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Understanding the mechanisms of decalin hydroprocessing using comprehensive two-dimensional chromatography Elodie Blanco, Luca Di Felice, Nelly Catherin, Laurent Piccolo, Dorothee Laurenti, Chantal Lorentz, Christophe Geantet, and Vincenzo Calemma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03472 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016
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Understanding the mechanisms of decalin hydroprocessing using comprehensive two-dimensional chromatography Elodie Blanco,† Luca Di Felice,†,• Nelly Catherin,† Laurent Piccolo,†,* Dorothée Laurenti,† Chantal Lorentz,† Christophe Geantet,† and Vincenzo Calemma‡ †
Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR5256 CNRS-Université Claude Bernard Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France. ‡ Eni S.p.A., R&M Division, Via F. Maritano 26, 20097 San Donato Milanese, Italy. ABSTRACT: Comprehensive two-dimensional gas chromatography (GC×GC) is a powerful technique for analyzing mixtures of hundreds of hydrocarbons. In the context of fuel upgrading through selective ring opening, we propose a methodology for GC×GC analysis of complex mixtures resulting from the hydroprocessing of a single model gas oil compound, decalin, over two different types of bifunctional catalysts based on a transition metal sulfide (NiWS on amorphous silica-alumina) or a noble metal (Ir on La,Na-Y zeolite). The reactions lead to several products families, the dominant ones being ring-opening products (ROPs) and skeletal-isomerization products (SkIPs). Firstly, it is shown that the ROP distribution can be characterized in terms of isomerization degree by using the cumulative distribution function of the GC×GC (GC Image) software. Secondly, in a more quantitative approach, the products families have been subdivided into chemical groups according to the isomerization degrees of the individual compounds, which were almost all tentatively identified by GC×GC-MS through the use of literature data. This allows us to thoroughly analyze the influence of the catalyst nature and the presence of H2S in the reactant feed on the products distribution, and thereby gain insight into the mechanism of decalin hydroconversion over bifunctional catalysts. In particular, it is shown that metal sulfidation suppresses the metal-catalyzed C-C hydrogenolysis pathway at the benefit of undesirable acid-catalyzed isomerization steps. The methodologic work presented here for decalin is believed to be applicable to other bicyclic (naphthenic or aromatic) compounds. classify the products into several families.23 However, possible co-elution along with incomplete MS databases lead to identification uncertainties, making the use of comprehensive twodimensional gas chromatography (GC×GC) indispensable.14,27,28 In a recent work,12 we have investigated the SRO of decalin under high pressure of hydrogen and in the presence of H2S (a classical contaminant of petroleum cuts) over NiWS catalysts supported on amorphous silica-alumina (ASA). It was found that 1-ring-opening products (1ROP) and skeletalisomerization products (SkIP) are the most representative compound families, accounting for 75-80% of the whole products distribution. In this paper, we report on a comparative GC×GC investigation of the products formed on NiWS/ASA and a reference Ir/zeolite catalyst (with or without H2S in the reactant feed) with the aim of gaining insight into the catalyst and conditions-dependent decalin hydroconversion pathways. The choice of catalysts was undertaken to extend the peak attribution to a large amount of products which can possibly be obtained from the hydroconversion of decalin and, further, two-ring hydrocarbons over a broad range of catalysts exhibiting various types and strengths of metallic and acidic functions. In particular, using GC×GC coupled with MS or flame ionization detection (FID), we focus on the identification and quantification of the individual molecules and related chemical groups inside each family of compounds.
