HY catalysts used for the hydrogenation and ring opening of naphthalene

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Applied Catalysis A: General 335 (2008) 230–240 www.elsevier.com/locate/apcata

Acidity and deactivation of Mo2C/HY catalysts used for the hydrogenation and ring opening of naphthalene Xuebin Liu, Kevin J. Smith * Department of Chemical and Biological Engineering, The University of British Columbia 2360 East Mall, Vancouver, BC, Canada, V6T 1Z3 Received 11 July 2007; received in revised form 10 November 2007; accepted 14 November 2007 Available online 28 November 2007

Abstract The selective ring opening (SRO) of naphthalene over a series of Mo2C/HY catalysts with 7–27 wt% Mo2C, is reported. Pulsed adsorption and temperature programmed desorption of n-propyl amine showed that increased Mo2C content of the Mo2C/HY catalysts significantly decreased the amount and strength of the catalyst acid sites, especially the Bro¨nsted acid sites necessary for SRO. Carbon deposition was the main cause of catalyst deactivation and thermogravimetric analysis of the used catalysts indicated that accumulation of carbonaceous species over the surface of the acidic HY support with time-on-stream led to deactivation of the Mo2C/HY catalysts, whereas no significant carbon deposition occurred on the Mo2C. Catalyst activity tests showed a synergistic effect between the Mo2C and the acidic support (HY) that was not readily obtained with a mechanical mixture of the two catalyst components. # 2007 Elsevier B.V. All rights reserved. Keywords: Ring opening; Hydrogenation; Catalyst; Molybdenum carbide; HY zeolite; Acidity; Carbon deposition; Deactivation

1. Introduction The Canadian oilsands, with 174 billion barrels of established bitumen reserves, has been increasingly recognized as a strategic, stable source of energy for North America [1] and consequently, bitumen-derived synthetic crude oil production from the Canadian oilsands is expected to double in the next 10 years. However, bitumen-derived heavy gas oil (HGO) has approximately 90% cycloparaffins plus aromatics, versus 60% in a typical ‘‘paraffinic’’ HGO [1]. The aromatic content of bitumen-derived HGO can be reduced through hydrogenation processes [2,3], but the cycloparaffinic content remains high, and these compounds have low cetane numbers. Therefore, selective ring opening (SRO) of the cycloparaffins without a reduction in carbon number of the product molecule is desired. Solid acid catalysts and noble metal catalysts (such as Pt, Pd, Ir, Ru and Rh supported on acidic, basic or neutral materials [2– 4]) catalyze ring opening of mono naphthenic ring compounds. However, high ring-opening selectivity is difficult to achieve over solid acid catalysts because isomerization and subsequent

* Corresponding author. Tel.: +1 604 822 3601; fax: +1 604 822 6003. E-mail address: [email protected] (K.J. Smith). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.11.028

hydrocracking (b-scission) reactions dominate. Supported noble metal catalysts are active for selective hydrogenolysis of naphthenes with higher ring strain such as cyclobutane, and cyclopentane, but they have much lower activity for SRO of sixmember ring compounds [5], which can be readily converted to ring opening products only after isomerization to fivemembered ring compounds over an additional acid site. Bifunctional catalysts are therefore widely adopted for SRO, in which highly dispersed metals account for hydrogenation/ dehydrogenation functionality together with that of hydrogen spillover [6] while the acidic supports provide cracking or isomerization functionality. Transition metal (Ni–Mo, Ni–W, Co–Mo) sulfides and noble metals supported on various mesoporous or macroporous zeolites [7–10] have mostly been investigated for SRO. Supported noble metal catalysts are very active and can catalyze SRO reactions below 200 8C [11]. However, they are readily poisoned by the S and N present in industrial feedstocks, although addition of a second noble metal increases the S tolerance of these catalysts [12–15]. By contrast, supported bimetallic sulfide catalysts have high tolerance to S, but their low activities require operation at high H2 pressure and high temperature. The catalytic activity of transition metal carbide catalysts resembles that of the noble metals for a number of reactions

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[16–19] including hydroprocessing reactions [20–24]. Our previous work [25] on SRO of naphthalene showed that the acidity of bulk molybdenum carbide and oxycarbide was too low to provide sufficient ring-opening selectivity, whereas molybdenum carbide supported on HY zeolite showed a promising ability for SRO together with a high level of S tolerance. The HY zeolite is considered to be one of the most appropriate zeolites for selective ring opening [10,26]. Zeolite acidity had a very significant effect on the catalytic performance of the supported Mo2C catalysts, especially with respect to catalyst deactivation [25]. However, the role of the Mo2C and the effect of the number and type of acid sites on the SRO of naphthalene are not well understood and needs further study. In the present study, the SRO of naphthalene over HY supported Mo2C has been investigated with an emphasis on the correlation between acidity, carbon deposition and activity. Our goal is to better understand the synergies between the carbide and the acidic support during SRO reactions, and to gain some insight into catalyst stability and causes of deactivation. 2. Experimental 2.1. Catalyst preparation Catalysts with 7, 13, 20 and 27 wt% Mo2C supported on HY zeolite were prepared by wet impregnation of the zeolite (ZeolystTM CBV720, SiO2/A12O3 = 30) using a quantified aqueous solution of ammonium heptamolybdate tetrahydrate (MoO3 81.0–83.0%, Sigma). After aging at 70 8C for 2 h, the impregnated support was dried at 120 8C for 12 h and then calcined at 500 8C for 4 h. Carburization of the calcined catalyst precursor was done in a U-shaped quartz tubular reactor (i.d. = 7 mm) by temperature programmed reaction in a CH4/H2 (UHP) flow (20% CH4 and total flow of 100 ml (STP)/ min). The catalyst precursor was heated to 700 8C at a rate of 5 8C/min and held at 700 8C for 4 h. The catalyst was then held for 2 h in a H2 (UHP) atmosphere to remove excess carbon before cooling to room temperature. Finally, the catalysts were passivated in a 1% O2 in He stream (150 ml (STP)/min) before being placed in sealed vials for later use. A bulk Mo2C was also prepared following the same temperature programmed reaction procedure and using MoO3 (+99.5%, Sigma–Aldrich) as the catalyst precursor. To investigate the synergistic effects between Mo2C and the solid acid support, an 11% Mo2C/HY catalyst was prepared by mechanically mixing appropriate amounts of the bulk Mo2C and the HY zeolite that had been treated in air at 700 8C for 6 h. 2.2. Catalyst characterization X-ray diffraction (XRD) was performed on the passivated catalysts using a Siemens D500 diffractometer with a Cu Ka X˚ . The analysis was performed ray source of wavelength 1.54 A using a scan range of 3–808 with a step size 0.048 and step time of 2 s. The phase identification was carried out after subtraction of the background using standard software. Crystallite size (dc) estimates were made using the Scherrer equation, dc = Kl/

