolite-Catalyzed Supercritical Dodecane Cracking: Effect of Supercritical Water Patricia Guerra, Azadeh Zaker, Pu Duan, Alex R. Maag, Geoffrey A. Tompsett, Avery B. Brown, Klaus Schmidt-Rohr, Michael T. Timko

PII:

S0926-860X(19)30485-5

DOI:

https://doi.org/10.1016/j.apcata.2019.117330

Reference:

APCATA 117330

To appear in:

Applied Catalysis A, General

Received Date:

23 August 2019

Revised Date:

4 October 2019

Accepted Date:

24 October 2019

Please cite this article as: Guerra P, Zaker A, Duan P, Maag AR, Tompsett GA, Brown AB, Schmidt-Rohr K, Timko MT, Analysis of Coke Formed during Zeolite-Catalyzed Supercritical Dodecane Cracking: Effect of Supercritical Water, Applied Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117330

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Analysis of Coke Formed during Zeolite-Catalyzed Supercritical Dodecane Cracking: Effect of

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Supercritical Water

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Patricia Guerra,† Azadeh Zaker,† Pu Duan,‡ Alex R. Maag,† Geoffrey A. Tompsett,† Avery B.

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Brown,† Klaus Schmidt-Rohr,‡ Michael T. Timko†

†Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester,

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Massachusetts 01609, United States

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‡Department of Chemistry, Brandeis University, Waltham, Massachusetts 02453, United States

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Graphical abstract

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Highlights

Supercritical water suppresses coke formation on ZSM-5 used for dodecane cracking.

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Both catalyst modification and direct chemical participation in the coke formation reaction network are implicated.

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Coke formed in the presence of supercritical water is less de-activating than coke formed in the presence.

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Abstract. Coking behavior of ZSM-5 was studied in the absence and presence of supercritical water (SCW) using dodecane cracking as a model reaction. SCW suppresses coke formation and inhibits conversion of coke precursors into polycyclic aromatic hydrocarbons. Interestingly, the thermal and structural characteristics of coke formed in the presence and absence of SCW appeared to follow similar reaction trajectories, with the primary difference being rate inhibition in the presence of SCW. Further experiments indicated that SCW must suppress coke by an

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indirect role of catalyst modification and by a direct role involving chemical or physical

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participation in the coke formation mechanism. These results provide new insight into the role of SCW on coke formation and motivate future work to identify strategies for stabilizing acid

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sites in the presence of liquid water.

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Introduction.

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Keywords: coke; zeolite; ZSM-5; hydrocarbon cracking; supercritical water.

Zeolites have been studied and used industrially for many important petroleum and biorefinery applications.[1-10] An important industrial consideration is catalyst lifetime.[11] Although

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several zeolite deactivation mechanisms exist,[12] coke formation in particular limits the lifetime of acidic zeolites.[9, 13-17] For example, Hopkins et al.[18] found that deposition of 0.5 wt% of coke on ultrastable Y zeolite reduced its hexane cracking activity by 50%. Pater et al.[19] observed 2% decrease in conversion per hour in their studies of isobutene alkylation, attributing their finding to coke accumulation. Coke formation especially plagues attempts to convert biorenewable molecules, such as pyrolysis bio-oils, into useful products, as reported previously 3

by several studies.[20-22] For example, Ma et al.[23] studied catalytic fast pyrolysis of lignin using ZSM-5, reporting coke yields ranging from 56.7 to 29.2 wt%, depending on the ratio of catalyst to feed. Zhang et al.[24] found that HZSM-5 increased coke yields by a factor of 5 in their studies of fast pyrolysis of corn cobs. Based on their studies of heptane cracking, Guisnet and Magnoux[17] suggested that coke deactivation involves blocking micropores and limiting access to reaction sites, while more

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recent work concluded that direct site poisoning also occurs in parallel with pore blockage.[25]

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An et al.[26] highlighted the specific role of methylbenzenium conversion to polycyclic aromatic hydrocarbons as a coke formation process in the methanol-to-olefin reaction.

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Considerable effort has been placed on developing approaches that minimize coke formation.

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Guisnet and Magnoux[17] recommended use of zeolites with pore structures lacking “trap cavities” and to tune zeolite acidity to be as weak as possible while still enabling catalysis of the

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desired reactions. Similarly, Huang et al.[27] performed calculations to conclude that catalyst designs that reduce ethylene adsorption/desorption energies can decrease coking rates. More

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recently, many studies have shown that reducing the diffusional path length for product exit from the zeolite pores can greatly reduce coke formation.[1, 5, 9, 14] Despite discovery of innovative materials designed to minimize coke formation, zeolite deactivation by coking is often unavoidable, and the design strategy of the fluidized catalytic

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cracker is based on the need for regular decoking,[12] typically by oxidation.[28] Guisnet and Magnoux[17] recommended a two-stage de-coking strategy consisting of sequential oxidation of hydrogen to form water at temperatures low enough that the steam product did not damage the catalyst, followed by carbon oxidation at higher temperatures. Although commercially

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operational, regular decoking increases capital costs, process complexity, and energy consumption, making new strategies potentially useful. Recently, Zaker et al.[29] studied zeolite catalyzed cracking of supercritical dodecane, reporting more than a 10× decrease in coke formation when supercritical water was added to the reaction mixture. The presence of supercritical water decreased dodecane conversion only marginally and the product mixture obtained by supercritical water assisted cracking consisted

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primarily of valuable short chain alkanes and single ring aromatics.[29] Preferential suppression

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of coke formation with minimal loss of activity is an interesting alternative for improving catalyst stability that utilizes unusual reaction conditions and a commercial zeolite, instead of the

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typical approach that requires custom-synthesized or modified catalytic materials.[5, 9]

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Many questions remain following the work of Zaker et al.[29] about the role of water on coke suppression. While steam has long been studied for reducing coke formation,[30] supercritical

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water has several properties that may make it especially effective for coke reduction. In particular supercritical water is both dense and highly miscible with hydrocarbons, properties

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which allow water can to access catalytically active sites, help remove heavy products from catalyst surfaces as they form, and potentially alter coke formation mechanisms.[31] On the other hand, Watanabe et al.[32] suggested that supercritical water can extract light products from bitumen as it cracks, resulting in enrichment of the oil phase with heavy compounds which

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subsequently undergo coking reactions.

