Fractionation of Coal Extracts Prior to Hydrocracking: An Attempt to

Energy & Fuels 2001, 15, 1153-1165

1153

Fractionation of Coal Extracts Prior to Hydrocracking: An Attempt to Link Sample Structure to Conversion Levels and Catalyst Fouling Isabel Suelves,† Maria-Jesus Lazaro,† Vanessa Begon, Trevor J. Morgan, Alan A. Herod,* and Rafael Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), Prince Consort Road, London SW7 2BY, U.K. Received January 23, 2001. Revised Manuscript Received May 23, 2001

Catalyst fouling during hydrocracking and conversions of larger molecular mass components have been investigated in terms of the structural features of a bituminous coal extract. The sample has been separated into two pairs of fractions: pentane-soluble (PS) and -insoluble (PI); toluenesoluble (TS) and -insoluble (TI). Differences between hydrocracked products and levels of carbondeposition on a commercial presulfided NiMo/γ-Al2O3 catalyst have been examined. Size exclusion chromatograms (SEC) showed MM-distributions of the samples decreasing in the order: TI > PI > TS > PS. This trend closely paralleled those given by TGA-derived boiling point distributions and the ordering of UV-fluorescence (UV-F) derived spectral shifts. In SEC, two columns with different operating ranges of molecular sizes were used. Results indicated that the largest molecular mass material did not pass through the column with the smaller molecular size range and was lost for analytical purposes. Within the range where probe mass spectrometry is capable of observation (up to ∼600 u), the hydrocracked products of all the fractions studied contained similar ranges of molecular species, in contrast with data from TGA, SEC, and UV-F. The differences between hydrocracked products from different fractions were confined to masses beyond the range of detection by probe mass spectrometry. A reliable correspondence was found between catalyst fouling levels and the concentration of >450 °C bp material in the feed. Our results are consistent with a model of the larger extract molecules, where large (>300 u) polycyclic aromatic (PCA) ring systems are embedded within a matrix held together by several different structural types of bridges. During hydrocracking, bridging structures between PCA ring systems break down although most PCA ring systems remain unaltered. It is thought that larger PCA groups liberated by the hydrocracking process are more likely to deposit on catalyst surfaces.

Introduction In recent batch catalytic hydrocracking experiments, we observed molecular mass distributions of coal extracts to shift toward smaller values during the first 5-10 min.1,2 The most rapid changes were observed before the catalyst (commercial NiMo/γ-Al2O3 or a Mocarbonyl catalyst precursor) was fully activated by sulfidation.3 The shifts in molecular mass (MM) could be reproduced in the absence of catalyst, indicating these initial changes to be pyrolytic in nature.3 Once the catalyst had been fully sulfidedsand the extract had undergone its initial reactive phasesproduct structures were observed to change continuously but comparatively slowly. Catalysts recovered from these experiments were tested for weight uptake due to fouling4 (“carbon lay-

down”). During the “heat-up” stage between ambient and 440 °C (i.e., with no holding at 440 °C), nearly 8% of the extract sample in the reactor was found deposited on the commercial NiMo/γ-Al2O3 supported catalyst. Much of this deposit could not be washed off, even with a strong solvent (NMP: 1-methyl-2-pyrrolidinone). At longer reaction times (1-2 h), the carbonaceous residue was found to harden (viz., became less reactive in TGAcombustion) and to slowly diminish in weight. During the hydrocracking of heavy hydrocarbon liquids, the loss of activity of supported NiMo and CoMo/ γ-Al2O3 catalysts is widely accepted to be “rapid”.5,6,7,8 It is commonly thought, however, that time-spans relevant to loss of activity usually extend to tens or hundreds of hours.9 Our experiments show this process to be rather more rapid.

* Corresponding author. E-mail: [email protected]. † Present address: Instituto de Carboquı´mica, Departamento de Energı´a y Medio Ambiente, CSIC, Marı´a de Luna 12, 50.015, Zaragoza, Spain. (1) Zhang, S.-F.; Xu, B.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1996, 10, 733. (2) Zhang, S.-F.; Xu, B.; Herod, A. A.; Kimber, G. M.; Dugwell, D. R.; Kandiyoti, R. Fuel 1996, 75, 1557-1567. (3) Begon, V.; Megaritis, A.; Lazaro, M.-J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1261-1272.

(4) Begon, V.; Warrington, S. B.; Megaritis, A.; Charsley, E. L.; Kandiyoti, R. Fuel 1999, 78, 681-688. (5) Alptekin, G.; Onsan, I. Z.; Kandiyoti, R. Bogazici University Journal 1979, 7, 1-22. (6) Thakur, D. S.; Thomas, M. G. Appl. Catal. 1983, 6, 283-292. (7) Stephens, H. P.; Stohl, F. V. ACS Div. Fuel Chem. Prepr. 1984, 29, 79-88. (8) Cloke, M.; Hamilton, S.; Wright, J. P. Fuel 1987, 66, 678-682. (9) Martin, S. C.; Snape, C. E.; Cloke, M.; Belghazi, A.; Steedman, W.; McQueen, P. Energy Fuels 1998, 12, 1228-1234.

