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Catalytic hydrodenitrogenation of asphaltene model compounds Judah M. J.-B. Mierau, Nancy Zhang, Xiaoli Tan, Alexander Scherer, Jeffrey Mark Stryker, Rik R. Tykwinski, and Murray R Gray Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01853 • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on September 27, 2015

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Catalytic hydrodenitrogenation of asphaltene model compounds Judah M. J.-B. Mierau1, Nancy Zhang2, Xiaoli Tan1, Alexander Scherer3, Jeffrey M. Stryker4, Rik R. Tykwinski3, and Murray R. Gray1, ∗ † 1. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada 2. Integrated Nanosystems Research Facility, University of Alberta, Edmonton, Alberta, T6G 2V4, Canada 3. Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Henkestraße 42, 91054 Erlangen, Germany 4. Department of Chemistry, University of Alberta, Edmonton AB T6G 1X3, Canada

*∗Author for correspondence: Email: [email protected] †

Current address: The Petroleum Institute, PO Box 2533, Abu Dhabi, UAE

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Abstract The catalytic hydrodenitrogenation of heavy petroleum fractions is important for the production of high-quality fuels, because the nitrogen-bearing compounds poison acidic catalysts and inhibit sulfur removal. Two families of synthetic nitrogen-containing model compounds representative of asphaltene molecular structures were catalytically hydrogenated over a commercial NiMo/γAl2O3 catalyst under industrial hydrotreating conditions, i.e., 370 °C and 18 MPa of hydrogen for 1 h, in a stainless steel batch reactor. The bridged compounds with pyridine as a centre ring gave cracking, hydrogenation, and hydrodenitrogenation products with selectivities that depended on the position of substituents on the central pyridine ring. In contrast, a series of fused cholestane-benzoquinoline compounds gave only hydrogenation of all-carbon aromatic rings.


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Introduction The removal of sulfur and nitrogen are important steps in the conversion of vacuum residues of petroleum and bitumen into high-quality transportation fuels. Removal of sulfur compounds, mainly by catalytic hydrotreating,1 has received the most attention due to the environmental impact of sulfur oxides. Removal of nitrogen compounds is important mainly because they poison acidic cracking catalysts,2 and inhibit the removal of sulfur.3-5 Most of the prior work on catalytic hydrodenitrogenation (HDN) of petroleum fractions has focused on the light gas oil fractions, because of their incorporation into diesel fuels and because representative test compounds are available. In the vacuum residue fraction, the nitrogen compounds are mainly alkylated and bridged benzologs of pyridine and pyrrole, with a smaller concentration of metal porphyrins.6, 7 When these fractions are processed with high-pressure hydrogen over molybdenum-based catalysts in commercial processes such as LC-Fining and H-Oil, the nitrogen concentration in the remaining residue is higher in the feed due to an accumulation of unreactive material.6, 8 A better understanding of the reactivity of high-boiling nitrogen compounds is required to improve processing technology for heavy oils and bitumens. Detailed study of HDN has mostly concentrated on compounds in the boiling range of middle distillates and heavy gas oils, including quinoline, acridine, indole, and carbazole.3 These studies show that the low rate of hydrodenitrogenation, relative to hydrodesulfurization, is partly due to the requirement for complete hydrogenation of the aromatic ring(s) proximate to the nitrogen atom to achieve its removal.3, 5 The effect of substitution of long side chains, saturated rings, and more complex pendant groups has not been systematically investigated, largely due to a lack of available compounds. The work on actual petroleum fractions shows that reactivity drops


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exponentially with increasing boiling point, requiring more severe conditions with vacuum residue and vacuum oils.1, 9 Chaudhary et al.10 suggested that tri-aromatic compounds inhibit HDN in heavy gas oils, but Kanda et al.4 found that self-inhibition by the nitrogen compounds themselves was dominant. In catalytic hydroconversion of residue fractions, the suppression of HDN is so severe that nitrogen accumulates in the residue fraction at levels above the feed concentration.6 The mechanism of nitrogen removal during HDN on commercial catalysts such as NiMo/γAl2O3 begins with the hydrogenation of the nitrogen-containing ring.11-13 This reaction can occur before or after hydrogenation of other aromatic rings in the structure of the hydrocarbon. Substitution of the nitrogen compound with alkyl groups makes the catalytic hydrogenation less selective toward the ring bearing the nitrogen atom.14 Only once the nitrogen-carbon double bonds are hydrogenated is the C-N bond weak enough to break, to give first an amine, followed by hydrogenolysis of the remaining C–N bond to eliminate NH3.12 Since hydrogenation of the nitrogen ring and any adjacent rings must occur before hydrogenolysis of the C-N bond, the equilibria of the ring hydrogenation, dehydrogenation, and double bond isomerization steps can affect rates of overall HDN.13 The largest compounds studied were unsubstituted 5,6- and 7,8benzoquinolines.15, 16 With more rings, a larger number of possible pathways for the initial hydrogenation were available and experiments at different hydrogen pressures gave different selectivities for hydrogenation of the nitrogen-bearing ring versus carbon-only rings. In general, however, the reactions followed the general scheme suggested by Prins17 that HDN of larger molecules should follow similar steps: extensive ring hydrogenation followed by C-N bond cleavage.