The foreseen shortage in fossil resources, together with the increasing demand in diesel fuel, lead the refiners to valorize non-conventional petroleum cuts, such as the Light Cycle Oil (LCO), which is produced from Fluid Catalytic Cracking. The LCO contains large amounts of polyaromatic hydrocarbons, and thus presents a low cetane number (CN), i.e. a poor combustion efficiency. The saturation of aromatic compounds into naphthenic ones through catalytic hydrotreating increases the CN, but selective ring opening (SRO) of naphthenes into paraffins is necessary to reach acceptable CN values.1,2 Research on SRO has long been carried out on model hydrocarbons consisting of single-ring naphthenic compounds, such as methylcyclopentane,3,4 cyclohexane5-7 and methylcyclohexane.8-11 The advantage of using these molecules is the small number of products, which facilitates the interpretation of reaction mechanisms. However, these compounds are poorly representative of the LCO composition. To account for this limitation, 2-ring probe molecules such as decalin and tetralin have been employed.8,12-26 However, even by starting from a single molecule (or two stereoisomers in the case of decalin), a complex mixture of ca. 200 hydrocarbons is formed upon reaction on bifunctional catalysts, due to the variety of reaction steps occurring on metal and/or acid sites. In this context, full separation and identification of the molecules are difficult tasks. One-dimensional gas chromatography (GC) using a 150-m capillary column coupled with mass spectrometry (MS) permitted to tentatively identify nearly 200 compounds and Page 1
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of 5 °C/min (0 min hold). 0.8 µl of pure sample was injected with a split ratio of 250. The carrier gas was high-purity helium flowed at a constant rate of 2 ml/min. Data treatment and matching of GC×GC-MS and GC×GC-FID were performed with GC Image 2.3 software (GC×GC Edition).31 Computer Language for Identifying Chemicals (CLIC) expressions were used for identification of compound families,32 and the NIST MS database (version 2.0) was used for the identification of 25 C10 molecules. The database was complemented with 148 mass spectra reported in the literature (see discussion below).33-45
EXPERIMENTAL Materials. A NiWS/F-ASA catalyst was prepared as detailed in a previous work.12 The ASA powder (Sasol SIRAL40) was modified by incipient wetness impregnation (IWI) of ammonium fluorine (NH4F), then dried overnight and calcined under air at 500 °C. The modified support was then impregnated (IWI) with Ni and W salts, and finally sulfided by exposure to a 10% H2S/H2 gas mixture at 400 °C. A 3 wt% Ir/LaNaY catalyst was prepared by IWI of a La-modified Naexchanged Y zeolite (Zeolyst CVB 760 PQ, SiO2:Al2O3 molar ratio 56) with an aqueous solution of H2IrCl6:HCl:CH3COOH, followed by drying at 80 °C and calcination at 360 °C.29 The La:Al and Na:Al molar ratios were 0.24 and 0.26, respectively, as determined by elemental analysis. The Ir dispersion (number of surface Ir atoms divided by total number of Ir atoms) was 69% as determined by H2 chemisorption, corresponding to ca. 1.5-nm-sized particles. Catalytic testing. The catalyst performances for decalin hydroconversion were evaluated in a flow fixed-bed reactor at 5.0 MPa of H2 , with a gas-phase decalin partial pressure of 16 kPa, a reaction temperature ranging from 270 to 380 °C and the possibility to feed H2S at relatively high concentration (0.8 vol% was employed in this work). The decalin cis-trans mixture (40-60%) was purchased from Sigma-Aldrich (reagent grade, 98%). The total weight-hourly space velocity (mass of decalin fed/mass of catalyst) was 0.8 h-1 for standard tests. The gas mixture was analyzed on-line with a HP 5890 Series II gas chromatograph equipped with a HP-1 column with a CP-Sil 5 CB stationary phase (25 m × 0.2 mm × 0.50 µm, N2 as carrier gas). The sampling loop volume was 250 µL. Technical details of the setup and the related GC analysis were provided elsewhere. 12 Decalin conversion was calculated as follows: Χ dec =
Dec (by − pass ) − Dec (out ) Dec (by − pass )
%
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RESULTS AND DISCUSSION Methodology for analyzing GC×GC chromatograms of decalin derivatives. In a previous work, five main product families were distinguished from the GC×GC-MS chromatograms of decalin hydroconversion products, as listed below and displayed on the chromatogram of Figure 1:12 1ROPs: 1-ring-opening products, i.e. C10 alkylmononaphthenes; AROPs: aromatic 1-ring-opening products, i.e. C10 alkylbenzenes; 2ROPs: 2-ring-opening products, i.e. C10 paraffins; SkIPs: skeletal isomerization products, i.e. C10 alkyldinaphthenes; DHPs: dehydrogenation products, i.e. all C10 unsaturated products except AROPs (tetralin, naphthalene, methyl-indans, etc.).