10.1021/ef010013d CCC: $20.00 © 2001 American Chemical Society Published on Web 08/17/2001

1154

Energy & Fuels, Vol. 15, No. 5, 2001

It is difficult to identify the original chemical nature of the carbonaceous material found adhering to catalyst surfaces. It seems possible, indeed likely, that the heaviest fractions of the coal extract would preferentially adhere to the catalyst.10,11,12 If so, the preferential depletion of larger molecular mass (MM) material from the sample solution would provide at least a partial explanation for the shifts we observed in molecular mass distributions during the first 5-10 min. However, the latter contribution must be seen as “partial” since the shifts to smaller molecular mass were also observed in the absence of catalyst: in reactors passivated by repetitive reuse. Size exclusion chromatography, our main tool for monitoring changes in molecular mass, is not quantitative and could not help distinguish between thermal cracking and precipitation on the catalyst. Within this framework, what seems fascinating is the persistent activity of the catalyst. There seems to be general agreement that some activity is retained, despite high levels of fouling. The phenomenon had been noted as early as 1976; Thomson and Webb13 have suggested that carbonaceous deposits may act as reservoirs and transmitters of hydrogen, shuttling hydrogen between catalyst surfaces and the bulk liquid phase. It seems, of course, difficult to visualize large coal derived molecules diffusing through the progressively hardening (presumably gradually dehydrogenating) layer of carbonaceous deposit. It becomes tempting to model the reactions of coal extract molecules during this process as consisting of pyrolytic bond scission in the bulk of the solution, followed by (a) free radical quenching through hydrogen donation by the donor solvent, or (b) direct hydrogen donation to product molecules through the carbonaceous film. This simplified picture would restrict the role of the catalyst to hydrogen dissociation (supplying both product and solvent) and possibly some direct hydrogenation, on catalyst surfaces, of small solvent molecules able to diffuse through the carbonaceous layer. This simple model also raises questions regarding what structural changes in the coal-derived liquid can be effected by the presence of the catalyststhe original object of the exercise! In particular, would catalyst activity help reduce the sizes of larger PCA ring systems imported into the extract from the parent coal? When using Point of Ayr coal as feedstock (carbon content: 83%; daf basis), laboratory scale hydrocracking experiments1,2 and pilot-plant trials alike14 have found a >450 °C boiling residue in the product mixture, that was chemically remarkably stable. In size exclusion chromatography, these materials appeared mostly under the resolved peak, indicating relatively low molecular masses [Figure 10 in ref 15]. The properties of this “liquefaction pitch” contrast sharply with those of a (10) Cillo, D. L.; Stiegel, G. J.; Tischer, R. E.; Narain, N. K. Fuel Process. Technol. 1985, 11, 273-287. (11) Yoshimura, Y.; Hayamiza, K.; Sato, T.; Shimada, H.; Nishijima, A. Fuel Process. Technol. 1987, 16, 55-69. (12) Stohl, F. V.; Stephens, H. P. Ind. Eng. Chem. Res. 1987, 26, 2466-2473. (13) Thomson S. J.; Webb, G. J. Chem. Soc., Chem. Commun. 1976, 526. (14) Harrison, J. S.; Kimber, G. M.; Gray, M. D. Proc. Int. Conf. Coal Sci. 1989, Tokyo, 655. (15) Zhang, S.-F.; Xu, B.; Moore, S.; Herod, A. A.; Kandiyoti, R. Fuel 1996, 75, 597-605.

Suelves et al.

coke-oven derived coal-tar pitch; the latter was found easy to hydrocrack,16 but the product was found to contain significant amounts of PCA ring systems of MM > 300, too large to go through a GC column.17 It seems reasonable to identify the residual “liquefaction pitch” with large polynuclear aromatic ring systems originally present in the coal. During extraction and the pyrolytic stage in the hydrocracker, these groups would have been stripped of substituent groups and crosslinking bridging structures. The occurrence of dehydrogenation reactions, leading to ring closure and increased size of PCA ring, systems is also likely. It seems difficult to visualize how the relatively ineffectual catalytic configuration described above could begin to reduce the PCA ring structures themselves to smaller units. The Point of Ayr Coal Liquefaction facility,14 the recycle solvent was found to contain 20% saturated hydrocarbons,18 causing loss of valuable hydrogen as well as of carbonaceous material; a “satcracker” was installed to recover some of these materials in usable form. Significantly, a self-conscious choice appears to have been made to use low rank coals (with attendant smaller PCA ring systems) as feedstocks, for coal liquefaction processes that are being deemed viable/ feasible in the 21st century [e.g., see refs 19 and 20]. The present study attempts to probe the links between structures of bituminous coal extracts (distributions of MMs and structural features) and, in particular, the presence of large molecules, with (a) catalyst fouling levels for a catalyst known to promote hydrodesulfurization and hydrocracking and (b) levels of conversion that could be expected for the larger molecular mass components of the extract samples. The work is not about the production of transport fuels, but concerns the behavior and fate of the large molecules known to be present in coal liquids from our development of SEC, UV-fluorescence spectroscopy, and MALDI-ms using NMP as solvent, in the processing of coal liquids.1-4,15-17 Changes in the characteristics of hydrocracked products and levels of carbon-deposition on a NiMo/γ-Al2O3 catalyst have been compared for the pentane-soluble and -insoluble fractions of a coal extract. A similar set of comparisons has been carried out with the toluenesoluble and -insoluble fractions of the same coal extract. Samples have been characterized by size exclusion chromatography and UV-fluorescence and probe-mass spectrometries. The boiling point distributions of the samples have been determined by thermogravimetric methods. Experimental Section The Sample: The sample was a coal liquefaction extract prepared by the “digestion” of Point of Ayr coal in recycle solvent (average residence time ∼1 h) and filtered to remove undissolved solids; it had been drawn directly from the stream (16) Begon, V.; Islas, C. A.; Lazaro, M.-J.; Suelves, I.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Eur. J. Mass Spectrom. 2000, 6, 39-48. (17) Herod, A. A.; Islas, C. A.; Lazaro, M.-J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201-210. (18) Wilson, R.; Parker, J. E.; Johnson, C. A. F.; Herod, A. A. Org. Mass Spectrom. 1987, 22, 115. (19) Okuma, O. Fuel 2000, 79, 355. (20) Kouzu, M.; Koyama, K.; Oneyama, T.; Aramaki, T.; Hayashi, T.; Kobayashi, M.; Itoh, H.; Hattori, H. Fuel 2000, 79, 365.