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The emphasis of studies on HDN of defined compounds has been on benzologs of pyridine and pyrrole without attached alkyl groups, saturated rings, or more complex pendant groups attached by alkyl bridges. In order to understand the behavior of the vacuum residue fraction, particularly the asphaltenes, the reactions of more complex representative molecules must be investigated. In this work, synthetic nitrogen-containing model compounds representative of asphaltene molecular structures were catalytically hydrogenated over a commercial NiMo/γAl2O3 catalyst under industrial hydrotreating conditions, i.e. 370 °C and 18 MPa of hydrogen for 1 h, in a stainless steel batch reactor. The objective of this study was to determine the reaction pathways for catalytic reaction of these compounds.

Experimental Section Materials Reactions of six model compounds divided into two group were examined; the pyrene/pyridine compounds consisted of two pyrene residues linked by ethandiyl (two-carbon alkyl) bridges to a centre pyridine ring, and the cholestane compounds feature a cholestane skeleton fused to a 5,6benzoquinoline structure with different pendant groups attached to the heteroaromatic ring (


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Table 1). Information on the synthesis and characterization of these compounds is provided in previous publications.18, 19 Pyrene, quinoline and phenanthrene were used in control experiments. The catalyst used in this study was commercial Shell S424 Ni-Mo/γ-Al2O3 hydrotreating catalyst, which typically contains 2-4 wt% Ni and 12-15wt% Mo, on a support with circa 0.4 mL/g pore volume, a surface area of 160 m2/g and a bulk density of 0.75 g/mL.20 All other chemicals and solvents were from Sigma Aldrich Co. and Fisher Scientific (Mississauga, ON).

Reaction Procedure Model compounds were dissolved in 1,2,3,4-tetrahydronaphthalene (tetralin) at room temperature by brief sonication, less than 5 min, to give an initial concentration of approximately 2.4 mg/mL. Generally, batches of feed sufficient for three experiments were prepared at one time, i.e. 1.5 mL of solution. The commercial catalyst pellets were ground by mortar and pestle into a powder and passed through a 710 µm sieve (Fisher Scientific No. 25). The larger particles were discarded. For pre-sulfidation, approximately 1 g of catalyst was loaded into a 15 mL batch tubular reactor, along with stainless steel mixing balls. Carbon disulphide was added (1.8 g), the reactor was purged with nitrogen, and pressurized with hydrogen to achieve a pressure of 18.5 MPa at the reaction temperature of 350 °C. The batch reactor was then immersed in a sand bath at 350 °C for 2 h. After the reaction was complete, the reactor was removed, cooled, and the catalyst was rinsed with toluene over a metal fabric mesh, then dried in a vacuum oven, under low vacuum at 80 °C for 30 min. The same treatment with CS2 was then repeated on the dried catalyst.


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Table 1. Structures of asphaltene model compounds Model Compound[a] 3,5-Bis(2-pyren-1-ylethyl)-pyridine (P-3,5pyridine-P) 2,6-Bis(2-pyren-1-ylethyl)-pyridine (P-2,6pyridine-P)

Molecular Structure

Molecular Weight (g/mol)

535.68 N

N

535.68

2,5-Bis(2-pyren-1-ylethyl)-3-methylpyridine (P-3-methyl2,5-pyridine-P)

549.25

Cholestane-phenyl

597.91


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cholestane-phenyln-butyl

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654.02

H H H

702.06

N

cholestanebibenzyl

[a]

P = pyrene Reactions of the model compounds were conducted in a microbatch reactor of local

construction, consisting of a 6.35 mm (1/4”) outer-diameter (OD) stainless steel tube, 5 cm (2”) in length, which was joined and capped with Swagelok fittings. The top of the reactor was attached to a 1.6 mm OD (1/16”) stainless steel tube, 8.26 cm (3¼”) in length, which was connected to a valve via a reducing union (Swagelok Severe-Service Union-Bonnet Needle Valve).21 The volume of the reactor was less than 1 mL. In a glove box, the reactor was loaded with presulfided catalyst (40 mg) and two stainless-steel balls for mixing, then the solution of reactant (0.40 g) was added by pipette along with 10 µL of CS2. The reactor was closed, removed from the drybox and pressure tested, then pressurized with hydrogen to 8,275 kPa of H2 at room temperature. The reactor was agitated in a sand bath at circa 60 Hz, at a temperature of 370 °C for 60 min with 17,850 kPa of pressure at reaction temperature (calculated). After the 8 ACS Paragon Plus Environment

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reactor had cooled, the product gases were purged into a fume hood. The reactor was then opened inside the glove box, the contents were rinsed out with toluene and the catalyst was recovered by filtration through a mesh. The product solution was filtered through a syringe with a fitted filter to remove fine particles of catalyst.

Sample Analysis Analysis of the reactant and product solutions was done by high-performance liquid chromatography (HPLC), matrix-assisted laser-desorption mass spectrometry (MALDI-MS) or gas chromatography (GC) depending on the experiment performed. Product samples from the catalytic hydrogenation of asphaltene model compounds were analyzed by HPLC to determine the conversion and by MALDI-MS to identify product species and determine ratios of products. GC was used to detect and quantify cracked products for the pyrene/pyridine family of model compounds. Control experiments performed with pyrene, quinoline and phenanthrene feeds were analyzed by GC for identification and quantification. HPLC analysis used an Agilent Technologies 1200 Series instrument with a Zorbax Eclipse PAH column of 4.6 x 150 mm with a C18 phase of 3.5 µm particles. The mobile phase consisted of 70–65% methanol and 30–35% methylene chloride.21 Benzo[a]pyrene was added as an internal standard. Analysis of control samples and cracked products was done with a Thermo Scientific – Trace GC Ultra gas chromatograph equipped with an AI3000 Auto-injector autosampler and a Trace DSQII – Mass Spectrometer. The flame ionization detector was used for quantification of samples. Product samples were diluted 1 : 40 by volume in carbon disulfide, and 2,4,5,6-tetrachloro-m-xylene was added as an internal standard. MALDI-MS analysis was done with an Applied Biosystems/MDS SCIEX 4800 Plus equipped with a MALDI TOF/TOF analyzer. A solution of 50 vol% acetonitrile, 0.1 vol% 9 ACS Paragon Plus Environment