Generally speaking, supercritical water may influence catalytic coking tendency either via a

direct chemical effect involving the reaction mechanism or by an indirect effect of catalyst modification. In terms of direct effects, water might interfere with coke formation through chemical or physical means, for example passivation of active sites by partially blocking them

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and preventing oligomerization reactions to form coke.[33] As for the indirect effects of catalyst modification, Maag et al.[34] performed a detailed study of ZSM-5 degradation in liquid water from 250 to 450 °C, finding evidence of both framework decrystallization to increase mesoporosity and acid site removal by dealumination. Since increased mesoporosity[14] and decreased acidity[17] are both associated with decreased coke production, water must clearly play an indirect role of catalyst modification to reduce coke formation. However, the relative

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importance of the two effects is not clear. Moreover, either the direct or indirect mechanism

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could be interesting technologically, since the direct mechanism would suggest a new approach to catalytic cracking that can reduce coke formation whereas the indirect mechanism might

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provide a new tool for catalyst modification.

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Differentiating between the direct and indirect roles of SCW on coke suppression was the focus of this study. Following the work of Maag et al.[34] and Zaker et al.,[29] dodecane

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cracking was chosen as the model coke forming reaction and ZSM-5 with a SiO2/Al2O3 ratio of 38 was used as the model zeolite. Reaction conditions were selected to ensure that the

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hydrocarbon reactants and supercritical water form a single phase, eliminating uncertainties introduced by fluid-fluid phase boundaries.[35] Coke formation experiments were performed at 400 °C; Zaker et al.[29] previously established that the rate of non-catalytic conversion of dodecane at 400 °C was more than 20× than the catalytic rate, meaning that thermal dodecane

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cracking reactions are insignificant at these conditions and simplifying data interpretation. The coke during dodecane cracking was then quantified and its structure characterized using

temperature programmed oxidation, vibrational spectroscopy, diffuse reflectance ultraviolet spectroscopy, and nuclear magnetic resonance. These data were then used to reconstruct coke formation trajectories in the presence and absence of SCW. Finally, tests using SCW-pretreated

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catalysts were compared with those performed using the original catalyst to differentiate the indirect effect of water on catalyst properties from its direct role in coke inhibition. The current study provides greater understanding of the role of supercritical water on coke formation. 2.0 Experimental Methods. 2.1 Materials. Dodecane (C12H26, 99.5% purity, TCI America) and ZSM-5 with approximately 2

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m particle diameter (p-38, Si/Al molar ratio equal to 38, ACS Materials) were purchased and used for cracking experiments. Zaker et al. previously reported the surface of the same catalyst

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as 320 m2 g–1 and its acid density as 630 mol g–1. Water used in cracking experiments was de-

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ionized (DI) to a resistivity of 18.1 MΩ•cm prior to use. He, N2, and air were used as carrier gases and for reactions (99.999% purity, Airgas). Dichloromethane (CH2Cl2, 99.9%, Fischer Chemical)

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was used as a solvent.

2.2 Dodecane Cracking Protocol. Zaker et al.[29] previously described the reactor and

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reaction protocol and only the most relevant details are provided here. The reactor was a stirred Inconel vessel (Parr Instrument Company, part number 4590 micro) with an internal volume of

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100 cm3 internal volume. The reactor is rated for operation at 500 °C and 0-34.5 MPa, and pressure transducer (0-35 MPa range, p/n 593HCP50AD), a thermocouple (p/n A472E2), a Parr magnetic drive (p/n A1120HC6), a rupture disk (p/n 526HCP50CTYZ50) for controlled pressure

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release as a safety feature, and an electric vessel heater (Parr Instrument, p/n A3240HC6EB). Temperature was controlled to within ±0.5 °C using a PID controller (Parr Instrument, p/n 4848). Standard experimental protocol were used for all reactions.[29] For experiments using SCW,

17 g of dodecane, 17 g of DI water and 0.85 g of ZSM-5 (5% of dodecane feed) were loaded into the reactor. For experiments performed in the absence of SCW, 34 g dodecane and 1.7 g ZSM-5 were loaded into the reactor. In both cases, the ZSM-5 loading was 5 wt% with respect to the

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initial dodecane loading. After loading, the reactor was sealed and purged with He to remove residual air. The reactor was then heated to 400 °C, maintained at this temperature for the desired reaction time (between 0.5 and 8 additional hours), and then rapidly quenched by submersion in an ice water bath. The reaction pressure was maintained at 24 ± 0.5 MPa, using a combination of reaction mixture vaporization, product formation, and initial addition of He, when required. The reaction conditions were selected to ensure a single fluid phase during the entire reaction, as per

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the predictions of He et al.[35]

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Mass balance closure was verified to within ± 0.5 g (about ±3%) by comparison of the total reactor weight before and after each experiment. Data from experiments which failed the mass

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balance closure test are not included here. All experiments were performed at least twice to

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ensure reproducibility.