Fractionation of Coal Extracts fed to the hydrocracker unit at the Point of Ayr Coal Liquefaction Pilot Plant formerly operated by British Coal. Solvent Fractionation. The extract sample was separated into (i) pentane-soluble and -insoluble fractions and into (ii) toluene-soluble and -insoluble fractions.21 For the toluene extraction, 1 g of coal extract was mixed with 20 mL of solvent. The mixture was then agitated in an ultrasonic bath for 30 min before the two fractions were separated by vacuum filtration. For the pentane extraction, 1 g of coal extract was first dissolved in 10 mL of toluene and agitated for 30 min in an ultrasonic bath. 200 mL of n-pentane was then added. After another 30 min in the ultrasonic bath, the mixture was left to settle overnight in the freezer; the precipitated insolubles settled at the bottom of the vessel. The pentane-solubles were separated from the insolubles by drawing with a syringe. The recovered fractions were dried in a vacuum oven at 70 °C for 5 h in order to eliminate traces of solvent before the hydrocracking step. In customary terminology, the pentane-soluble (PS) fraction corresponds to “"oils” and the pentane-insoluble (PI) fraction to mixed asphaltenes and preasphaltenes. The toluene-soluble fractions (TS) correspond to oils plus asphaltenes, and the toluene-insoluble (TI) fraction to preasphaltenes. The pentane-insolubles and the toluene-insolubles were both recovered as dry powders following the drying step; no solvent smell was detected in any of the “soluble” fractions. 62 wt % of the PoA coal extract was recovered as the “toluene-soluble” fraction and 19% as “toluene-insoluble”. Similarly 53% of the extract was recovered in the “pentane-solubles” against 33% in the “pentane-insoluble” fraction. 19 and 14% of the original sample were lost during the toluene and pentane separation steps, respectively. These losses are thought to correspond to loss during the drying step of materials volatile at 70 °C in a vacuum. This was considered as acceptable since the intended work focused primarily on aspects of hydrocracking of the larger coal derived molecules. Catalytic Hydrocracking. The procedure has been described elsewhere.1,15,22 Briefly, 200 mg of sample was reacted in the presence of 100 mg commercial NiMo/γ-Al2O3 catalyst (PBC-90D) and 1 g tetralin. The tetralin was distilled to avoid introducing impurities into the system. This catalyst was presulfided by adsorption of a proprietary heavy-sulfur containing organic compound. it was supplied as 6 mm dia cylindrical pellets; in laboratory experiments it was crushed to less than 250 µm to improve contact with sample. Activation of the catalyst takes place by H2S release during early stages of the reaction. The reaction was carried out at 440 °C under 190 bar H2 pressure, for 60 min. After the reaction, a 4:1 chloroform:methanol mixture (vol:vol) was used to wash the reactor. Used catalyst and any undissolved materials were filtered off. The mass of product was weighed after drying off the solvent by warming under a stream of nitrogen gas. Recovered solids were washed in NMP to recover residual heavy product; the catalysts were examined by TGA, before and after washing with NMP to evaluate the recovery and to indicate the deposition of NMP-insoluble material. Size-Exclusion Chromatography. Samples were characterized by size exclusion chromatography (SEC) using 1-methyl-2-pyrrolidinone (NMP) as eluent.23 Two polystyrene/ polydivinylbenzene columns (Polymer Laboratory Ltd., UK) of different properties were used: a 3 µm particle size Mixed-E column and a 5 µm particle size (larger porosity) Mixed-D column. The linear calibration range of the Mixed-E column extends to 30-40 000 u, while that of the Mixed-D column extends to about 200 000 u at the exclusion limit. Above the (21) Herod, A. A.; Stokes, B. J. Fuel Process. Technol. 1990, 24, 45. (22) Zhang, S.-F. Ph.D. Thesis, Imperial College, University of London, 1995. (23) Lafleur, A. L.; Nakagawa, Y. Fuel 1989, 68, 741.