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trifluoroacetic acid with the remainder water was then added to the matrix compound α-cyano-4hydroxycinnanic acid (HCCA) to achieve a saturated solution (at concentration of ca. 5 mg/mL). Product samples were mixed with this solution supernatant in a 1 : 5 ratio by volume, and then pipetted onto a 384 Opti-TOF 123 mm x 81 mm stainless steel plate. The MS Reflector Positive setting was used, with the laser intensity set to approximately 3300. Multiple spectra from various wells of the same sample were collected to ensure representative and repeatable results. Identified and verified product peaks were integrated, and isotopic peaks were identified.22

Results The normal conditions for the HDN experiments in this study were 370 °C for 60 min at 17.85 MPa of initial hydrogen pressure at reaction temperature. The micro-batch reactor was heated to within 2% of the final temperature within 2.5 min, and cooling to 100 °C was complete in 2.5 min. The activity of the catalyst and the ability of the analytical methods to detect reaction products were verified by comparison to the HDN of quinoline and hydrogenation of phenanthrene and pyrene (data not shown).21 The standard error for HPLC determination of the concentration of the reactants was 0.07 mg/mL. GC analysis gave an average error in repeat determinations of product concentrations of 3.5%. The average repeatability of determination of ratios of peak heights by MALDI-MS was 12%.

Steady State Activity in Sequential Batch Catalytic Reactions The small quantities of available reactants, which were prepared by custom synthesis, and the gradual loss of catalyst limited the number of successive experiments that could be run with each subsample of catalyst to 3-4. The catalyst was reused in each successive batch reaction in order to allow the catalyst to reach an initial stable activity, and to account for the possibility of strong


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adsorption of reactants and products. Given that the overall rate of HDN tends to be first order in nitrogen concentration,6 the following equation holds: = −

(1)

where is the ratio of mass of catalyst to volume of liquid feed in g/L. Then integrating and solving for conversion, X, the result is: 1 − = −

(2)

where k is the reaction rate constant with units L/(gcat·s). This apparent first-order rate constant for a given model compound will depend on the temperature, hydrogen pressure, and catalyst activity. The data of Figure 1 show a representative plot of rate data as a function of cumulative reaction time for one model compound. After an initial low reactivity internal in the first 60 min, the subsequent rate constants were equivalent within the experimental error.


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Reaction rate constant, k (L/(g cats))

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7.5x10-6 7.0x10-6 6.5x10-6 6.0x10-6 5.5x10-6 5.0x10-6 4.5x10-6 4.0x10-6 3.5x10-6 0

20

40

60

80

100 120 140 160 180 200

Cumulative reaction time, t (min)

Figure 1. Reaction rate constant of P-3-methyl-2,5-pyridine-P at 370 °C versus cumulative reaction time. Each rate constant was determined from a 60 min experiment. The dashed line shows the steady-state rate constant.

The reaction rate constants for conversion of the parent compounds are listed in Table 2. The compound P-2,6-pyridine-P gave erratic results, and did not give an initial steady state conversion. Therefore, the reaction rate constant for this compound cannot be determined accurately, but falls in the range of 3.19 – 6.52 x 10-6 L/(gcat·s). Three model compounds gave conversions of over 99%, therefore, for these compounds the data of Table 2 indicate a minimum rate constant. Rather than attempting experiments for much shorter reaction times or at lower temperatures, and faced with limited quantities of reactants, we focused instead on comparing the selectivity of the HDN reactions at fixed reaction conditions.


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The total conversion of each model compound was determined in all cases by HPLC. An overall material balance was not possible because analytical standards for all products were not available, and the sample sizes were too small for gravimetric determination. In general, the possible products from conversion of the model compounds would be cracking, oligomerization giving coke formation, hydrodenitrogenation, and hydrogenation yields. When coking occurs with model compounds of the pyrene or cholestane types, intermediate addition products are also readily detectable by MALDI-MS due to the addition of cracked fragments to the parent compound.19, 23, 24 A control experiment on P-3,5-pyridine-P without the addition of catalyst showed clear evidence for products of mass 675-750 Da in the MALDI-MS spectrum due to addition reactions (data not shown)21. No addition products of mass greater than hydrogenated parent compound were detected with any of the model compounds in experiments with catalyst, therefore, the yields of coke were insignificant in these catalytic hydrotreating experiments. Analysis of the products was used to apportion the conversion by each reaction pathway. Cracked products from the pyrene compounds were determined directly by GC – MS and GC – FID. The remaining moles of converted feed were by hydrogenation and HDN by mass balance: [moles hydrogenated + HDN] = [moles feed converted]-[moles cracked]

(3)

The MALDI – MS spectra were examined to determine all product peaks, then each peak was assigned either to hydrogenation or HDN to calculate the fraction of each type among the detected products: =

∑$ #%& ! "# ∑) (%&' ! "(

*+ = 1 −

(4)

(5)


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As indicated in Table 2, the cholestane family of model compounds exhibited the highest activity, almost double that of the pyrene/pyridine family of model compounds. This result was unexpected, given the number of side groups and saturated rings in the cholestane compounds.