Following each experiment, the reactor was de-pressurized and liquid, solid, and gas products

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were collected. Zaker et al.[29] previously reported liquid and gas analysis methods and data. Table SI-1 provides dodecane conversion data for reference. The focus of this work is

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characterization of the coke present on the recovered solids. The solid was separated from the liquid product using filtration, then rinsed with dichloromethane solvent until the recovered liquid was colorless. The filtered solid was oven dried (100 °C) for 3 h to remove residual solvent and stored in air-tight containers for further characterization. A representative coked

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ZSM-5 sample was subjected to 24-h soxhlet extraction using dichloromethane and the extraction liquid analyzed using gas chromatography (GC). Degraded ZSM-5 catalyst powder was produced by placing 2 g of catalyst and 20 g of DI in

the same vessel as used for dodecane cracking. The vessel was heated to 400 °C for either 1 for 2 h and then quenched. The solid product was recovered as described previously.

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2.3 Coke Characterization Methods. Coked ZSM-5 was characterized using: Temperature Programmed Oxidation (TPO), Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR), Diffuse-Reflectance UV/Vis Spectroscopy (DRUVS), 13C Nuclear Magnetic Resonance (NMR), and UV Raman microscopy. TPO was performed using a thermogravimetric analyzer (model 209 F1 Libra, Netzsch). Samples were heated from room temperature to 800 °C at 10 °C min–1 in an alumina crucible. The oxygen and nitrogen flow rates were set to 4 and 8 cm3 min–1,

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respectively. Analyses were performed in duplicate to confirm reproducibility (± 0.5%). Average

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ATR-IR spectra with 4 cm–1 resolution were obtained (Vertex 70, Bruker) over the range from 4500 to 600 cm–1 by averaging 512 individual scans. ATR-IR spectra were obtained in duplicate

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and average spectra are presented here. DRUVS analysis was performed using a Thermo

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Scientific Evolution 300 spectrophotometer equipped with a Praying Mantis diffuse reflection cell. BaSO4 was used as the white reflectance standard and spectra were plotted using the

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Kubelka and Munk model.[36, 37] DRUVS analysis was performed in duplicate and average results are presented here. UV Raman spectroscopy (inVia, Renishaw) was performed using an

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excitation beam of 244 nm and at a maximum power of 5 mW. Three spectra were obtained from different sample locations and averaged spectra are presented here. Solid-state

13C

NMR was

performed with multiple cross-polarization (multiCP) as first described by Johnson and SchmidtRohr.[38] Samples were packed directly into 4-mm zirconia rotors and NMR spectra were

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measured (Biospin DSX400, Bruker) at 100 MHz using 14-kHz magic-angle spinning (MAS). 3.0 Results and Discussion The objective of the current study was to understand better the coke inhibition effect of SCW and to investigate structural differences in the coke formed in the presence and absence of SCW. Based on the previous reports by Maag et al.[34] and Zaker et al.,[29] SCW is expected to play

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an indirect role in reducing coke formation by modifying the catalyst to make it less prone to coking. The central question addressed therefore was determining if SCW also inhibits coke formation via a direct effect by altering the coke formation mechanism or rate. To differentiate between the direct and indirect roles of coke suppression, ZSM-5 was used as a catalyst for the dodecane cracking reaction. For consistency with the notation for supercritical water (SCW), supercritical dodecane is referred to as “SCD” since the hydrocarbon reactant was

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also in its supercritical state at reaction conditions. Reaction times ranged from 0 to 8 hr, where 0

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implies heating of the reactor to the final temperature followed by rapid cooling. Conversion ranged from 10 to 90% under these conditions, permitting a wide range of coked samples to be

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generated. Table SI-1 provides dodecane conversion data for reference. So that conversion

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covered a similar range, longer reaction times were examined for SCW conditions, since the presence of SCW reduced dodecane cracking rates.

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The study then consisted of three main parts. In the first part, the coked samples were analyzed using a variety of complementary methods to identify trends in coke characteristics in the

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presence and absence of SCW. The objective of the first part of the study was identification of coke formation rates and structural characteristics in the presence and absence of SCW. In the second part of the study, the structural data were organized to construct coke formation trajectories in the presence and absence of SCW. In the third part of the study, ZSM-5 was

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subjected to SCW pretreatment under non-reactive conditions, recovered, and then used as a catalyst in the absence of SCW to differentiate between the direct and indirect effects of SCW suppression of coke formation on ZSM-5. 3.1 Coke Analysis. Photographs of the coked ZSM-5 samples are provided in the Supporting Information as Figure SI-1. Whereas the samples produced in the absence of SCW are uniformly

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black, samples produced in its presence are instead light gray. This visual difference is qualitative evidence of the coke inhibition effect of SCW. To make this determination quantitative, the coke content of the zeolite samples was measured using TPO. Figures 1a and b contains representative TPO differential thermograms obtained from analysis of coked samples formed in the presence (1a) and absence (1b) of SCW. The thermogram obtained for the sample used in the presence of SCW shows two main features, one at 220-280

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°C and the other centered at 550 °C. In comparison, the thermogram of the sample obtained in

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the absence of SCW is dominated by a single feature centered at approximately 550 °C.