Energy & Fuels, Vol. 15, No. 5, 2001 1155 exclusion limits, higher molecular mass (MM) polystyrene standards are resolved but show a different linear relationship between MM and elution volume. A polystyrene MM-standard of mass 1,850,000 u was found to elute at about 11 min in the Mixed-E column and at about 9 min in the Mixed-D column.24,25,26 The system was operated with a solvent flow rate of 0.45 mL min-1 at a temperature of 85 °C (Mixed-E column) and a flow rate of 0.5 mL min-1 at a temperature of 80 °C (Mixed-D column). Detection was carried out using two different instruments in series: a variable wavelength Perkin-Elmer LC250 UV detector set at 450 nm and an Applied Biosystems 1000 S diode array detector, set at 280, 300, 350, and 370 nm; the data were recorded simultaneously. UV-Fluorescence Spectroscopy. A Perkin-Elmer LS50 luminescence spectrometer has been used as described elsewhere.27,28,29 Briefly, the spectrometer was used in static-cell mode with NMP as solvent. The latter has little fluorescence, but is opaque at 254 nm, becoming partly transparent at 260270 nm. A quartz cell of 1 cm light path length was used. The reproducibility of the spectroscopic measurements was found to be within the noise levels of the spectra.30 Results were obtained by scanning three types of spectra: (i) emission spectra obtained by scanning the available range of wavelengths for a fixed excitation wavelength, (ii) excitation spectra obtained by scanning the available range of excitation wavelengths while the emission was monitored at a fixed wavelength, and (iii) synchronous spectra obtained by varying simultaneously both the excitation and emission wavelengths with a constant wavelength difference (20 nm). Only synchronous spectra are shown below. Thermogravimetric Analyses. Two different instruments were used for the determination of boiling point distributions and extents of catalyst fouling by the laydown of carbonaceous material. Boiling Point Determinations. A Perkin-Elmer Corporation TGA-7 thermal analysis system was used for the boiling point determinations. It consists of a thermal gravimetric analyzer with a 1020 system controller. The ultra-microbalance is able to detect weight changes as small as 0.1 mg, and the furnace allows operation from ambient to 1000 °C with a heating rate ranging between 0.1 and 200 °C min-1. The sample was redissolved in chloroform/methanol solvent and introduced onto the sample pan; for some samples, the material was transferred to the TGA pan by spatula without solvent. The instrumental conditions were a helium flow of 60 mL min-1, a temperature profile of 30 °C held for 15 min to evaporate the chloroform and methanol, a temperature rise of 4 °C min-1 to 50 °C held for 120 min to evaporate the tetralin and the materials with similar boiling points and then a temperature rise of 4 °C min-1 to 247 °C held for 10 min to evaporate products with boiling points 450 °C bp) materials present in the original coal extract were found to be concentrated in the pentane-insoluble (PI) and the toluene-insoluble (TI) fractions. The original extract sample contained 31% of the >450 °C bp material. Actual masses of >450 °C bp material in each fraction were calculated using the proportions found by TGA and the mass of each fraction recovered by solvent separation (see Experimental Section). Back calculating, the total amount of >450 °C bp material in the pentanesoluble fraction plus that in the pentane-insoluble fraction came to 39.4%. The equivalent sum for the toluene-soluble and -insoluble fractions was 42.6%. During fractionation by toluene and pentane, losses of 19 and 14%, respectively, of original sample were recorded (see Experimental Section). The apparent increase in the proportion of >450 °C bp material in the separated fractions appears linked to losses of light ends during solvent removal. Size-Exclusion Chromatography of the Coal Extract and Its Fractions. Figure 1 presents SEC-chromatograms of the original coal extract and its TS and TI fractions. The profiles were obtained using the Mixed-E (Figure 1a) and Mixed-D columns (Figure 1b), respectively. Detection was by UV absorption at 350 nm. At first glance, results obtained using these two analytical columns appear broadly similar. In line with Table 1, signal for larger MM material excluded from column

Figure 1. Size exclusion chromatograms of the original coal extract (1) and of its toluene-soluble (2) and toluene-insoluble (3) fractions: In Figure 1a chromatograms were obtained with the smaller particle packed “Mixed-E” column, and in Figure 1b, with the larger particle packed “Mixed-D” column.

porosity was greater in the TI fraction, compared to the TS fraction. The TS fraction gave more intense signals for material eluting between 15 and 25 min, and only small amounts of early eluting material were observed. Closer scrutiny showed a significant difference between data from the two chromatographic columns. In Figure 1a, the excluded peak for the coal extract was greater than that of the toluene-insoluble fraction (curves 1 and 3); this is unexpected and contrasts with that of Figure 1b. Data from the Mixed-D column showed significantly greater signal intensity for excluded material in curve 3 (toluene-insolubles), relative to the coal extract, curve 1. The largest molecular mass materials in the toluene-insoluble fraction could not apparently all be eluted through the Mixed-E column, although all of the sample did dissolve in NMP. The Mixed-E column is packed with smaller particles (3 µ diameter) with finer porosity than the Mixed-D column. The results indicate that the Mixed-E column filtered out some early eluting material. Adsorptive effects may be ruled out since the two columns were packed with chemically similar material. These observations lead to a somewhat fortuitous identification of fine column porosity as a factor in partial loss of large MM material in an SEC column.