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Table 2. Conversions, yields and apparent reaction rate constant for asphaltene model compounds at 370 °C, 60 min with 17.85 MPa H2

Model Compound

Conversion by all pathways (%)

Overall apparent reaction rate constant, k (L/(gcat·s))

Yield of cracked products (wt %)

Yield of hydrogenation products (wt %)

Yield of HDN products (wt %)

Pyrene/pyridine family P-2,6-pyridine-P

69 ± 2

3.2 – 6.52 x 10-6

17

38

13

P-3-methyl-2,5-pyridine-P

84 ± 1

7.1 ± 0.1 x 10-6

37

2

45

23

10

65

ND

ND

>99

P-3,5-pyridine-P

>99

>9.5 x 10-6

Cholestane family Cholestane-phenyl

98 ± 1

15.7 ± 0.1 x 10-6

Cholestane-phenyl-n-butyl

>99

>15.7 x 10-6

ND

ND

>99

Cholestane-bibenzyl

>99

>15.7 x 10-6

ND

ND

>99

ND = not detected


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Yield of Cracked Products The cracked fragments from the pyrene/pyridine family were determined by GC – MS in combination with GC – FID. The two major cracked products identified by GC – MS for all pyrene/pyridine model compounds were pyrene and 1methylpyrene. Secondary products from cracking were partly hydrogenated pyrene and 1-methylpyrene. The yield of cracked products differs for each compound in this family, ranging from approximately 20–40 %. Cracking of both ethandiyl bridges in the pyrene/pyridine compounds is possible, but no peaks were detected for pyridine or alkyl-substituted pyridine compounds, such as dimethyl-pyridine. Consequently, breakage of both bridges was not significant under these reaction conditions. The data of Table 2 indicate that no cracking of the cholestane compounds was observed. Comparison of the MALDI–MS spectra of the feed and product samples did not indicate any product peaks consistent with the m/z for products with mass losses from cracking of side chains, or loss of cycloalkyl rings or pendant groups.

Yield of Hydrodenitrogenation and Hydrogenation Products The hydrodenitrogenation and hydrogenation yields shown in Table 2 were determined by combining the results from equations (3) through (5): [moles hydrogenated] = fHydrogenated [moles hydrogenated + HDN] (6) [moles HDN] = fHDN [moles hydrogenated + HDN]

(7)


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To assign products in the MALDI-MS spectra, the spectra of the product mixture was compared to the spectrum of the feed, and then analyzing the potential product peaks by MALDI-MS/MS to confirm consistency with hydrogenation and/or HDN (See Supporting Information for detailed method and list of m/z assignments). As an example of this analysis, the MALDI–MS/MS spectrum of the hydrogenation product of P-3-methyl-2,5-pyridine-P at m/z = 554.28 is shown in Figure 2. The unreacted parent compound gave a peak at M+1, or m/z = 550.28 due to the donation of a proton from the matrix compound, therefore, the m/z = 554.28 of this ion suggests addition of two hydrogen molecules by catalytic hydrogenation (M+5). The 120.07 m/z fragment ion is consistent with the centre nitrogen bearing ring of methylpyridine, after the breaking of the carbon-carbon bonds of the two bridging chains, evidence that the heteroaromatic ring does not preferentially hydrogenate. Pyrene fragments are consistent with m/z at 203.06 Da (not hydrogenated), 205.07 Da (hydrogenated once, +H2), and 207.1 Da (hydrogenated twice, +2H2) m/z (+2H2). Also observed in this range are isotope peaks, due to 13C and 2H. Cleavage of the ethandiyl bridge between the two alky carbons would give methylpyrene fragments of 215 < m/z < 220 depending on protonation in the MS. Removal of a pyrene or hydrogenated pyrene would give a remaining fragment peaks in the range 349 < m/z < 354, depending on protonation, while removal of a methylpyrene fragment would account for fragments in the range 335 < m/z < 340. Rearrangement of the ions by cyclization could further broaden the range of possible fragment m/z. These MS data indicate 17 ACS Paragon Plus Environment

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that the product peak at m/z = 554.28 was a mixture of hydrogenated products, one compound with addition of one H2 to each of the pyrene rings, and one compound with addition of 2 H2 to one of the pyrenes.

Figure 2. MALDI-MS/MS spectrum of P-3-methyl-2,5-pyridine-P hydrogenation product peak m/z = 554.28

Once the product peaks were identified and assigned, the MALDI-MS spectra were used to calculate the fractions of hydrogenated and HDN products (equations 4 and 5). The isotope peaks for the feed compound and the products were determined, then the total integrated peak height for each component of the mixture was determined from the sum of the isotope peaks, less any contributions from the spectrum of the parent compound. Based on the assumption that the


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structurally similar compounds have comparable ionization efficiencies, these peak areas were used to calculate the fractions of the hydrogenated and HDN products. The modified integrated peak area spectra for the pyrene/pyridine family of model compounds P-3,5-pyridine-P, P-3-methyl-2,5-pyridine-P and P-2,6-pyridine-P are shown in Figure 3, Figure 4 and Figure 5, respectively.