231

bcosu where the constant K was taken to be 0.9, l is the wavelength of radiation, b is the peak width in radians and u is the angle of diffraction. X-ray photoelectron spectroscopy (XPS) analysis was done with a Leybold Max 200 spectrometer using AlKa radiation as the photon source, generated at 15 kV and 20 mA. The pass energy was set at 192 eV for the survey scan and at 48 eV for the narrow scan. The catalysts were analyzed after the passivation step without further treatment and all XPS spectra were corrected to the C1s peak at 285.0 eV. Transmission electron microscopy (TEM) images of the catalysts were obtained using a Fei Tecnai 20 scanning transmission electron microscope operating at 200 kV. Catalyst samples were ground to a fine powder, dispersed in ethanol and sonicated for 2 h. A drop of the catalyst suspension was placed on a 200 mesh copper grid coated with formvar and carbon, and left to dry before analysis. BET surface areas were measured by a single-point N2 adsorption at 196 8C using a Flowsorb 2300 (Micromeritics) analyzer. A 30% N2/70% He mixture, fed at 15 ml/min (STP) was used for single-point surface area measurements. Samples were degassed at 250 8C for 2 h prior to making the measurements. Carbon deposition on the catalysts after reaction with naphthalene was investigated by thermogravimetric analysis (TGA) using a TGA-50 thermogravimetric analyzer (Shimadzu, Japan). About 5–10 mg of the used catalyst was loaded into an alumina crucible and heated to 800 8C at a rate of 5 8C/ min in an air atmosphere (flow rate = 15 ml (STP)/min). The weight gain and loss was quantified and differential TGA (DTGA) was accomplished using standard TGA software. In the present work, n-propylamine (nPA) was used to titrate the type, strength and quantity of acid sites on the Mo2C/HY catalysts. nPA is protonated by Bro¨nsted acid sites and decomposes to propylene and ammonia in a well-defined temperature range via a reaction similar to the Hofmannelimination reaction. In the nPA-temperature programmed desorption (TPD) profiles, it is normally accepted that the peaks emerging before 300 8C are due to nPA desorption and are attributed to Lewis acid sites [27,28]. The peaks above 300 8C are due to nPA cracking on Bro¨nsted acid sites. The relative ratio of Lewis and Bro¨nsted acid sites can be obtained by deconvolution of the TPD profile using stoichiometry and the amount of nPA desorbed below 300 8C. Pulsed adsorption and TPD of nPA was conducted in a stainless steel reactor (i.d. = 9 mm) containing 0.5 g of catalyst that was pretreated in He (30 ml (STP)/min) at 100 8C for 1 h to remove water followed by a 1 h treatment in H2 (30 ml (STP)/ min) at 400 8C to activate the passivated Mo2C and finally a 2 h flush in He (30 ml (STP)/min) at 500 8C to remove adsorbed H2. A flow of He (30 ml (STP)/min) saturated at room temperature with nPA (Aldrich, 99.8%) passed through heated gas lines to a 1 ml sample loop that injected the saturated nPA vapor into a He (30 ml (STP)/min) stream flowing through the reactor containing the pre-treated catalyst already cooled to 25 8C. The gas leaving the reactor was monitored by a thermal conductivity detector (TCD). Pulses of nPA were injected