(1)
where Dec(by-pass) and Dec(out) are the sums of cis and trans decalin peak areas when decalin is by-passed off the reactor and at the reactor outlet, respectively. GC×GC analysis. A two-dimensional GC×GC Agilent 6890N chromatograph equipped with either an Agilent 5975B mass detector (single quadrupole, mass/charge range: 50-300 amu, up to 22 scans/s) or a flame ionization detector (FID) was used to analyze the condensed phase collected at the outlet of the reactor with a cryostat kept at 0 °C. This analytical system has been described in detail in reference.30 A 30 mlong Phenomenex ZB1MS column (ID 0.250 mm, film thickness 1 µm) was connected in series with a 3 m-long Varian VF17MS column (ID 0.100 mm, film thickness 0.2 µm) by means of a cryogenic two-stage thermal modulator (Zoex Corporation, modulation period 11.63 s, N2 as cryogenic fluid, 300 ms hot air jet). The temperature of the first GC column was programmed from 60 °C (1 min hold) to 150 °C (0 min hold) at a rate of 0.6 °C/min, and then to 300 °C at a rate of 5 °C/min (0 min hold). For the second column, the temperature was programmed from 60 °C (1 min hold) to 300 °C at a rate
Figure 1. Chemical families identified on GC×GC-MS chromatograms. Catalyst and specific conditions: NiWS/F-ASA, 0.8% H2S, 380 °C, Xdec = 80%. Retention time ranges: 50 - 120 min (1st dimension) and 1.8 - 11.3 s (2nd dimension).
Since, in the presence of H2S in the reactant feed and for any catalyst, SkIPs and 1ROPs represent together from 60 to 80% of the products, a methodology for identifying these products is desirable in order to appreciate the differences in the reactions mechanisms. Following the approach of Kubička et al.,46 a first tentative identification of chemical groups inside the 1ROP family has been carried out based on the dominant MS m/z fragment which results from the corresponding alkyl-group loss (the molecular weight of the starting C10 molecule is 140): methyl-1ROP (m/z = 125), ethyl-1ROP (m/z = 111), propyl1ROP (m/z = 97) and butyl-1ROP (m/z = 83). Figure 2 shows Page 2
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differences in the relative intensities of the peaks were observed. Figure 3 shows cumulative 1ROP distributions for decalin hydroconversion over two types of catalysts (Ir/LaNaY vs NiWS/F-ASA) under two different conditions (with or without H2S). As previously mentioned, the more branched is the product, the smaller should be the mesh number. Hence, in order to reach a high cetane number (linear products, wth low isomerization degree), the distribution should be shifted toward higher mesh numbers (i.e. to the right-hand side of the graph), as is the case for Ir/LaNaY in H2S-free conditions (black straight curve). On the other hand, the addition of H2S shifts the cumulative distribution to lower mesh numbers (black dashed curve), suggesting that more branched products are formed. In the case of NiWS/F-ASA (gray curve), a similar distribution of (branched) products as for Ir/LaNaY under H2S-free conditions is observed for mesh numbers between 0 and 35. However, the cumulative distribution joins that of Ir/LaNaY (in the presence of H2S) for higher mesh numbers. Overall, NiWS/F-ASA leads to somewhat more linear products than Ir/LaNaY in the presence of H2S (meshes 70-90).
1ROP peaks based on specific criteria, accounted for in GC Image software’s CLIC expressions, along with typical molecule structures of each group. Figure 2a shows that the methyl1ROP group (criterion: peak at m/z = 125 > 20% in intensity of the main peak) has the tendency to elute first, i.e. is located at the left-hand side of the 1ROP region. The ethyl-1ROP (m/z = 111 > 50%, Figure 2b) and propyl-1ROP (m/z = 97 > 40%, Figure 2c) appear more scattered, and consistently contain more compounds, than the methyl-1ROP group. Finally, Figure 2d shows the result of the search for m/z = 83 > 50%, and corresponds to the last eluted compounds, i.e. butyl- and pentyl-1ROP groups.