5000 HDN Parent Compound Hydrogenation

4000

Integrated Peak Area

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3000

2000

1000

0 520

525

530

535

540

545

550

Mass/charge ratio (m/z)

Figure 3. Modified integrated peak area spectrum of P-3,5-pyridine-P


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2500

2000


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HDN Parent Compound Hydrogenation

1500

1000

500

0 535

540

545

550

555

560

565


Figure 4. Modified integrated peak area spectrum of P-3-methyl-2,5pyridine-P


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1800 HDN Parent Compound Hydrogenation

1600


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1400 1200 1000 800 600 400 200 0 520

525

530

535

540

545

550


Figure 5. Modified integrated peak area spectrum of P-2,6-pyridine-P

The pyrene/pyridine family of model compounds reacted along both HDN and hydrogenation reaction pathways to different extents depending on the model compound involved. This result is derived by comparing the modified integrated peak area spectra of P-3,5-pyridine-P (Figure 3) and P-2,6-pyridine-P (Figure 4). A simple change in the bridging locations on the centre heteroatomic ring produced a significant difference in the amount of HDN observed. The model compounds P-3-methyl-2,5-pyridine-P and P-3,5-pyridine-P exhibited similar behaviour despite having greater differences in chemical structure than that observed between P-3,5-pyridine-P and P-2,6-pyridine-P. Both substances yielded predominantly hydrogenation products with only one HDN product, 7 Da lighter 21 ACS Paragon Plus Environment

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than the parent compound, corresponding to hydrogenation of the heteroatomic ring and removal of the nitrogen from the now saturated ring by ammonia elimination. The model compound P-2,6-pyridine-P produced the most HDN of any compound investigated and had an HDN yield significantly greater than the other two pyrene/pyridine compounds. The other product peaks attributed to HDN in Figure 5, those lower molecular weight peaks not 7 Da smaller than the parent compound, represent HDN products that have been further hydrogenated or dehydrogenated and all were confirmed via MALDI–MS/MS. No more than four hydrogen addition reaction steps (+ 4H2) to the parent compound was detected.

The data of Figure 6 show the modified integrated peak area spectrum of cholestane-phenyl. As in the pyridine/pyrene family, the height of each bar represents the relative concentration of a product, with the parent peak included for comparison. None of the cholestane family gave evidence for HDN, therefore, selectivity for this reaction was zero. Product compounds showed as many as five additions of H2 to the parent compound, without nitrogen removal. Past this range, either the product peaks in the MALDI spectra became too small to be not verifiable by MALDI–MS/MS. The other two compounds, cholestane-phenyl-nbutyl (Figure 7) and cholestane-bibenzyl (Figure 8) give equal evidence for multiple hydrogenation steps without HDN. The significant difference in the series arises in cholestane-phenyl-n-butyl, which also gave peaks consistent with dehydrogenation of the parent compound. 22 ACS Paragon Plus Environment

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18000 Parent Hydrogenation

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Figure 6. Modified integrated peak area spectra of cholestane-phenyl


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3000

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Figure 7. Modified integrated peak area spectrum of cholestane-phenyl-nbutyl


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4000

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Figure 8. Modified integrated peak area spectra of cholestane-bibenzyl

Hydrogenation of the nitrogen-bearing ring in the model compounds would generate alkylamine compounds as intermediates. Such cyclic amine products were of particular concern for the cholestane compounds which did not give detectable HDN. None of the MALDI–MS/MS spectra of the detected product peaks clearly indicated formation of an alkylamine. Fourier transform infrared spectroscopy (FT – IR) was used to check the spectra of the product mixtures, but no signal was detected for N-H stretch at ~3400-3200 cm-1 (data not shown). From these results we conclude that no significant amounts of amine accumulated from the hydrogenation of the cholestane compounds, and that the hydrogenation


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of these compounds was in the other aromatic rings, not in the heteroaromatic ring.

Discussion Pyrene/Pyridine Family of Model Compounds Based on the MALDI-MS/MS data and the yields in Table 2, the schematic of Figure 9 shows the main reaction products from P-2,6-pyridine-P, as a representative member of this family. The other two compounds gave the same reaction pathways, albeit to different extents. Model compound P-3-methyl-2,5pyrdine-P yielded nearly double the amount of cracked products as the other two model compounds, which could be partly the result of steric hindrance in adsorbing onto the catalyst surface, which would suppress hydrogenation and make cracking more significant as a fraction of the total yield. The major cracked products were the same for all three pyrene/pyridine model compounds, including pyrene, methylpyrene and smaller amounts of dihydropyrene. Only methyl pyrene is shown in Figure 9 as a representative product. The cracked products identified in this study follow expected cracking behaviour from prior studies of alkylpyenes, via breakage of the two carbon-carbon bonds in the ethyl bridge to give pyrene and 1-methylpyrene.19, 25, 26 The schematic of Figure 9 shows the hydrogenation having occurred at the pyrene group, specifically at the 9,10position. This representation is supported by three results: •

MALDI – MS/MS spectra of hydrogenation products showing fragment ions consistent with partially saturated pyrene groups (see


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•

Figure 2).

•

Detection of fragment ions in MALDI – MS/MS spectra of hydrogenation products for the intact pyridinic centre ring, and no detection of partially hydrogenated intermediate species.