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repeatedly until the TCD signal from the outlet gas showed no further adsorption of the injected nPA. The total acidity of the catalyst was calculated by determining the total uptake from the pulse of nPA vapor, assuming that one nPA molecule is adsorbed per acid site [29]. Following the pulsed adsorption experiments, the system was purged for 1 h in a He flow (30 ml (STP)/min) to remove residual amine. Subsequently, TPD of nPA from room temperature to 700 8C at a heating rate of 5 8C/ min was performed to determine the type and relative ratio of acidic sites of each of the catalysts. Blank experiments were also completed to eliminate effects of nPA physisorption on the gas lines and reactor wall. The Lewis acid sites were estimated by quantifying the TPD peak area occurring below 300 8C, where the product of desorption is nPA alone. The difference between the total acidity and the Lewis acidity is taken as the Bro¨nsted acidity. To confirm this value of Bro¨nsted acidity, the TPD peaks were deconvoluted (see Fig. 4) and using the known stoichiometry of the nPA decomposition on Bro¨nsted acid sites (C3H7NH2 + H2 ! NH3 + C3H8) and the known thermal conductivities of NH3 and C3H8, the amount of NH3 and C3H8 produced and hence the amount of nPA desorbed from Bro¨nsted acid sites was calculated. The calculated stoichiometric ratio of NH3 and C3H8 was close to 1 as expected. 2.3. Catalyst activity tests The SRO performance of each of the catalysts was determined in a stainless steel fixed-bed reactor (i.d. = 9 mm) at 300 8C and a total pressure of 3.0 MPa with naphthalene as the model reactant. Catalyst (about 0.5 g without dilution with inert material) was loaded into the isothermal section of the reactor with either end of the bed packed with glass beads (1 mm diameter). The passivated catalyst was first activated in pure H2 (60 ml (STP)/min) at 450 8C for 1 h. After cooling to the reaction temperature, a 5 wt% solution of naphthalene (99%, Acros Organics) in heptane (Fisher Scientific, HPLC grade) was pumped, using a Gilson Model 0154E metering pump (0.16 ml/min), into a stream of flowing H2 (19 and 28 ml (STP)/min for a H2/ naphthalene ratio of 20 and 30, respectively) that entered the reactor. The feed weight hourly space velocity (WHSV) was 1 h1. Each catalyst test was performed over a period of 5 h and liquid products of the reaction were collected each hour and analyzed using a Varian Star 3400 Gas Chromatograph equipped with a flame ionization detector (FID). Component separation was achieved using a capillary column (Varian CP8969, 30 m length and 0.53 mm i.d.). Component identification was confirmed using the same column and a GC–MS (Agilent 6890/5973N). To facilitate the discussion, reaction products were grouped as follows: (a) Hydro: tetralin and decalin (cis and trans isomers), (b) ROP: ring-opening products, mainly alkylcyclohexanes, alkylbenzenes and alkylindenes, (c) Poly: aromatics and naphthenes with more than 10 carbon atoms, mainly alkyltetralins. The catalyst tests were randomly repeated to confirm experimental reproducibility and in each of the experiments the carbon balance was better than 95%. Both diagnostic tests and calculation was used to confirm

that at the chosen conditions, the catalyst activity data were free of both internal and external heat and mass transfer effects. 3. Results 3.1. Catalyst characterization The powder X-ray diffractograms of Fig. 1 confirm the presence of the Mo2C phase on the HY supported catalysts, although for the 7 wt% Mo2C/HY, the Mo2C peak intensity is near the diffractometer’s detection limit. Fig. 1 also shows that the crystallinity of the HY zeolite was maintained after loading the Mo2C, indicating that there was no obvious framework collapse associated with the high-temperature preparation procedure. The physiochemical properties of the supported Mo2C catalysts are summarised in Table 1 with the HY zeolite and bulk Mo2C included for reference. The presence of Mo2C significantly reduced the surface area of the supported catalysts compared to that calculated from the surface area and weight fraction of each of Mo2C and HY. The Mo2C was distributed over the HY with a particle size of 19  4 nm, as determined by X-ray line broadening, meaning that a significant quantity of the Mo2C crystallites must be too large to be located inside the channels of the HY zeolite. Similar observations have been made for Mo2C/HZSM-5 catalysts [30–33], although in all cases the presence of Mo2C within the channels of the zeolite cannot be excluded [32,33]. The calculated grain size of the HY zeolite (at 2u = 6.38 and 15.98, the two strongest reflections from the HY zeolite) decreased gradually with increased Mo2C loading, suggesting that the presence of Mo2C had some influence on the long-range order of the HY zeolite crystallites. TEM micrographs are shown in Fig. 2A and B for the 7% Mo2C/HY and the 27% Mo2C/HY catalysts, respectively. A range of Mo2C particle sizes less than 20 nm in size are apparent, with the Mo2C well dispersed over the zeolite. The volume average particle size determined from Fig. 2A and B was 4  1 nm and 9  3 nm, respectively.

Fig. 1. X-ray powder diffraction pattern of bulk Mo2C, support (HY zeolite) and supported catalysts thereof with various Mo2C loading.

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Table 1 Physicochemical properties of supported molybdenum carbide catalysts Catalyst

SBET 2

m /g

Acidity (mmol/g)a Lewis

Particle size (nm) Bro¨nsted

Total

HY 6.38

HY 7 wt% Mo2C/HY 13 wt% Mo2C/HY 20 wt% Mo2C/HY 27 wt% Mo2C/HY Bulk Mo2C a b c

711 606 525 417 309 6.8

0.19(23) 0.15(22) 0.10(20) 0.08(17) 0.08 (25) 0.009 (44)

0.65 (77) 0.52 (78) 0.40 (80) 0.39(83) 0.24 (75) 0.011 (56)

0.84 0.67 0.50 0.47 0.32 0.02

XPS Mo2C

b

– 44 41 40 31 –

15.98 – 66 63 60 45 –

b

39.38 – – 19 22 15 28

Mo/(Si + Al)

Si/Al

– 0.03 0.06 0.10 0.13 –

– 26.6 28.0 21.9 13.3 –

c

The numbers in brackets are the percentage of Lewis and Bro¨nsted acid sites. Corresponding to the particle size of HY zeolite in specified directions. Corresponding to the particle size based on the (1 0 1) lattice plane of Mo2C in XRD spectra.