Fractionation of Coal Extracts

Figure 2. Size exclusion chromatograms of the original coal extract, and of its pentane-soluble and -insoluble fractions: (a) chromatograms obtained with the smaller particle packed “Mixed-E” column and (b) chromatograms obtained with the larger particle packed “Mixed-D” column. Curve 1: original coal extract. Curve 2: pentane-soluble fraction. Curve 3: pentane-insoluble fraction.

Returning to the chromatograms, Figures 2a,b, present qualitatively similar results for the coal extract and its pentane-soluble/-insoluble fractions. Trends found in Figure 2a confirmed the partial loss of early eluting material in the Mixed-E column. As in Figure 1, the pentane-soluble fraction (Figure 2b, curve 2) gave a small excluded peak and a very intense retained peak. The Mixed-D chromatogram of the pentane-insoluble fraction (Figure 2b, curve 3) gave a more intense excluded peak than that of the coal extract. Taken together, the SEC data on the sample fractions followed the trend indicated by the boiling point distributions (Table 1) and showed MM distributions diminishing in the order TI > PI > TS > PS. UV-Fluorescence Spectroscopy of the Coal Extract and Its Fractions. We have already discussed the interpretation of UV-fluorescence (UV-F) spectra of coal tars,29 coal extracts and their hydrocracking products.31 Figure 3a presents synchronous UV-F spectra of the original coal extract and its toluene separated fractions. The spectrum of the TS fraction (Curve 2) exhibited a small shift to shorter wavelengths (hypsochromic) compared to the original coal extract (Curve 1). Similarly, the TI fraction (Curve 3) showed a large shift to longer wavelengths (bathochromic) compared to the original extract. These shifts are consistent with the concentration of molecules with more complex structures and larger polynuclear aromatic ring systems in (31) Begon, V.; Megaritis, A.; Lazaro, M.-J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1261-1272.

Energy & Fuels, Vol. 15, No. 5, 2001 1157

Figure 3. (a) Comparison of the synchronous UV-fluorescence spectra of the original coal extract with its toluenesoluble and -insoluble fractions. Curve 1: original coal extract. Curve 2: toluene-soluble fraction. Curve 3: toluene-insoluble fraction. (b) Comparison of the synchronous UV-fluorescence spectra of the original coal extract with its pentane-soluble and -insoluble fractions. Curve 1: original coal extract. Curve 2: pentane-soluble fraction. Curve 3: pentain-insoluble fraction.

the TI fraction. The spectra have been presented in peak-normalized mode, to show observed shifts more clearly. Intensities of the TI spectrum and that of the PI fraction (see below) were low. This is consistent with the low fluorescence quantum yields, attributable to the presence of large and complex PCA ring systems. Similar trends were observed for the pair of pentanesoluble/-insoluble fractions. The bathochromic shift of the PI fraction was less marked and the hypsochromic shift of the PS fraction was larger compared to the toluene-separated fractions. The ordering of spectral shifts thus closely paralleled the hierarchy of boiling point distributions (Table 1) and SEC-derived MM distributions (Figures 1b and 2b): TI > PI > TS > PS. Probe Mass Spectra of the Samples. Figure 4 presents probe mass spectra of (a) the PS fraction, (b) the PI fraction, and (c) of the TI fraction. All three samples were recovered after a solvent-drying step, thought to cause loss of some of the light ends. Because the present study is primarily concerned with the fate of larger MM material during hydrocracking, the loss of light ends does not affect the main thrust of the work. The mass spectrum of the PS fraction (Figure 4a) showed an upper mass limit of about m/z 560, with m/z 202 pyrene isomers and m/z 216 methyl pyrenes as major components. The spectrum was highly complex and showed evidence for the range of well-known polycyclic aromatic hydrocarbons and hydrogenated aromatics.16,32 (32) Herod, A. A.; Kandiyoti, R. J. Chromatogr. A 1995, 708, 143160.

1158


Suelves et al.

Figure 4. Probe mass spectra of (a) the pentane-soluble, (b) the pentane-insoluble, and (c) the toluene-insoluble fractions.