•

Detection of dihydropyrene and other cracked hydrogenated pyrene products from all three model compounds via GC – MS (data not shown).

The hydrogen addition is represented at the 9,10-position as the most likely location on the pyrene group.


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N

+

Cracking 17% HDN N

55%

Hydrogenation 13%

H

N

H H

H

Figure 9. Schematic of the main products from each of the detected pathways of the catalytic reaction of pyrene-2,6-pyridine-pyrene (P-2,6-pyridine-P), with the corresponding selectivity on a mass basis

The only model compound in the pyrene/pyridine family to exhibit significant HDN was P-2,6-pyridine-P. The relative lack of HDN products for P-3-methyl2,5-pyridine-P can be attributed to steric interference from the alkyl substitution of the heteroaromatic ring, as noted above, which would inhibit adsorption and reaction in the centre ring. The data of Table 3 suggest that the basicity of the


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centre ring might play a role, based on the basicities of pyridine analogs. Model compound P-2,6-pyridine-P would be more basic (pKb of 7.33 for 2,6dimethylpyridine) than P-3,5-pyridine-P (pKb of 8.19 for 3,5-dimethylpyridine). Comparison to data from Cox and Berg,27 who used Raney nickel to hydrogenate substituted pyridines, showed that 2,6-dimethylpyridine had an overall reaction rate constant more than six times larger than 3,5-dimethyl-pyridine under the same conditions and feed concentration, consistent with the results in Table 2 and Table 3 for HDN. Alshareef, et al.,19 found that the 3,5-isomer gave more thermal cracking than the 2,6-isomer, consistent with these data. All the HDN products identified for all three model compounds of the pyrene/pyridine family were consistent with the HDN mechanism supported in literature, i.e., initial saturation of the nitrogen-containing ring followed by hydrogenolysis of the C–N bond(s) and eventual elimination of ammonia. The main HDN product would then be represented in a MALDI–MS spectrum by a product peak 7 Da less than the parent compound (-14 for N, +7 hydrogens from hydrogenation/protonation). This result is confirmed by both the appropriate modified integrated MALDI–MS spectra for each model compound and by MALDI–MS/MS spectra suggesting only structures consistent with the illustrated HDN products.


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Table 3. Comparison of overall reactivity by all pathways, the yield of HDN products (%) and core pyridine ring basicity for the pyrene/pyridine family Conversion

Yield of

Model

by all

HDN

Model pyridine

Compound

pathways

Products

ring

(%)

(%)

P-2,6pyridine-P

69 ± 2

pKb

7.33

38

2,6-dimethylpyridine

P-3-methyl2,5-pyridine-

84 ± 1

2

7.47

P 2,3,5-trimethylpyridine

P-3,5pyridine-P

>99

10

8.19 3,5-dimethylpyridine


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A comparison of Figure 3 through Figure 5 shows that the selectivity for the hydrogenation of the model compounds P-3,5-pyridine-P and P-3-methyl-2,5pyridine-P were similar, whereas for P-2,6-pyridine-P (Figure 4), hydrogenation without HDN was a minor pathway for reaction. The MALDI-MS/MS data suggest that these hydrogenation reactions occurred at least partly on the pyrene rings. Up to four molecular hydrogen addition steps were detected for all three pyrene/pyridine model compounds and both P-2,6-pyridine-P and P-3-methyl-2,5pyridine-P showed no preference in abundance for these four hydrogenation products. In contrast, from Figure 3, it is evident that the most abundant hydrogenation product for P-3-methyl-2,5-pyridine-P was from a single H2 addition, with diminishing abundance over increasing steps of hydrogenation. The schematic of the main products detected from the catalytic hydrogenation of the cholestane family of model compounds is illustrated in Figure 10, for the example case of cholestane-phenyl. No HDN or cracked products were detected for the cholestane family of model compounds. From Figure 3 to Figure 5, it is evident that the most abundant product generated for all three model compounds was a singly hydrogenated compound, with significantly smaller amounts of further hydrogen addition products. The most likely H2 addition location would be to the 9,10-positions on the 5,6-benzoquinoline structures, retaining the greatest aromatic stabilization, as illustrated in Figure 10. No amines were detected by FTIR, therefore, hydrogenation of the nitrogen-bearing rings was not significant and the subsequent hydrogenation positions are shown generically in the remaining carbon aromatic ring of the 5,6-benzoquinoline structure.


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Figure 10. Schematic of the main products from the reaction pathway detected from of the catalytic reaction of cholestane-phenyl. Addition of H2 to (I), 2H2 to (II) and 3H2 to (III).

No nitrogen removal by HDN was found to take place for the cholestane family compounds, based on the following evidence. First, no peaks consistent with HDN products were identified in the MALDI–MS spectra for these compounds. Second, no evidence for cyclic amines was observed from FT–IR spectra, as discussed above. The published reaction networks for 5,6-benzoquinoline from Shabtai, et al.,15 and Moreau, et al.,16 show that hydrogenation of the heterocyclic 32 ACS Paragon Plus Environment