XPS analysis was used to estimate the distribution of the oxidation states of Mo, C and O on the Mo2C/HY catalysts, by decomposition of the Mo3d, C1s and O1s spectra, respectively. The de-convoluted XPS spectra for 27% Mo2C/HY are shown in Fig. 3 and the XPS results for all the supported Mo2C catalysts are summarized in Table 2. Curve-fitting of the Mo 3d region was accomplished using a mixed Gaussian–Lorentzian function (15% Lorentzian) and a non-linear least-squares fitting algorithm with Shirley background subtraction applied. An intensity ratio of 2/3 (3d3/2/3d5/2), a 3d3/2–3d5/2 split of 3.2 eV, and a full width at half maximum (FWHM) of 1.8 eV for the metal carbide and 2.4 eV for the oxide were used. As shown in Fig. 3A, a characteristic species with Mo 3d5/2 binding energy (BE) of 229.1 eV, higher than 228.2 eV reported for the Mo2C [34] and lower than 229.7 eV reported for Mo4+ [36], was identified. During deconvolution, the width of this peak was 1.8 eV, somewhat lower than the 2.4 eV usually associated with molybdenum oxide. Assigning this peak to Mo2C or Mo4+ species alone did not give reasonable results. Therefore we attribute the peak to a mixture of Mo4+ and partially oxidized Mo2C species, herein designated as Mod+. Two other molybdenum species, with Mo 3d5/2 binding energy of 231.2 and 233.3 eV, were assigned to Mo5+ and Mo6+, respectively,

consistent with literature values of 230.8 and 232.5 eV, respectively [34,36]. No peak corresponding to Mo metal with Mo 3d5/2 BE at 228.0 eV [34,35] was detected. The percent Mod+ species increased with increased Mo content (Table 2), as did the ratio of Mo/(Si + Al) determined by XPS and reported in Table 1. Clearly, a significant portion of the surface Mo species are present in the form of carbides or oxycarbides, although >50% molybdenum exists as Mo5+ and Mo6+ species after passivation. Table 2 also confirms the presence of the Mo2C species on the 7% Mo2C/HY catalyst (17% of total surface Mo species), which was not clearly evident by XRD (Fig. 1). The C1s and O1s XPS spectra were each de-convoluted into three XPS peaks as shown in Fig. 3B and Fig. 3C, respectively. The peaks at C1s BE of 284.1  0.3 eV and O1s BE of 531.3  0.3 eVare attributed to carbon bonded to oxygen and metal (designated as C–O–Mo) and oxygen bonded to metal (designated as O–Mo) in molybdenum oxycarbide [36]. As expected, the amount of carbon and oxygen species in an oxycarbide state increased with increased Mo2C loading (Table 2). The other two peaks of the C1s XPS spectra, at BE 285.0 eV and 286.0 eV, are assigned to adventitious carbon (amorphous carbon or adsorbed hydrocarbon designated here as (CH2)n) [37,38], and carbon atoms involved in carbonate

Fig. 2. TEM micrographs of (A) 7%Mo2C/HY and (B) 27%Mo2C/HY catalysts.

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Fig. 3. Superposition of experimental XPS spectra and spectra after deconvolution for 27% Mo2C/HY catalyst. (A) Mo3d, (B) C1s, (C) O1s. Table 2 XPS results for HY-supported molybdenum carbides with different Mo loading Catalyst

7% Mo2C/HY 13% Mo2C/HY 20% Mo2C/HY 27% Mo2C/HY

C 1s

O 1s

Mo 3d5/2 d+

5+

6+

Oxycarbide

Adventitious

Carbonate

Mo

carbide

Mo oxide

Mo oxide

Oxycarbide

Al2OSiO4

Carbonate

283.9 284.0 284.1 284.4

284.9 284.9 285.0 285.3

286.0 286.0 286.0 286.6

229.1 229.1 229.1 229.1

(17) (31) (38) (47)

231.2 231.2 231.2 231.2

233.3 233.3 233.3 233.3

531.0 531.3 531.6 531.5

533.0 533.1 533.2 533.1

534.0 534.2 534.2 534.2

(14) (15) (17) (38)

(56) (54) (53) (47)

(30) (31) (30) (15)

species (C–O, no C O at 288.5 eV was found in the present work) [39], respectively. The O1s XPS peaks at BE 534.0 eV and 533.0 eV are attributed to oxygen atoms involved in carbonate species and silicon–oxygen tetrahedron of the zeolite, respectively. The latter species account for about 60% of the total oxygen and decreases significantly at high Mo2C loading (Table 2). The total acidity and the ratio of Lewis to Bro¨nsted acid sites measured for the Mo2C/HY catalysts are summarized in Table 1. The total acidity of the Mo2C/HY catalysts decreased significantly with increased Mo2C content. Bro¨nsted acid sites prevailed over Lewis acid sites for all of the supported catalysts, whereas for the bulk Mo2C, both acid sites were present in approximately equal amounts, albeit in much lower amounts compared to the supported catalysts. With increased Mo2C loading from 7 to 20% (Table 1), the percent of Lewis acid sites decreased marginally (from 22 to 17%) whereas a further increase in Mo2C content from 20 to 27% resulted in a decrease in the percent of Bro¨nsted acid sites from 83 to 75%. Deconvolution of a typical nPA TPD profile is shown in Fig. 4, in which peaks I, II and III were assigned to desorbed nPA (Lewis acid sites), propylene and ammonia (Bro¨nsted acid sites), respectively. The ratio of propylene and ammonia desorbed in Fig. 4 was 0.9  0.1, in good agreement with the stoichiometry of the Hofmann degradation reaction. The fourth peak in Fig. 4 appearing at high-temperature was attributed to a small fraction of NH3 desorbed from Bro¨nsted acid sites that likely re-adsorbs and reacts with Mo2C or Mo2COx at high temperature, forming new species such as N2, CH4 and/or H2O. The nPATPD profiles from the various catalysts are plotted in Fig. 5. The first peak for all samples, assigned to Lewis acid sites, emerged at the same temperature (200 8C in Fig. 5) with reduced

(33) (30) (23) (23)