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(c)

50 40

30

hard coke

Coke (wt%)

20 10 0 50 40 30

soft coke

hard coke

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2 4 6 Reaction Time (h)

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Figure 1. Temperature programmed oxidation (TPO) data obtained from analysis of ZSM-5 used for dodecane cracking in the presence (D/W) and absence (D) of SCW: a) representative

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differential thermograms of samples obtained in the presence of SCW from 1 to 8 h as indicated,

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b) representative differential thermograms of samples obtained in the absence of SCW, c) coke content determined by integration of the raw thermograms. The top panel was obtained in the

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absence of SCW and the bottom in the presence. Reaction conditions: 400 °C, 24 ± 0.5 MPa. Interestingly, the minor feature appearing at 220-280 °C in the thermogram of the sample prepared under SCW conditions is not apparent in the thermogram of the sample prepared in its absence. Attributing the feature at 220-280 °C is not obvious. We confirmed that simply

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exposing ZSM-5 to dodecane followed by TPO analysis results in a single sharp feature appearing at approximately 220 °C, which is distinct from the thermogram of the coked zeolite. Accordingly, and consistent with prior studies of coked zeolites,[16, 39] we attributed the feature appearing at 260 °C to “soft coke”. Consistent with previous studies,[16] the feature appearing at 550 °C can be attributed to “hard coke”. The magnitude of this feature is much greater in the absence of SCW than the presence, a 12

clear indication that SCW reduces hard coke formation.[40] Moreover, the maximum hard coke volatilization temperature (visible in the TPO thermograms in both Figure 1a and b) increases with increasing reaction time, an indication of increasing thermal stability that is related to other structural trends such as increasing aromaticity.[16] The TPO thermograms were integrated to quantify the amounts of coke residing in soft and hard forms, following methods previously reported in the literature.[41] Figure 1c provides the

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results, plotted as the coke content as wt% of the total solid recovery as a function of reaction

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time. In the absence of water, dodecane conversion reached >90% after 2 h and the reaction was not studied for times greater than 2 h for this reason. In comparison, dodecane conversion

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continued to increase for reaction times as great as 8 h in the presence of SCW, and hence Figure

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1a provides coke content data in the presence of SCW for reaction times from 0.5 to 8 h. At all reaction times, the coke formed in the absence of SCW is of the hard variety and its

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content is more than 10-times greater than observed in the presence of SCW. In fact, even after the shortest reaction time (0.5 h), considerable coke formation was observed in the absence of

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SCW, followed by a steady increase with increasing reaction time. In the presence of SCW, on the other hand, total coke increased with increasing reaction time for the initial 3 h, at which time the coke content remained steady at approximately 10 wt% of the total solid product. Similarly, the coke formed in the presence of SCW was primarily of the soft variety for reaction times less

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than 2 h, and then transitioned to a mixture consisting of nearly equal amounts of the two coke varieties for reaction times greater than 2 h. TPO provides quantitative information on coke content and some qualitative information about

thermal stability that provides indirect evidence of structural trends. Figures 2-5 provide additional compositional analysis in the form of ATR-IR (Figure 2 and 3), DRUV (Figure 4),

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and UV Raman (Figure 5) spectra. Of these, ATR-IR provides information about aliphatic and aromatic C-C and C-H structural components; DRUV and UV Raman provide information about the size or aromatic domains. Considering first ATR-IR spectra, Figure 2 provides data in the range from 3200-2800 cm–1, the part of the spectrum in which various C-H stretching features appear.[42] In the presence of SCW, the ATR-IR spectra are dominated by bands associated with aliphatic C-H stretches,

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especially those attributed previously to CH3 groups.[43] For a reaction time of 8 h, the ATR-IR

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spectra of coke formed in the presence of SCW contain a minor feature associated with the aromatic C-H stretch. In contrast, the aromatic C-H stretch dominates the ATR-IR spectra of

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coke formed in the absence of SCW at all reaction times.

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Figure 3 provides ATR-IR data in the fingerprint region from 1800 to 1400 cm–1. Figure 3 indicates that coke formed in the absence of SCW is much more highly aromatic than the coke

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formed in the presence of SCW, as suggested by the relative intensities of the C-C aromatic bands appearing at 1600 and 1470 cm–1 in the two sets of samples. Interestingly, the ATR-IR (a)

(b)

aromatic C-H asym CH3

Reflectance (a.u.)

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Reflectance (a.u.)

aromatic C-H asym CH3 sym CH3 8

2 1

2

0.5

1

3200

3100 3000 2900 –1 Wavenumber (cm )

2800

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3200

3100 3000 2900 –1 Wavenumber (cm )

2800

Figure 2. The C-H stretch region of Attenuated Total Reflectance Infrared (ATR-IR) spectra obtained from analysis of ZSM-5 coked in the presence (a) and absence (b) of SCW. Reaction conditions: 400 °C, 24 ± 0.5 MPa, reaction times in hours shown directly on plotted data. (b)

aromatic C-C

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4 3 2

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1 0.5

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1700 1600 1500 –1 Wavenumber (cm )

1800

1400

1700 1600 1500 –1 Wavenumber (cm )

1400

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1800

aromatic C-C

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Reflectance (a.u.)

Reflectance (a.u.)

aromatic C-C aromatic C-C carbonyl

H3

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(a)

1

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Figure 3. The fingerprint region of Attenuated Total Reflectance Infrared (ATR-IR) spectra obtained from analysis of ZSM-5 coked in the presence (a) and absence (b) of SCW. Reaction

(a)

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conditions: 400 °C, 24 ± 0.5 MPa, reaction times in hours shown directly on plotted data.