The spectra of the pentane- and toluene-insolubles (Figure 4b,c) gave lower signal intensities, in line with the expected relative scarcity of smaller molecules in these fractions.16 Figure 4b (PI fraction) showed molecular ions of pyrene (m/z 202) and other polycyclic aromatic hydrocarbons up to m/z 400. Figure 4c (TIfraction) showed a series of alkyl-, cyclo- and bicycloalkyl fragment ions from m/z 40 to at least m/z 155. The clusters of peaks, separated by 14 mass units, showed the greatest intensity for the bicyclo-alkane fragment and the least intensity for the alkane fragment. This can be seen clearly at m/z 81 (bicyclo fragment), which was of greater intensity than m/z 83 (cycloalkyl fragment); in turn, this was of greater intensity than m/z 85, the alkyl fragment. In the absence of any aromatic fragment ions or any recognizable aromatic molecular ions, it may be presumed that the molecular ions detected (m/z 236, 256, 368, 496, 524, 538) correspond to cyclo alkane structures as follows: m/z 236, C17H32, a bicyclo alkane; m/z 256, C19H28, a hexacycloalkane; m/z 368, C27H44, a hexacycloalkane; m/z 496, C36H64, a pentacycloalkane; m/z 524, C38H68, a pentacycloalkane and m/z 538, C39H70, a pentacycloalkane. Discussion: Structural Features of the Coal Extract and Its Fractions. In earlier work on pyrolysis tars from a rank ordered set of coals,29 samples were characterized by overlaying data from SEC and UVfluorescence spectroscopy. UV-F spectra were found to shift to longer wavelengths as SEC-derived MMs of

samples increased. These results were interpreted as showing increasing concentrations and sizes of polynuclear aromatic ring systems in larger molecular mass material. We speculated that more than one PCA ring system could be embedded in some of the larger molecules. This was not new. Many previous workers have discussed the concept of aromatic entities being found embedded in molecular units larger than the aromatic entities themselves.33,34,35 However, these early discussions have been carried out in terms of molecular masses of up to about 1000 u and individual PCA ring systems containing not more than 3-4 aromatic rings. Since the late 1980s, much larger molecular masses have been identified in coal derived “liquids” than had been assumed in these earlier models [cf., e.g., refs 26, 36, and 37]. Recently, we presented statistical analyses of MALDI-TOF mass spectra of the pyridine-insoluble fraction of a coal tar pitch, showing a “safe” uppermass limit of signal in the vicinity of 95 000 u.24,38 Recent results from size exclusion chromatography suggest the (33) Given, P. Fuel 1960, 39, 147. (34) Howard, J. B. Chemistry of Coal Utilisation, 2nd supplementary vol.; ed Elliott, M. A., Ed.; Wiley: NY, 1981; Chapter 12, p 665. (35) Shinn, J. H. In Proceedings of the 1985 International Conference on Coal Science, 28-31 Oct 1985; Sydney; Pergamon: Sydney. 1985; pp 738-741. (36) Boenigk, W.; Haenel, M. W.; Zander, M. Fuel 1990, 69, 1226. (37) Lapucha, A. P.; Larsen, J. W. Am. Chem. Soc. Div. Fuel Chem. 1992, 37(3), 1221. (38) Lazaro, M. J.; Herod, A. A.; Domin, M.; Zhuo, Y.; Islas, C. A.; Kandiyoti, R. Rapid Commun. Mass. Spectrom. 1999, 13, 1401-1412.


presence of much larger species in coal extracts and a coal-tar pitch.39,40 With regard to the sizes of PCA ring systems embedded in these large molecules, the lack of detail in UV-F spectroscopy has not hitherto allowed speculation, except to say that these appear to be significantly larger than the 4-5 ring PCA systems imaged in earlier work. More recently, we characterized a coal tar pitch17 and the same coal extract as in the present work39 in some detail. The samples were fractionated by planar chromatography and results from the pyrolysis-GC-MS and 13C NMR of the fractions correlated with their SEC chromatograms. In the lighter fractions (more mobile in planar chromatography), all the expected smaller polynuclear aromatic hydrocarbons could be identified. However, greater aliphatic content was identified in the heavier fractions, which 13C NMR still showed to be predominantly aromatic. However, the pyrolysis-GC-MS of the (heaviest) fractions, immobile in pyridine, showed only aliphatic and light alkyl aromatic fragments. It appears that PCA ring systems contained in the heavy fractions of both samples were either embedded in structures that did not volatilize (and charred directly) or were too large (larger than, say, coronene) to pass through the chromatographic column. These results lend support to the concept of polynuclear aromatic ring systems embedded in molecules held together by numerous bridging systems, probably made up of ether, aliphatic, hydroaromatic and alkylaromatic structures. Once again, the major difference with previous models is in the perceived ranges of the masses of the molecules themselves and in the sizes of the polynuclear aromatic ring systems embedded within them. In Figure 4, the low intensities of signal from smaller molecules in the spectra from the PI and TI fractions are consistent with this model, which is needed to explain product distributions obtained by hydrocracking these samples. Description of the Hydrocracked Products. Attempts to quantify light ends by TGA derived boiling point distributions proved unreliable as the method requires the retention of light ends for an accurate mass balance. However, the materials of greater interest for this study (>450 °C bp) could be quantified satisfactorily and will be discussed below. Size Exclusion Chromatography. Figure 5a compares SEC chromatograms of the original coal extract and its hydrocracked products, obtained with the “Mixed-D” column and recorded at 350 nm. Compared with the chromatogram of the original coal extract, that of the hydrocracked product showed a large shift to longer elution times (smaller MMs); signal due to material excluded from column porosity was also much reduced. Synchronous UV-F spectra (Figure 5b) of the two samples showed a parallel shift to shorter wavelengths by the spectrum of the hydrocracked product. If, as we think, the reduction in the size and concentration of PCA ring systems were relatively small, the spectral shift would correspond to the breakdown of five(39) Islas, C. A.; Suelves, I.; Carter, J. F.; Herod A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 1766-1782. (40) Herod, A. A.; Lazaro, M.-J.; Suelves, I.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Energy Fuels 2000, 14, 1009-1020.