(50) (39) (39) (31)

(0) (4) (10) (11)

(58) (63) (63) (53)

(42) (33) (27) (36)

intensity as the Mo loading increased. This indicates that Mo2C addition decreased the amount of Lewis acid sites but did not affect their strength. By contrast, both intensity and position of the peaks between 300 and 500 8C, corresponding to Bro¨nsted acid sites, changed significantly as Mo2C loading increased. As shown in Fig. 5, besides a sharp decrease in intensity, the desorption temperature of these peaks decreased from 409 8C for the HY zeolite to temperatures as low as 310 8C for the Mo2C/ HY catalysts. Mo2C obviously decreased the acidity of the Mo2C/HY catalysts, especially the Bro¨nsted acidity. Previous reports on the TPD of amines do not show desorption products above 527 8C [27,29,40] because deso-

Fig. 4. Demonstration of decomposition of n-propylamine TPD profile (7% Mo2C/HY).

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Fig. 5. n-Propylamine TPD profiles for support (HY zeolite) and HY supported catalysts with various Mo2C loadings.

Fig. 6. n-Propylamine TPD profiles of bulk Mo2C (reduced and unreduced) and 27% Mo2C/HY catalysts.

rption from Bro¨nsted acid sites is completed at lower temperatures. The peaks observed at this high temperature in the present work do not reflect the catalyst acid properties. However, closer inspection of the TPD profile at this temperature provides some insight into the synergistic effects between Bro¨nsted acid sites and Mo2C, even though the reactant is nPA rather than naphthalene. The small peak at 550 8C in the TPD profile of the HY zeolite (Fig. 5) is caused by a small portion of NH3, generated by the decomposition of nPA over Bro¨nsted acid sites, being strongly adsorbed on the HY zeolite. This peak disappeared on the 7 wt% Mo2C/HY but a new, wider peak at 630 8C appeared in its place. Clearly, the NH3 desorbing from Bro¨nsted acid sites, migrates and readsorbs on the Mo2C sites nearby and reacts with Mo2C resulting in new species such as N2, CH4 (over Mo2C) and/or H2O (over Mo2CO). With increased Mo2C loading, the production of these species would increase, resulting in a decrease in the desorption temperature and widening of the peaks. These observations are further confirmed by the TPD profiles of the bulk Mo2C catalyst in Fig. 6. Without the aid of

HY zeolite, the last peaks were greatly weakened since NH3 production over Bro¨nsted acidic sites was small. Finally, it is observed that the shape of the last peak from the TPD of bulk passivated (unreduced) Mo2C catalyst is quite different to that of activated (reduced) bulk Mo2C catalyst. This is because less H2O is produced over the metal carbide than the metal oxycarbide, which is known to exist even after H2 reduction at high temperatures [41]. 3.2. Catalyst performance for SRO of naphthalene Preliminary experiments with the Mo2C/HY catalysts (not shown), using the heptane solvent as feed, demonstrated that nearly all light hydrocarbon products generated during reaction with naphthalene were a consequence of heptane cracking. Furthermore the production of light hydrocarbons from heptane was significantly hindered by the presence of naphthalene in the feed. This result was expected since naphthalene is much more nucleophilic than heptane toward both Bro¨nsted acid and metal sites. Hence, the low molecular weight products of cracking

Table 3 Catalytic performance test of various catalystsa Experiment

Mo2C wt%

Conv. (%)

E1 E2 E3 E4 E5 E6 E7 E8

HY 7 13 20 27 Bulk Mo2C 20 b 11b,c

24 86 91 91 84 74 92 48

Selectivities (%)

Yield (%)

Decalin

Tetralin

Hydro

RO

Poly

Hydro

RO

0 1.9 2.4 2.6 3.1 16.2 2.8 0.9

31.2 60.5 61.0 61.7 63.2 82.7 55.6 56.1

31.2 62.4 63.4 64.3 66.3 98.9 58.4 57.0

9.6 23.8 24.3 22.4 23.0 0.8 32.9 27.5

59.2 13.8 12.3 13.3 10.7 0.3 8.7 15.5

7.5 53.7 57.7 58.5 55.7 73.2 53.7 27.4

2.3 20.5 22.1 20.4 19.3 0.6 30.3 13.2

a Reaction conditions: time-on-stream: 1 h; catalyst weight: 500 mg; T: 300 8C; P: 3 MPa; WHSV (referred to the sum of H2 and Naphthalene): 1 h1; feed: H2 and a solution of 5 wt% naphthalene in heptane; H2/naphthalene = 20 (mole) unless otherwise noted. b H2/naphthalene = 30. c Catalyst prepared by mechanically mixing bulk Mo2C and HY Zeolite.

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were not included in the calculation of the product distribution during SRO of naphthalene over Mo2C/HY catalysts. The initial Mo2C/HY catalyst activities (after 1 h time-onstream) for SRO of naphthalene are shown in Table 3. The HY support had the lowest ability to convert naphthalene (E1 of Table 3) with a naphthalene conversion of 24% and a product distribution of 31% tetralin and 60% polymerization products. GC–MS analysis of the polymerization products showed that alkylnaphthalene dominated, together with smaller amounts of methyltetralins and trinucleated aromatics. There was obvious deactivation over the HY zeolite as the conversion of naphthalene decreased from 24 to 5% after 4 h time-on-stream (Fig. 7, 0% Mo loading). At the same time, the selectivity to tetralin increased and the selectivity to polymerization products decreased with the selectivity to ROP slightly reduced. By contrast, bulk Mo2C (E6 of Table 3) activated naphthalene with a relatively high conversion (74%) but generated the hydrogenation products tetralin (83%) and decalin (16%) (trans/cis = 5). The high selectivity to hydrogenation products (99%) was retained through the entire experiment (Fig. 7, 100% Mo loading) with tetralin (97%) dominating at the end of the experiment (not shown). The naphthalene conversion decreased gradually from 74 to 58% due to a strong interaction of the naphthalene with surface metals that limits the hydrogenation of tetralin [42]. This interaction is likely a simple site blocking mechanism since no carbon deposition was found over the Mo2C catalyst, as discussed below. The SRO of naphthalene over the Mo2C/HY catalysts is clearly different from that over the HY zeolite or the Mo2C alone. For the Mo2C/HY catalysts, the conversion of naphthalene and ROP yield were higher than that obtained over the HY zeolite or the bulk Mo2C. Tetralin was the main product in all cases but its selectivity as well as the selectivity to