220

(b)

220

8 6 4 3

260

330

Intensity

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Intensity

260

440 2 1

2 1 200

300 400 Wavelength (nm)

500

0.5 200

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300 400 Wavelength (nm)

500

Figure 4. Diffuse Reflectance Ultraviolet (DRUV) spectra obtained from analysis of ZSM-5 coked in the presence (a) and absence (b) of SCW. Reaction conditions: 400 °C, 24 ± 0.5 MPa, reaction times in hours shown directly on plotted data. (a)

(b) 1640

1080

1210

1640

1120 1230

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1460

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2

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Intensity

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Intensity

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1

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0.25

800

1000

1200

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1000 1200 1400 1600 1800

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Wavenumber (cm )

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Figure 5. UV Raman spectra obtained from analysis of ZSM-5 coked in the presence (a) and absence (b) of SCW. Reaction conditions: 400 °C, 24 ± 0.5 MPa, reaction times shown directly

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on plotted data.

spectra of the coke formed in the presence of SCW contains a minor side band at approximately 1720 cm–1; the typical attribution of IR features in the range from 1750 to 1700 cm –1 is carbonyls.[44] Therefore, ATR-IR analysis suggests the presence of carbonyl functionality in the

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coke formed in the presence of SCW, but not in its absence. Surface bound carbonyls may be consistent with intermediate formation during coke gasification.[45] Figure 4 provides DRUV spectra obtained for coked ZSM-5 generated in the presence (a) and

absence (b) of SCW. DRUV is sensitive to the aromatic structure of coke and can be used to gain insight into the number of rings present in aromatic structures.[46] DRUV spectra of coke

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formed in the presence of SCW consists primarily of 1-, 2-, and 3-ring aromatic compounds, as suggested by prominent bands appearing only at 220 and 260 nm and consistent with previous literature attributions.[47] The additional features at 330 and 440 nm in Figure 4b indicate that coke formed in the absence of SCW consists of aromatic structures containing 4 or more rings.[47] The presence of multi-ring PAHs in the ZSM-5 coked in the absence of SCW is consistent with the general mechanism suggested previously by Guisnet and Magnoux.[16]

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UV Raman microscopy provides additional detail about aromatic content. Figure 5 provides

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representative UV Raman spectra obtained in the presence (Figure 5a) and absence (Figure 5b) of SCW. In both cases, the Raman spectra are dominated by prominent bands at approximately

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1640 cm─1, which can be assigned to the asymmetric breathing mode of aromatics and sometimes

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generically termed the “G band”.[48] A major difference in the two sets of spectra shown in Figure 5a and 5b is the relative prominence of the secondary feature centered at approximately

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1440 cm─1 in the coke formed in the presence of SCW and 1460 cm─1 in its absence. The band in the range from 1440-1460 cm─1 is mostly associated with symmetric breathing modes of

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aromatic molecules, and is sometimes generally termed the “D band”.[49] In both Figures 5a and 5b, the D band feature is broad and clearly consists of multiple sub-features, suggesting that it arises from many compositionally differentiated sources within the coke. In addition, the D band is much more prominent in the coke formed in the absence of SCW; for PAH structures

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consisting of fewer than 10 rings,[48] D band intensity increases with increasing number of aromatic rings. Therefore, the UV Raman spectra indicate that the coke formed in the absence of SCW consists of larger aromatic clusters than the coke formed in the presence of SCW. The final analysis performed on the coked zeolite was quantitative

13C

NMR.[38]

13C

NMR

provides information on the local environments of carbon atoms in the coke and can be used to

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infer whether they participate in aromatic or aliphatic bonds. Unlike ATR-IR, the pulse sequences used for

13C

NMR analysis provide nearly quantitative data. Representative spectra

are shown in Figure 6. Figure 6a contains the NMR spectrum obtained for ZSM-5 used in the absence of SCW (1 h reaction time). As expected from previous considerations, >90% of the carbon atoms of the ZSM-5 used for hydrocarbon cracking in the absence of SCW participate in aromatic bonds. The remaining carbon atoms are present as methyl groups, consistent with

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previous analysis of coked  zeolite.[50]

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Figures 6b and 6c provide NMR spectra obtained from analysis of ZSM-5 used for dodecane cracking in the presence of SCW at reaction times of 1 h (Figure 6b) and 8 h (Figure 6c),

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respectively. Again, as expected from previous analysis, the coke formed in the presence of

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SCW is primarily aliphatic, with a minor contribution from aromatic carbons. The abundance of aromatic carbons formed in the presence of SCW increases when reaction time is increased from

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1 to 8 h. NMR does not indicate the presence of carbonyl groups in the coked formed in the

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presence of SCW. The combined information from the two methods suggests that carbonyl is a

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Figure 6. 13C NMR spectra of coke in ZSM-5 formed in the absence (a) and presence of SCW for 1 hour (b) and 8 hours (c). Spectra of nonprotonated C and mobile segments obtained after 68 s of gated 1H decoupling are shown by thinner red lines.

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minority component (<5%) and is consistent with IR being more sensitive for detection of the carbonyl group than is NMR.[44] Interestingly, the aliphatic portion of the NMR spectrum obtained from analysis of the SCW

coked samples is remarkably similar to the spectrum expected for dodecane,[44] the original reactant molecule. The dodecane apparently persists in the ZSM-5 structure even after the

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dichloromethane rinsing protocol all recovered samples were subjected to, prompting a question as to why the dichloromethane treatment did not remove the residual dodecane. To investigate the presence of dodecane further, coked ZSM-5 samples were subjected to an aggressive 24-hour soxhlet extraction using dichloromethane and the extract analyzed; consistent with NMR analysis, dodecane was the primary component of the extracted sample. On the other hand, the simple dichloromethane rinse, similar to that used for recovering coked ZSM-5 from

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the reaction mixture, completely removed dodecane from ZSM-5 which had been exposed to the

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liquid hydrocarbon under non-reactive conditions. Apparently, therefore, dodecane is physically trapped inside coked ZSM-5, a phenomenon which can be explained either by hydrocarbon

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adsorption or partial pore blockage during use.