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ring was prerequisite for HDN. Moreover, the product distribution provided by Shabtai, et al.,15 for the HDN of 5,6-benzoquinoline showed appreciable amounts of 1,2,3,4-tetrahydrobenzoquinoline in the product mixture, a logical precursor to nitrogen removal. The representations of Figure 10 are consistent with the work of Jokuty and Gray,14 who suggested that polycyclic substituted nitrogen compounds can undergo significant hydrogenation away from the N-bearing ring. In contrast to the simple benzoquinoline, ring fusion to the cholestane fragment clearly inhibits hydrogenation of the nitrogen-containing ring, suppressing HDN. The reactivity of the cholestane compounds, considering the conversion from all pathways in Table 2, are comparable to one another despite significant differences in the functional group attached to the heteroatomic ring. The pseudo-first order reaction rate constant could only be determined for the model compound cholestane-phenyl, because the product concentrations of other two cholestane model compounds were too low to provide a reliable measurement by calibration curve. Two compounds gave MALDI-MS peaks suggestive of dehydrogenation reactions: P-2,6-pyridine-P (Figure 5) and cholestane-phenyl-n-butyl (Figure 7). The main HDN product from P-2,6-pyridine-P gives an m/z circa 528, which was observed as the largest signal. Higher mass would correspond to further hydrogenation of the pyrenes, but peaks of lower masses, circa m/z=526 and m/z=524, suggest competitive dehydrogenation. Similarly, the signals at m/z 2 and 4 mass units lower than the parent compound cholestane-phenyl-n-butyl (Figure 7) suggest loss of molecular hydrogen. Dehydrogenation reactions of 33 ACS Paragon Plus Environment

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these compounds are unlikely under the experimental conditions, since dehydrogenation of the saturated rings of similar cholestane compounds was not observed even under thermal cracking conditions.23 Although the MALDI-MS suggests peaks from dehydrogenation, this result is not a robust conclusion and awaits verification. Comparing the conversions by all pathways between the two model compound families and the reaction rate constants of cholestane-phenyl and P-3-methyl-2,5pyridine-P, it is clear that the cholestane family is inherently more reactive than the pyrene/pyridine family. Prins17 suggests that larger nitrogen compounds tend to react more readily than smaller nitrogen compounds because of the larger adsorption constants and weaker aromaticity of the larger nitrogen compounds. Therefore, if adsorption behaviour of the model compounds is dominated by the basic nitrogen centre of the heteroatomic ring, then a qualitative comparison of reactivity can be drawn from already-published data on the reactivity of 5,6benzoquinoline and alkyl-substituted pyridines. Kinetic data from Cox and Berg27 show that acridine is more reactive than many alkyl-substituted pyridines. For example, the first order reaction rate constants for 3-ethyl-4-methylpyridine, 2,4dimethylpyridine and 3,5-dimethylpyridine are 0.114 h-1, 0.272 h-1and 0.271 h-1, respectively; compared to 0.399 h-1 for acridine. Furthermore, results from Katti and Gates28 demonstrated that 5,6-benzoquinoline was more reactive than acridine at 400 °C and 13.3 MPa. Therefore, the higher reactivity of the cholestane family as compared to the pyrene/pyridine family is consistent with prior work on the kinetics of poly-nuclear aromatic pyridine benzologs.


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The other factor to potentially affect catalytic conversion is the diffusion of the reactants within the catalyst matrix. A ratio of molecular size to pore size of 0.1 or more gives restricted diffusion, with a ratio of 0.6 giving a 10-fold reduction in diffusivity.29 The majority of the pore volume in commercial hydrotreating catalysts is in pore diameters the range 6-15 nm.30 The cholestane family of compounds are relatively rigid, with little difference in size due to different conformations, with a length of approximately 2.1 nm and width of 1 nm from xray crystallographic data.18 The pyrene/pyridine family of compounds have similar maximum dimensions, but the flexible ethandiyl bridges enable a variety of smaller partly-folded conformations. Both types of compounds would be expected to give hindered diffusion in the smaller pores of the alumina support, but the effect would be larger with the more rigid cholestane family, which does not correlate to their higher reactivity for hydrogenation (Table 2). All of the model compounds are comparable in size to the thickness of many of the Ni-Mo sulfide crystallites observed in hydrotreating catalysts, in the range 1-4 nm.30 Consequently, we would expect all of these large molecules to occupy large areas of the metal sulfide crystallites once they are adsorbed, but this characteristic does not help to explain the strong differences in reactivity.

Implications This study is the first to investigate the behaviour of large multi-ring aromatic nitrogen compounds undergoing catalytic hydrogenation/hydrogenolysis under conditions representative of industrial hydrotreating. The data presented in Table 2 and the pathway schematics illustrated in Figure 9 and Figure 10 provide strong


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evidence that these molecules crack and hydrogenate easily, but do not undergo HDN readily, even under conditions generally favourable for nitrogen removal. Moreover, this behaviour contrasts the reaction networks and other data reported for smaller polycyclic aromatic nitrogen compounds such as benzoquinoline, acridine, and quinoline. The asphaltene model compounds do not readily undergo hydrogenation at the N-bearing rings to form saturated amine intermediates, which is opposite that observed for smaller nitrogen compounds, as delineated in reaction networks for 5,6-benzoquinoline reported by Nagai, et al.,15 and Moreau, et al.,16 and quinoline by Satterfield and Yang31. As suggested by Jokuty and Gray,14 larger nitrogen compounds such as the asphaltene model compounds studied in this work do not undergo nitrogen removal without extensive hydrogenation and cracking of the rest of the compound, likely in a multi-step process. The implication of these results is that hydrotreament of these large polycyclic aromatic nitrogen-containing asphaltene compounds will be dominated by hydrogenation and cracking reactions, not by direct HDN of the nitrogen ring, and that investigation into the behaviour of the small nitrogen analogues give only limited insight to the varied and complex reactivity observed for the larger asphaltene model compounds.