decalin over supported catalysts was lower than that observed over the bulk Mo2C catalyst. As the Mo2C content increased from 20 to 27% on the Mo2C/HY catalysts, the naphthalene conversion decreased from 91 to 84%. Increasing the H2/ naphthalene ratio facilitates hydrogenation over both metal and acidic sites (by spill over hydrogen), that in turn accelerates the conversion of hydrogenated products to ROP (E4 and E7 of Table 3). The selectivity to polymerization products is significantly reduced over the Mo2C/HY catalyst in comparison with HY alone, however, there was still about 9% polymerization products (E7 of Table 3) even with a H2/naphthalene ratio of 30, and this decreased gradually with time-on-stream. The above comparison of SRO over HY zeolite, bulk Mo2C and Mo2C/HY clearly demonstrates the existence of synergistic effects between the two components of the catalysts. The synergistic effects between Mo2C and HY (acid sites) cannot be reproduced by a simple mechanical mixture of the two components. As shown in Table 3, the naphthalene conversion over the mechanical mixture is much lower than the Mo2C/HY catalyst (compare E3 and E8 of Table 3), even though the latter has the benefit of a higher H2/naphthalene ratio. Fig. 7 shows that the conversion of naphthalene increased gradually with increased Mo2C loading and reached a maximum at a Mo2C loading of 20 wt%. The conversion then decreased with a further increase in Mo2C content. The catalyst selectivity, however, remained relatively constant as the Mo2C loading increased. These observations suggest that within the range investigated, increasing Mo2C loading increased the number of bifunctional active centers available for hydrogenation, isomerization and ring opening. However, at higher Mo2C loading (27%), although the acid centers decreased and the Mo dispersion increased (Table 1), the number of bifunctional centers must decrease resulting in a decrease in conversion to the hydrogenated products, and a decrease in the ROP yield. 3.3. Characterization of the used catalysts by TGA

Fig. 7. SRO performance of various catalysts at 1 h (solid symbols) and 5 h (hollow symbols) (4 h for HY zeolite) of time-on-stream. SRO of naphthalene was carried out under the same reaction conditions as described in Table 4 except for time-on-stream. ($, §) conversion; (&, &) hydrogenation selectivity; (~, ~): ROP selectivity; (*, *) polymerization selectivity.

Coking is usually the main reason for deactivation of solid acid catalysts, especially in the presence of aromatic hydrocarbons. To better understand the stability of the HY supported Mo2C catalysts for SRO, TGA/DTGA of various used catalysts was performed and compared with those of unused catalysts (Fig. 8 and Table 4). The DTGA profiles of Fig. 8 show two weight loss peaks at approximately 100 8C and 795 8C, attributed to the removal of trace water and the sublimation of MoO3, respectively. Between 100 and 600 8C peaks associated with both a weight gain and a weight loss are shown in Fig. 8. Comparing the DTGA profile of the used 27% MoC2/HY (Fig. 8D) catalyst with that of the un-used sample (Fig. 8E), the weight gain peak between 100 and 350 8C is assigned to the oxidation of the molybdenum carbide and/or oxycarbide to MoO2 or MoO3, whereas, the weight loss peaks between 350 and 550 8C are attributed to the removal of carbon deposited on the catalyst during the SRO reaction of naphthalene since there is no weight loss peak in the un-used sample. There are three weight gain peaks in the DTGA profile of the un-used bulk Mo2C (Fig. 8A), among

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Fig. 8. DTGA profiles of various catalysts. (A) un-used bulk Mo2C; (B) used 7% Mo2C/HY; (C) used 20% Mo2C/HY; (D) used 27% Mo2C/HY; (E) un-used 27% Mo2C/HY; (F) used mechanically mixed 11% Mo2C/HY; (G) used HY zeolite.

which the first two correspond to the oxidation of surface Mo2C and/or Mo2COx species, while the largest peak at about 500 8C is attributed to the oxidation of bulk molybdenum carbide and oxycarbide species. Due to the slow diffusion of oxygen and the amount of Mo2C associated with the large bulk particles, the