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To study dodecane adsorption, the 13C mobility of coked ZSM-5 samples was investigated. The insensitivity of the majority of hydrocarbon signal to gated decoupling showed that it arose from

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mobile segments, suggesting that dodecane was not fixed in place. Therefore, pore blockage may be more likely than adsorption to the zeolite surface as an explanation for the persistence of

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trapped dodecane. Pore blockage may be due either to closure of pores during the harsh SCW treatment,[34] formation of aromatic coke at the pore mouths or similar locations that restrict dodecane removal,[17] or some combination of the two. 3.2 Coke Formation Trajectory Construction. The previous section clearly establishes that

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coke formed in the presence of SCW is primarily aliphatic, consistent with dodecane trapped within the pores and a minority component of aromatic compounds with ≤ 3 rings, whereas the coke formed in the absence of SCW was primarily aromatic, consisting of multi-ring PAHs with methyl side groups. The analysis is consistent with the consensus coke formation sequence[16] – 1) adsorption of hydrocarbon precursors to the zeolite, 2) formation of aromatic constrictions that

20

trap dodecane as “soft coke”; 3) conversion of trapped dodecane into hard coke as aromatic compounds consisting of 1-3 rings, 4) conversion of simple aromatic compounds into multi-ring PAH coke, and 5) transition from pore-bound coke to an external coke layer once internal pores are filled. The initial hydrocarbon sorption step is consistent with the mechanism suggested by Li and Stair[51] from their studies of propene coke formation on zeolite Y, but is not observed directly here as aromatic blockages have already formed even for the least coked samples.

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Consistent with this observation, the least coked samples contain approximately 1-2 wt% coke

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and Chen et al.[52] suggested that pore blockage occurs once coke loading reaches 2 wt%.

Interestingly, the final coking step, formation of PAHs with more than 3 rings, is not evident

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from DRUV spectra obtained for coked samples formed in the presence of SCW. The lack of

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multi-ring PAH formation in the presence of SCW is consistent with the observation by De Lucas et al.[53] that the threshold for external coke formation is formation of approximately 5

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wt% coke.

Conversely in the absence of SCW, evidence for the first three coke formation steps is limited,

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consistent with the rates of these reactions being much greater than the sample acquisition time. Therefore, the data presented in Figures 1-6 leave open two important questions: 1) does the presence of SCW fundamentally alter the coke formation mechanism, or simply inhibit reaction progress? and 2) can the effect of SCW be attributed to the well-known catalyst degradation

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previously reported in the presence of liquid water phases,[34] or does the explanation involve a direct participatory role of water in the coke formation mechanism? Answering the first question requires determination of the most relevant chemical timescale in

the system. Two time scales were considered – the time scale for dodecane cracking, as quantified by dodecane conversion at different times, and the time scale for coke formation, as

21

quantified by coke composition measured at different times. Of these, the coke formation time scale is more relevant to coke composition; the initial dodecane cracking step occurs prior to coke formation, making the connection between dodecane cracking and coke composition less direct than that between coke formation and coke composition. Accordingly, Figure 7 contains various structural parameters derived from Figures 1-4, all plotted as functions of coke content. These plots provide important information to help answer the question of the effect that SCW

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has on the coke formation pathway.

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Figure 7a provides data extracted from Figure 1, specifically the temperature of the normalized maximum hard coke volatilization rate, , observed for a given sample. As mentioned

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previously, the temperature T of the maximum hard coke volatilization rate is associated with

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structure, for example the degree of coke aromatization.[16] For clarity of presentation, a scaled temperature,  has been defined to vary from 0 to 1.0 using the relationship,  = (T – Tmin)/(

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Tmax – Tmin) where Tmax is the temperature of the maximum sample volatilization rate observed for any sample (i.e., the temperature of the peak in the TPO thermogram obtained for coke

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produced after 2 h of reaction time in the absence of SCW) and Tmin is temperature of the minimum sample volatilization rate observed for any sample (i.e., the temperature of the peak in the TPO thermogram obtained for coke produced after 0.5 h in the presence of SCW), and T is simply the temperature of the maximum volatilization rate for the sample of interest. An

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interesting trend is evident in Figure 7a, as  is constant at values less than 0.02 for coke content less than 3%, and then rapidly increases to nearly 1.0 for coke content greater than 3%. The transition cannot be ascribed to the presence or absence of SCW, since data points obtained from coke formed in the presence in SCW appear on both sides of the transition. Accordingly, Figure 7a clearly points to SCW inhibiting the coke formation rate, but not altering the final product.

22

(b) 1.0

 (normalized)

0.8 0.6 0.4 0.2 0.0 2

(c)

4

6 8

2

150

100

50

0 2

4

6 8

1

4

10 Coke (wt%)

Coke (wt%)

4

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8

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6

4

2

0 2

4

6 8

2

10

4

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1

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Aromatic 1-3 rings/ Aromatic 4+ rings

2

10

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1

200

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Aliphatic C–H/Aromatic C–H (ATR-IR)

(a)

Coke (wt%)

Figure 7. Coke structural characteristics extracted from the raw data provided in Figures 1-3, (a)

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normalized maximum hard coke volatilization temperature  = (T – Tmin)/( Tmax – Tmin), as determined from TPO thermograms provided in Figure 1; (b) aliphatic to aromatic C-H ratio extracted from ATR-IR data provided in Figure 2; (c) ratio of small aromatic molecules (1-3 rings) to large aromatic molecules (≥ 4 rings) estimated from the DRUV spectra provided in Figure 3.