Conclusions The pathways for the catalytic hydrogenation of the pyrene/pyridine model compounds include cracking, hydrogenation and hydrodenitrogenation, depending on the model compound observed. The pyrene/pyridine family of asphaltene


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model compounds exhibit a strong preference for the hydrogenation of the bridged pyrene groups in contradistinction to the hydrogenation of the centre pyridinic ring. Alkyl substitution and bridging locations on the heteroatomic ring were significant factors in affecting reactivity and HDN selectivity. There were no observed cracking or HDN reactions for the cholestane family of asphaltene model compounds.

Acknowledgements The authors are grateful for the support of the Natural Sciences and Engineering Research Council of Canada and the Institute for Oil Sands Innovation at the University of Alberta, and to the Integrated Nanosystems Research Facility, University of Alberta, for use of analytical equipment.

References 1. Topsøe, H.; Clausen, B. S.; Massoth, F. E., Hydrotreating Catalysis. Springer: Berlin, 1996. 2. Mills, G. A.; Boedeker, E. A.; Oblad, A. G. J. Am. Chem. Soc. 1950, 72, 1554-1560. 3. Furimsky, E. Catal. Rev. Sci. Eng. 2005, 47, (3), 297-489. 4. Kanda, W.; Siu, I.; Adjaye, J.; Nelson, A. E.; Gray, M. R. Energy Fuels 2003, 18, 539-546. 5. Kabe, T.; Ishihara, A.; Qian, W., Hydrodesulfurization and Hydrodenitrogenation: Chemistry and Engineering. Kodansha: Tokyo, 1999. 6. Gray, M. R., Upgrading Oilsands Bitumen and Heavy Oil. University of Alberta Press: Edmonton, AB, 2015. 7. Strausz, O. P.; Lown, E. M., The Chemistry of Alberta Oil Sands, Bitumens, and Heavy Oils. Alberta Energy Research Institute: Calgary, AB., 2003. 8. Zhao, S.; Kotlyar, L. S.; Woods, J. R.; Sparks, B. D.; Chung, K. H. Energy Fuels 2001, 15, 113-119. 9. Trytten, L. C.; Gray, M. R.; Sanford, E. C. Ind. Eng. Chem. Res. 1990, 29, 725-730. 10. Choudhary, T. V.; Parrott, S.; Johnson, B. Catal. Comm. 2008, 9, 1853– 1857. 11. Furimsky, E.; Massoth, F. E. Catalysis Reviews 2005, 47, 297-489. 37 ACS Paragon Plus Environment

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12. Prins, R.; Jian, M.; Flechsenhar, M. Polyhedron 1997, 16, (18), 32353246. 13. Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021-2058. 14. Jokuty, P. L.; Gray, M. R. Ind. Eng. Chem. Res. 1992, 31, 1445-1449. 15. Shabtai, J.; Yeh, G. J. C.; Russell, C.; Oblad, A. G. Ind. Eng. Chem. Res., 1989, 28, (139-146). 16. Moreau, C.; Durand, R.; Zmimita, N.; Geneste, P. J. Catal. 1988, 112, 411-417. 17. Prins, R. Adv. Catal. 2001, 46, 399-464. 18. Scherer, A.; Hampel, F.; Gray, M. R.; Stryker, J. M.; Tykwinski, R. R. J. Phys. Org. Chem. 2012, 25, 597-606. 19. Alshareef, A. H.; Scherer, A.; Tan, X.; Azyat, K.; Stryker, J. M.; Tykwinski, R. R.; Gray, M. R. Energy Fuels 2012, 26, 1828-1843. 20. Ying, Z.-S.; Gevert, B.; Otterstedt, J.-E. Ind. Eng. Chem. Res. 1995, 34, (5), 1566-1571. 21. Mierau, J. M. B.-J. Patterns and Pathways of Hydrogenation of Asphaltene Model Compounds. University of Alberta, 2011. 22. Manura, J. J.; Manura, D. J. Supplies and Services for Mass Spectrometers, Gas Chromatographs & Liquid Chromatographs. http://www.sisweb.com/mstools/isotope.htm 23. Alshareef, A. H.; Scherer, A.; Stryker, J. M.; Tykwinski, R.; Gray, M. R. Energy Fuels 2012, 26, 3592-3603. 24. Alshareef, A. H.; Scherer, A.; Tan, X.; Azyat, K.; Stryker, J. M.; Tykwinski, R.; Gray, M. R. Energy Fuels 2011, 25, 2130-2136. 25. Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989, 28, 645-653. 26. Freund, H.; Matturro, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuels 1991, 5, 840-846. 27. Cox, K. E.; Berg, L. Chem. Eng. Progr. 1962, 15, (12), 54-55. 28. Katti, S. S.; Gates, B. C. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 618-626. 29. Lee, S. Y.; Seader, J. D.; Tsai, C. H.; Massoth, F. E. Ind. Eng. Chem. Res., 1991, 30, 29-38. 30. de Jong, K. P.; van den Oetelaar, L. C. A.; Vogt, E. T. C.; Eijsbouts, S.; Koster, A. J.; Friedrich, H.; de Jongh, P. E. J. Phys. Chem. B 2006, 110, (21), 10209-10212. 31. Satterfield, C. N.; Yang, S. H. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 11-19.


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