membered rings, aliphatic, alkyl, hydroaromatic, etc., substituent groups. Figure 6a presents analogous SEC chromatograms of the TI and TS-fractions (curves 1,3) and their hydrocracked products (curves 2 and 4) recorded at 350 nm. A major shift in MM distributions was observed between the TI fraction (curve 1) and its hydrocracked product (curve 2), where the relatively large “excluded” peak of the TI fraction was much reduced after the reaction. Analogous but smaller changes were observed between the TS fraction and its product; the main change was a shift to a higher intensity retained peak. Figure 6b presents the analogous SEC chromatograms of the pentane-separated fractions and their hydrocracked products recorded with the Mixed-D column and detection at 350 nm. The results were qualitatively similar to those in Figure 6a. UV-Fluorescence Spectroscopy of the Hydrocracked Products. Parts a and b of Figure 7 present the overlaid synchronous UV-F spectra of the toluene and pentane separated fractions and their hydrocracked products, respectively. For all fractions, hydrocracking reactions caused shifts of the spectra to shorter wavelengths (hypsochromic). The shifts were larger in the case of the PI and TI fractions and their hydrocracking products. The shift to shorter wavelengths was also larger for the TS product compared to PS product. These results are consistent with trends observed for changes in SEC chromatograms outlined above. Once again, if the reduction in the size and concentration of PCA ring systems were relatively small, the spectral shift would correspond to the breakdown of five-membered rings, aliphatic, alkyl, hydroaromatic, etc., substituent groups. Probe Mass Spectra of the Hydrocracked Products. The summed mass spectra of products from the hydrocracking of the two solvent-soluble fractions (Figure 8a,b) were similar, differing only in the relative intensity at the high mass end of the range. Products from the PS fraction showed less intensity above m/z 300, compared to the hydrocracking product of the TS fraction. Differences at the lower end of the mass scale were expected to be small: the solvent drying stage depletes light ends in a common pattern. The mass-spectra of the products from the solventinsoluble fractions (Figure 8c,d) showed the presence of tetralin, as residue from the hydrocracking procedure. The series of peaks corresponding to products from the coal-derived material observed in the spectra were similar to each other. Partly, this is due to similarities between the two samples: the PI fraction contained the “asphaltenes” and “preasphaltenes” while the TIs contained the “preasphaltenes”. The latter spectra were also quite similar to those for hydrocracked products obtained by processing the solvent-soluble fractions (Figure 8a,b). Thus, within the range where probe mass spectrometry is capable of observation, the hydrocracked products of all the fractions studied appeared to contain similar ranges of molecular species. However, both SEC profiles and UV-fluorescence spectra have shown differences in structural features and in MM distributions. It was concluded that differences between hydrocracked products from the different fractions were confined to higher ranges of masses, above the range of detection

1160


Suelves et al.

Figure 5. The unfractionated original coal extract and the hydrocracked product (a) size-exclusion chromatograms on the Mixed-D column and (b) synchronous UV-fluorescence spectra. Curve 1: whole sample. Curve 2: hydrocracked product.

by probe mass spectrometry (material found charred on the probe after the analysis). The py-GC-MS analyses of fractions of the original coal extract have indicated the presence of long alkyl chains in the larger MM fraction of the sample.39 The spectra in Figure 8a,b gave no indication of the detectable presence of aliphatic material in the hydrocracked product; it is likely that the aliphatic groups, forming part of the original molecular structure, were converted to light alkanes during the breakup of large molecules during the catalytic hydrocracking process and were lost during product recovery.

>450 °C Boiling Fractions and Carbonaceous Deposits on the Catalysts. Catalyst fouling levels: Due to similar boiling ranges of lighter components in the hydrocracked product and the donor solvent tetralin (and its own reaction products),2,41 the recovery of lower boiling products could be quite variable. The >450 °C bp material has been recovered (to within about ( 5% error) either in the product solution, or as deposited on the catalyst. Table 2 lists the masses of >450 °C bp material found in each of the original samples and (41) Brodzki, D.; Djega-Mariadassou, G.; Li, C.-Z.; Kandiyoti, R. Fuel 1994, 73, 789-794.