Signifies data collected in the presence of SCW.

absence of SCW.

23

Signifies data collected in the

Figure 7b provides C-H aliphatic and aromatic data extracted from fitting the ATR-IR spectra shown in Figure 2, following literature protocol.[43] As expected from qualitative consideration of the ATR-IR spectra and the quantitative NMR spectra showed in Figure 6, the aliphatic/aromatic ratio is approximately 200:1 for coke formed in the presence of SCW at reaction times less than 1 h. With increasing reaction times, the relative quantity of aromatic C-H monotonically increases, again with a sharp transition at approximately 3 wt% coke. For coke

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formed in the absence of SCW, the aliphatic/aromatic ratio reaches a value less than 1, consistent

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with aromatic cores and methyl side groups. As with the TPO analysis shown in Figure 7a, Figure 7b suggests that SCW mainly plays an inhibiting role without altering the coke formation

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pathways themselves.

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Figure 7c provides data extracted from the DRUV spectra shown previously in Figure 3 as the ratio of different types of aromatic structures, those consisting of 1-3 rings and those consisting

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of more than 4 rings. Here, DRUV bands at 220 and 260 were used to determine the relative abundance of the small PAH clusters and the bands at 330 and 440 nm were used for the larger

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clusters, as described in the literature.[47] Unlike Figures 7a and 7b, the data in Figure 7c do not exhibit a critical threshold and instead the relative abundance of large aromatic clusters increases monotonically with increasing coke content. Interestingly, however, a significant gap exists between the most heavily coked sample prepared in the presence of SCW and the least heavily

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coked sample prepared in its absence. Efforts to prepare samples with intermediate coke content failed, either because coking was too rapid in the absence of SCW or because coke content reached a maximum value of about 5% in the presence of SCW; increasing reaction time to values greater than 8 h did not increase the coke content of samples prepared in the presence of SCW. Nonetheless, Figure 7c suggests a continuum of coke structure, captured by coke content

24

and with a trajectory that is independent of the presence of SCW. Accordingly, Figure 7c supports previous conclusions drawn from TPO and ATR data that the presence of SCW inhibits coke formation rates without modifying pathways that lead to determination of structure. 3.3 Effect of SCW Pretreatment on Coke Formation. The analysis provided in Figure 7 suggests that SCW inhibits coke formation without disrupting its structure, at least when analyzed using coke content as the scaling factor. This left the final question to be answered:

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does water participate in coke formation pathways directly via some physical or chemical effect,

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or only indirectly by modifying acid sites and/or textural properties such as mesoporosity or external surface area. In fact, Zaker et al.[29] and more recently Maag et al.[34] reported that

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exposure to SCW can decrease Brønsted acid site (BAS) density of ZSM-5 by >90% and modify

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micropore structure, alter surface textural properties, and decrease particle thickness. All of these changes can be expected to reduce coke formation.

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Based on previous work on ZSM-5 degradation which showed that SCW exposure reduces BAS density, increases external surface area, and decrease particle thickness,[29, 34] SCW must

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reduce the coking tendency of ZSM-5 via the indirect role of catalyst modification. The question then becomes whether any direct role can be differentiated from the indirect effect. Several plausible coke suppression mechanisms have been proposed based on results obtained under steaming conditions. For example, An et al.[26] suggested that the water byproduct of the

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methanol-to-olefin reaction prevented conversion of methylbenzenium carbenium ions into hard coke on the H-ZSM-5 surface. Phillips and Datta[33] studied the ethanol dehydration reaction catalyzed by ZSM-5 in the presence of water vapor, suggesting that water adsorbed to the surface inhibited bimolecular recombination reactions and hence coke formation. These studies have not

25

considered SCW conditions, meaning that the current study can provide new guidance into coke formation and suppression. A major challenge with differentiating indirect from direct water effects is that SCW treatment results in changes in many different ZSM-5 properties.[34] Moreover, these properties change in parallel during the course of the reaction, any one of which – or their combination – might plausibly play a role in coke formation rates. In fact, narrowing down which property or

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properties exerted the strongest effect on coke formation was not the aim of this work, which

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was instead focused on understanding the role of water on coke formation.

To circumvent these complicating factors, a different approach was adopted. Instead of

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attempting to analyze all potential effects of SCW treatment on BAS density, particle thickness,

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external surface area, etc. – as Maag et al.[34] previously reported under SCW conditions – the current study instead generated a catalyst sample by pretreatment in SCW and then used this

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material for dodecane cracking in the absence of water. Pretreating the material in SCW generates a sample that duplicates the full range of acidity and textural properties associated with

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SCW-exposed ZSM-5. Performing the coke formation test in the absence of SCW means that the properties of the catalyst do not change during the course of the reaction,[29] making interpretation straightforward compared with the moving target presented by reaction in the presence of SCW.