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area and position (approximately 500 8C) of this peak is significantly higher compared to the first two peaks. By contrast, the weight gain peaks for the supported catalysts (Fig. 8B–E) are much smaller and appear at much lower temperature (<350 8C). This is because Mo2C and/or Mo2COx species are well dispersed over the surface of the zeolite and the ratio of surface to bulk species is higher compared to the unsupported bulk Mo2C. This can be further confirmed by the DTGA of the 11% Mo2C/HY sample (Fig. 8F) prepared by mechanically mixing the bulk Mo2C and the HY support, in which the slow oxidation of the bulk Mo2C produces an extremely wide weight gain peak. The data of Fig. 8 show that there is only one large weight loss peak that occurred during the DTGA of the HY support after reaction with naphthalene, and it appeared at 525 8C (Fig. 8G). Undoubtedly, this peak corresponds to the removal of carbon by combustion. With addition of the Mo2C to HY, the weight loss peak splits into two weak peaks, emerging at 375 and 475 8C, respectively (Fig. 8B–D). The former weight loss peak is likely related to the carbon located on the interface between the Mo2C and the HY support, which is a consequence of intermediate products and is easily removed by reaction in air. The latter is much less reactive and is attributed to carbon generated by acid catalyzed polymerization reactions. Due to a limited interaction between the Mo2C and the HY zeolite in the mechanically mixed Mo2C/HY catalyst, the peak positions attributed to weight loss shift to higher temperature (450 and 500 8C, respectively) and the weight loss peaks of Fig. 8F are dominated by carbon species difficult to remove that appear more like the weight loss peaks for the HY support alone (Fig. 8G). The above observations suggest that carbon deposition mostly occurs on the acidic support and this carbon plays a crucial role in the deactivation of the HY supported Mo2C catalysts. Further details on the effect of Mo2C on carbon deposition over Mo2C/HY catalysts are shown in Table 4. Firstly, most of the deposited carbon comes from naphthalene rather than the heptane solvent (S6 of Table 4) and the carbon deposition

Table 4 TGA analysis of used catalysts (unless otherwise noted) No.

Sample

Weight gain (%) a

Weight loss (%) b

Note b

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

Unused bulk Mo2C HY Bulk Mo2C 7% Mo2C/HY 13% Mo2C/HY 13% Mo2C/HY 20% Mo2C/HY 20% Mo2C/HY 20% Mo2C/HY 11% Mo2CinHY 11% Mo2CinHY 27% Mo2C/HY Unused 27% Mo2C/HY

31.7 0 31.4 0.8 2.0 1.0 2.8 2.2 2.9 0.5 0.6 4.3 5.4

0 16.3 0 10.9 8.1 0.8 7.0 7.0 5.2 8.8 8.7 5.4 0

As prepared – Passivated in air at RT – – Solvent heptane as feed – 4 h on stream Rc = 30 Rc = 30, mechanical mixture of bulk Mo2C and HY Rc = 30, for 3 h time-on-stream – Used as prepared

a b c

The weight gain and weight loss were evaluated within a temperature range 100 and 600 8C. SRO of naphthalene was carried out under the same reaction conditions as described in the Table, for a 5 h time-on-stream, or as noted. R: ratio of H2/naphthalene.

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reaches a stable amount after approximately 3 h time-on-stream (S7/S8 and S10/S11 of Table 4) in agreement with the rapid catalyst deactivation observed and confirming that carbon deposition is the main reason for the loss in catalyst activity. Secondly, carbon deposition is significantly inhibited by increased Mo2C loading. The weight loss decreased from 16% for HY (S2 of Table 4) to 5.4% for 27% Mo2C/HY (S12 of Table 4). Moreover, S7 and S9 of Table 4 also demonstrate that the carbon deposition could be moderated by increasing the H2/ naphthalene ratio. However, although with a high H2/ naphthalene ratio, carbon deposition over the mechanically mixed Mo2C/HY catalyst (corresponding to a weight gain of 8.8% – S10 of Table 4) was still higher than that obtained over the supported Mo2C/HY catalyst (8.1%, S5 of Table 4) demonstrating that the synergy between the Mo2C and HY support could not be obtained by simply mixing the two components. 4. Discussion Analysis by XRD and XPS confirmed that Mo2C was successfully synthesized on all the Mo2C/HY catalysts without changing the structure of the HY zeolite support (Fig. 1 and Table 2). The increase in the Mo/(Si + Al) ratio determined by XPS (Table 1) with increased Mo2C loading indicated that Mo2C was well dispersed over the surface of the supported catalysts. Furthermore, the XRD data (Table 1) showed that the Mo2C particle size (about 20 nm) remained nearly unchanged with increased Mo2C content. Hence there was no obvious Mo2C sintering over the Mo2C/HY catalysts within the Mo2C loading range investigated. Previous studies of Mo2C/HZSM-5 catalysts used for the aromatization of CH4 [30–33] have suggested that on these catalysts, Mo-containing clusters about 1 nm in size are formed inside the HZSM-5 channels [33] whereas Mo2C with a particle size 2–15 nm is formed on the HZSM-5 surface [33]. Note, however, that most studies of Mo2C/HZSM-5 have focused on low loading Mo2C (<10%). In the present work, the much higher Mo2C loadings implies that a significant fraction of the Mo2C is located outside the zeolite channels. Mo2C plays an important role in determining the acidity of the Mo2C/HY catalyst. Mo2C reduces significantly the amount of acid sites compared to the HY support and the acid site strength, especially the Bro¨nsted acid sites (Table 1 and Fig. 5). Since there was no collapse of the HY crystal lattice and the size of Mo2C particle was too large for it to occupy Bro¨nsted acid sites located in the zeolite channels, the reduction of catalyst acidity with increased Mo2C loading must be mainly due to ionexchange between hydrogen and molybdenum ions during the catalyst preparation [32,33]. The Mo2C, mostly dispersed on the catalyst surface, also contributed to the reduction in Lewis acid sites by blocking some extra framework Lewis acid sites on the zeolite. SRO is favored over zeolites with a low density of Bro¨nsted acid sites, since strong and high density Bro¨nsted acidity usually leads to extensive cracking, dealkylation [43] and carbon deposition. On the other hand, Bro¨nsted acidity is