100
Cumulative 1ROP distribution (%)
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Figure 2. CLIC-filtered images of 1ROP families. Catalyst and conditions: same as Figure 1. Corresponding chemical groups: (a) methyl-1ROP; (b) ethyl-1ROP; (c) propyl-1ROP; (d) butyl- and pentyl-1 ROP. See text for details.
On the basis of normal configuration of the two GC columns (apolar × polar), it is expected that molecules with higher degree of branching elute faster.28 For example, among a same ethyl-1ROP group, trimethylethylcyclopentanes elute faster than dimethylethylcyclohexanes, which themselves elute faster than diethylcyclohexanes. In addition, with the same number of substituents, the compounds with the biggest alkyl substituent elute faster, e.g. methyl-iso-propylcyclohexanes elute faster than diethylcyclohexanes. This approach evidences the general trend that more isomerized 1ROPs (with low cetane number) elute faster than linear ones (with higher cetane number), and this feature can be used to quickly characterize the quality of the obtained 1ROP mixture. The GC×GC-FID technique was used for calculating the socalled “cumulative distribution function” of 1ROP compounds, C1ROP. A uniform grid consisting of n = 100 meshes along the horizontal axis (see Figure S1) was applied to the 1ROP region in the GC×GC chromatogram, using the GC image software. The associated cumulative function is calculated as follows:
C1ROP n
∑in i1 V1ROP i
Vtot
Ir/LaNaY, H2S-free Ir/LaNaY, 0.8% H2S
80
NiWS/F-ASA, 0.8% H2S 60
40
20
0 0
20
40
60
80
100
Mesh number
Figure 3. Cumulative distribution of 1ROPs at similar products yields (YROP =12%) for reaction over Ir/LaNaY without (270 °C) or with (310 °C) H2S, and NiWS/F-ASA with H2S (380 °C).
Such a procedure was only applied to 1ROPs because of the ease of using the isomerization criteria. Noticeably, GC×GC analysis allows this type of investigation because 1ROPs are spatially well separated from all other chemical families, i.e. the 1ROP family elution area does not overlap with any other chemical families on two-dimensional chromatograms. This is not the case when conventional GC is used and significant overlapping zones appear among C10 chemical families.47 As seen above, the cumulative distribution curves allow one to compare quickly the 1ROP distributions in terms of branching, which has a crucial influence on the cetane number, and thus enable easy catalyst screening. However, in order to gain insight into the reaction mechanisms, a more detailed analysis of products is necessary, as presented below. In the case of SkIP identification, as reported by Kubička et al.46,48,49 and our group, 14,27 the widely employed NIST mass spectra database is very limited for C10-bicycloalkanes (25 compounds). Therefore, literature references were used in order to collect 79 additional spectra, which were added to the MS database. This allowed us to tentatively identify the fol-
(2)
where V1ROP(i) and Vtot are the volume of detected 1ROP peaks at each i mesh and the total 1ROP peak volume, respectively. Under the assumption that all C10 compounds have the same response in FID, it was assumed that the distribution is quantitative. Note that the chromatograms obtained from MS and FID for a same sample were very similar; only small Page 3
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Figure 4. Structure and abbreviated name of SkIP groups.
lowing decalin derivatives (the corresponding chemical groups are presented in Figure 4 with their abbreviated name): - methylbicyclo[4.3.0]nonanes, 17 isomers (Figure 4-I)36 - dimethylbicyclo[3.3.0]octanes, 12 isomers (Figure 4- II)37 - methylbicyclo[3.3.1]nonanes, 6 isomers(Figure 4-III)
For 1ROP identification, as for the case of SkIPs, literature references38-41,44,51 were used to collect 69 additional spectra (belonging to methyl-1ROP and ethyl-1ROP groups), which were added to the MS database. The remaining products (