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With this approach in mind, ZSM-5 was subjected to pretreatment in SCW at 400 °C without

addition of dodecane, conditions which Maag et al.[34] previously reported result in rapid reduction of BAS density and modification of the crystalline structure and textural properties of ZSM-5. Two SCW pretreatments were evaluated – a mild one consisting of a 1 h treatment and a more aggressive one consisting of a 2 h treatment and resulting in a 90% reduction of BAS

26

density.[29] For comparison, Zaker et al.[29] reported 90% BAS reduction after treatment in dodecane-water mixture at 400 °C, meaning that the two SCW pretreated samples bracket the range of degradation expected for ZSM-5 used under reactive SCW conditions. The SCW-pretreated catalysts were then used for dodecane cracking in the absence of SCW for direct comparison with the original materials. The two SCW-pretreated catalysts were analyzed after use for dodecane cracking using TPO to quantify hard coke content and

NMR to

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provide structural detail.

13C

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TPO analysis indicated that the coke contents of the two SCW-treated samples were 8.7±0.5 and 6±1 wt%, for the mildly and aggressively pretreated ZSM-5 respectively. The coke contents

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obtained at the same reaction conditions (400 °C and 2 h) but for the untreated catalyst were

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37±2 (absence of SCW) and 2.1±0.4 (presence of SCW) wt%, respectively. The fact that the SCW-pretreated catalysts yield coke amounts much greater than found in the presence of SCW

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clearly indicates that SCW must play more than the indirect role of catalyst modification to explain coke suppression. A direct chemical role in reducing coke formation is implicated. 13C

NMR. Figure 8 provides the

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The coked, SCW-pretreated catalysts were analyzed using

corresponding spectra for the 1 h SCW treatment (Figure 8a) and the 2 h SCW treatment (Figure 8b). The main features of the

13C

NMR spectra obtained from the coked SCW-pretreated

catalysts are intermediate to those obtained using original materials in the presence and absence

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of SCW. Specifically,

13C

NMR indicates that the coke formed on SCW-pretreated catalyst is

primarily aromatic, but with an aliphatic component and a prominent dodecane signature – i.e., a combination of the characteristics of the coke formed using unmodified ZSM-5 in the presence and absence of SCW. As expected, the more aggressive pretreatment results in an NMR

27

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13C

NMR spectra obtained from analysis of SCW-pretreated ZSM-5 coked after

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Figure 8.

dodecane cracking (2 h reaction time at 400 °C); (a) 1 h SCW pretreatment resulting in 50%

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BAS density compared with the original and (b) 2 h SCW pretreatment resulting in 10% BAS

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density compared with the original.

spectrum closer to that obtained in the presence of SCW and the milder pretreatment results in

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coke more similar to that obtained in the absence of SCW.

Both TPO and NMR analysis of the SCW pretreated, coked ZSM-5 samples indicate that the

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characteristics of coke formed in the absence of SCW but using SCW pretreated catalyst was intermediate to that formed using fresh catalyst in the presence (heavy coking, highly aromatic) and absence (light coking, mainly aliphatic) of SCW. SCW therefore plays the expected indirect

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role on coke formation, presumably by reducing BAS density and/or increasing mesoporosity or external surface area. SCW may have potential as a rapid zeolite modification technique for careful tuning of BAS density and accessibility to minimize coking tendency of ZSM-5. More interestingly, tests with SCW pretreated catalyst indicate that the presence of water results in coke yields and structures that cannot be explained by catalyst modification and instead implicates an direct role for SCW in coke inhibition. Several possible mechanisms of coke

28

inhibition exist, including suppression of oligomerization reactions, as suggested by Phillips and Datta,[33] or disruption of the carbenium pathway proposed by An et al.[26] The identification of a direct role of coke inhibition by SCW motivates future studies to elucidate the mechanism and to identify catalyst or reaction engineering strategies to stabilize the zeolite against SCW to exploit the benefits of coke reduction while minimizing deactivation due to dealumination and

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decrystallization.[34]

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4.0 Conclusions.

Coking is one of the primary zeolite deactivation mechanisms. In this work, the coke inhibition

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effect of SCW was investigated using dodecane cracking catalyzed by ZSM-5 as a model reaction. Coked ZSM-5 was obtained in the presence and absence of SCW and analyzed using a

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variety of thermal and spectroscopic techniques. The presence of SCW reduced coke formation

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by an order of magnitude, and shifted it primarily from hard coke to soft coke. The hard coke was characterized by stability to oxidation and PAH content. The soft coke was more readily

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oxidized and consisted of aliphatic and single-ring aromatic compounds. In the presence of SCW, coke initiation seemed to involve trapping dodecane inside the pores, followed by conversion to aromatic products. Coke properties were plotted as function of the coke content of the catalyst, showing that coke formation in the presence and absence of SCW follows a

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common trajectory, and that SCW inhibits coke reaction progress. A major question in this work was understanding the role of SCW in coke suppression. Two

possibilities were considered. Either SCW plays a direct role, physical or chemical, in the coke formation pathway, or SCW modifies the catalyst acidity and/or structural properties to reduce coking tendency. SCW treated catalysts were prepared in the absence of dodecane to simulate the effect of SCW exposure on catalyst properties. The SCW-pretreated materials were then used

29

to catalyze dodecane cracking in the absence of SCW, revealing that SCW must inhibit coke formation by both of the postulated coke inhibition mechanisms. These results provide greater understanding of the role of SCW in coke formation by zeolites and motivate future work to extend the stability of zeolites in SCW. Acknowledgments. Saudi Aramco was the primary funding source for this study (6600023444).

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Dr. Ki-Hyouk Choi and his team at Saudi Aramco encouraged this work and provided valuable technical input. PG was supported by a CNPq graduate study fellowship. Petroleum Research

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Fund supported ARM’s contribution.

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