necessary for isomerization reactions, a key step in SRO of naphthenes with six-member rings. When Mo2C loading increased from 20 to 27% (Table 1), the percent of Bro¨nsted acid sites decreased from 83 to 75%, indicating that excess Mo2C decreased the number of bifunctional active centers by replacing or occupying acidic centers. Therefore, the SRO activity decreased at high Mo2C loading (E4 and E5 of Table 3). Similarly, fewer active centers were obtained by a simple mechanical mixture of HY zeolite and bulk Mo2C (E3 and E8 of Table 3). Mo2C also plays an important role in suppressing carbon deposition during naphthalene conversion over the Mo2C/HY catalysts. As shown in Fig. 9, the weight loss measured by DTGA after reaction with naphthalene, decreased significantly with increased Mo2C loading. As already mentioned, Mo2C moderates the Bro¨nsted acid sites of the Mo2C/HY catalyst, and consequently coke precursors from acid-catalyzed polymerization reactions (Table 3) are greatly reduced. Moreover, Mo2C has excellent hydrogenation ability and can offer spillover hydrogen to the support as well. These factors improve the saturation of unsaturated compounds and facilitates their desorption from both acid and metal sites. In this way, carbon deposition over the Mo2C/HY catalyst was further reduced. The identical weight gain and weight loss for the used and un-used bulk Mo2C (S1 and S3 of Table 4) indicated that there was minimal carbon deposition on the bulk Mo2C catalyst, consistent with the fact that there were no polymerization products and only hydrogenation products from the reaction of naphthalene over bulk Mo2C (Table 1). On the supported catalyst, the data of the present study also suggest that carbon deposition on the Mo2C is minimal. Unlike the weight loss, the weight gain of the used supported Mo2C catalysts increased linearly with Mo2C loading of the Mo2C/HY catalyst (see Table 4 and Fig. 9), suggesting that the Mo2C did not contain any carbon deposits that reacted in the temperature range of the observed weight gain, i.e. between 100 and 350 8C. This is

Fig. 9. The effect of Mo2C loading on weight gain and weight loss in TGA of used catalysts.

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further confirmed by the small difference in weight gain between the un-used and used 27% Mo2C/HY reported in Table 4 (S12 and S13). All of these observations indicate that after 5 h reaction, the Mo2C supported on HY is relatively unchanged with little carbon deposition on the Mo2C. Since the carbon removal apparently only occurs at temperatures >350 8C on the HY and the Mo2C/HY, and the amount of carbon deposited was highest on the HY zeolite (S2 of Table 4, and Fig. 8) we conclude that carbon deposition on the HY zeolite is responsible for the deactivation of Mo2C/HY catalyst. The product distribution of the HY zeolite (E1 of Table 3) could be interpreted by the bimolecular mechanism proposed by Sato et al. [7] for hydrocracking of tetralin on a NiW/USY catalyst. Accordingly, the absence of hydrogenation-oriented metal sites prevents an effective supply of hydrogen to the zeolite acid sites and a carbenium ion of alkylbenzene is formed on a Bro¨nsted acidic site from one tetralin molecule, which is a consequence of hydride transfer reactions of naphthalene over

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acidic sites [25]. This carbenium ion molecule then attacks one naphthalene molecule leading to alkylnaphthalene and multinuclear aromatics that are further transformed into coke by chain reactions. The above bimolecular mechanism is shown in Fig. 10B. In the case of reactions over a bifunctional catalyst, a monomolecular pathway [43,44] would be followed, and some side reactions such as dealkylation and secondary cracking would also occur. As Fig. 10A shows, the naphthalene is hydrogenated to tetralin and small amounts of decalin on Mo2C sites. The hydrogenated intermediates migrate to acid sites where further hydrogenation by spillover hydrogen migrating from Mo2C sites and isomerization and rupture of saturated naphthenic rings occurs, leading to the desired ROP. According to the product distribution of the supported Mo2C catalyst, the bimolecular path (responsible for coking and deactivation of zeolite) over the acidic support still occurs and competes with the monomolecular path, although it is

Fig. 10. Proposed reaction schemes for the main products from naphthalene on Mo2C/HY catalysts. A: acid sites; M: metal sites; ISO: isomerization; DS: desorption; RO: ring opening.

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significantly inhibited by the activated hydrogen generated over the Mo2C sites of the Mo2C/HY catalyst. Due to the carbon deposition caused by the bimolecular path and consequent deactivation of the HY support, further conversion of tetralin (readily produced on Mo2C sites) to ROP is hindered. Meanwhile, the bimolecular path itself is inhibited over deactivated acid sites leading to a decrease in polymerization species with time-on-stream (Fig. 7). 5. Conclusions Bifunctional active centers for SRO of naphthalene were formed by dispersing Mo2C onto the external surface of an HY zeolite. The synergy between the Mo2C and the HY could not be readily obtained from a mechanical mixture of bulk Mo2C and HY. The number of active centers increased with increased Mo2C loading up to 20 wt% Mo2C/HY. Mo2C significantly reduced not only the amount but also the strength of Bro¨nsted acid sites, which decreased the carbon deposition and consequent catalyst deactivation. Carbon deposition was the main reason for catalyst deactivation. Minimal carbon deposition occurred over the Mo2C, whereas carbon deposition via a bimolecular mechanism that occurs on the surface of the acidic support leads to catalyst deactivation. Hydrogenation of naphthalene to tetralin over Mo2C readily occurs whereas further hydrogenation, isomerization and ring-opening on acid sites via a monomolecular mechanism are critical for obtaining high SRO yields. Acknowledgements Funding for the present study from Natural Resources Canada through the Climate Change Technology and Innovation Initiative is gratefully acknowledged. The authors also acknowledge the support of Dr. Zbigniew Ring, CANMET Energy Technology Center, Devon, Alberta. References [1] [2] [3] [4]

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