Table 2. Recovery of >450 °C Boiling Material: Reaction Time ) 60 Mins for All Runsa

sample original coal extract hydrocracked product of original coal extract conversion PI fraction hydrocracked product of PI fraction conversion PS fraction hydrocracked product of PS fraction conversion TI fraction hydrocracked product of TI fraction conversion TS fraction hydrocracked product of TS fraction conversion a

>450 °C bp of liquid produced of recovered (mg) (basis: 200 mg feed) I

catalyst wt. loss by TGA (mg) before wash (basis: 100 mg catalyst) II

catalyst wt. loss by TGA (mg) after wash (basis: 100 mg catalyst) III

washings (mg) (basis: 100 mg catalyst) IV

62.2 21.4

not applicable 19.5

not applicable 19.4

not applicable 0.1

62.2 40.9

178.6 111.6

not applicable 28.8

not applicable 22.9

not applicable 5.9

34% 178.6 140.4

37.2 13.8

not applicable 16.5

not applicable 13.2

not applicable 3.3

22% 37.2 30.3

182.4 170

not applicable 32.8

not applicable 28.0

not applicable 4.8

19% 182.4 202.8

81.4 43.6

not applicable 18.5

not applicable 12.1

not applicable 6.4

negative -16.6% 81.4 62.1

total >450 °C bp of material recovered (mg) V

23%

PS: pentane-soluble. PI: pentane-insoluble. TS: toluene-soluble. TI: toluene-insoluble.

Figure 6. Size exclusion chromatograms of (a) the toluenesoluble and -insoluble fractions and of the hydrocracked products of both fractions and (b) the pentane-soluble and -insoluble fractions and of the hydrocracked products of both fractions, on the Mixed-D column. Curve 1: solvent insolubles. Curve 2: solvent solubles. Curve 3: hydrocracking product of insolubles. Curve 4: hydrocracking product of solubles.

masses of >450 °C bp material after 60 min hydrocracking (440 °C, 190 bar H2 with NiMo/γ-Al2O3 presulfided supported catalyst). Amounts of carbonaceous material burnt off the spent catalyst have also been shown (column II). This allowed the total amount of >450 °C bp material in the reactor to be calculated.

Figure 7. Synchronous UV-fluorescence spectra of a) the toluene-soluble and -insoluble fractions and the hydrocracked products of both fractions and b the pentane-soluble and -insoluble fractions and the hydrocracked products of both fractions. Curve 1: solvent insolubles. Curve 2: solvent solubles. Curve 3: hydrocracking product of insolubles. Curve 4: hydrocracking product of solubles.

Deposits on reactor walls could be neglected. Conversions (column V) calculated by adding data in columns I and II are notional, since it cannot be proven that all the deposits originated from >450 °C bp material. As expected, Table 2 (column I) shows the unreacted TI and PI fractions to be principally made up of >450 °C bp material and column II indicates that far greater accumulations of carbonaceous material took place on the catalyst in the presence of the two heavier fractions. The data are consistent with results from a fixed bed

1162


Suelves et al.

Figure 8. Probe mass spectra of the hydrocracked products of (a) the pentane-soluble, (b) the toluene-soluble, (c) the pentaneinsoluble, and (d) the toluene-insoluble fractions.

catalytic reactor9 and show a good correlation between catalyst fouling levels and the concentration of >450 °C bp material in the feed. We recently described how the amount of carbonaceous deposit adhering to catalyst particles changes with time.4 A rapid surge in carbon deposition takes place when using fresh catalyst and fresh feed. Using fresh feed with reused catalyst gave rise to additional but smaller surges in deposition. Translated to a steadystate operation, this behavior would correspond to continuous deposition and desorption of heavier material in the fresh feed, around a harder nucleus of more thoroughly dehydrogenated carbonaceous deposit. The fresh sulfided catalyst showed4 a weight loss by TGA of

5.2%; we have assumed that in the present work, the sulfidation of the catalyst would be converted into an active catalyst material after use in the reactor corresponding to involatile material of the feed. Therefore, the weight losses from the recovered catalyst in Table 2 (column II) have not been corrected for the weight loss from the fresh catalyst. If any correction was appropriate, the conversions in Table 2, column V would be increased. The SEC profiles of material recovered by NMP washing of spent catalyst (already washed with chloroform/methanol) are presented in Figure 9, showing material at shorter elution times than in the original samples or their hydrocracked products. With one



Figure 9. Size exclusion chromatograms of the NMP washings from the used catalysts (a) whole original coal extract, (b) pentanesoluble, (c) toluene-soluble, (d) pentane-insoluble, and (e) toluene-insoluble fractions.

exception, the amount of NMP-soluble (but otherwise solid) material trapped within the catalyst and/or the carbonaceous layer was between 3 and 6% of the catalyst (1.5-3% of the feed). NMP-soluble material recovered from catalyst used with the PI and TI fractions contained a greater proportion of excluded material (the first peak of the partly resolved doublet at 9 min), than did the material recovered from catalyst used with the PS and TS fractions. The relative insolubility of these “NMP washings” make it difficult to perceive them as material diffusing in and out of the carbonaceous layer, but rather like a “softer” fillingsperhaps acting as the H-shuttling medium. The relative proportions of >450 °C bp material deposited from the PS and TS fractions was greater that those of the TI and PI fractions, suggesting preferential deposition on catalyst surfaces even at low concentrations, but the data are not conclusive. Conversion of Larger Molecular Mass Components. So far, data from this and previous studies have shown that deposition of carbonaceous material on catalyst particles takes place rapidly and in significant amounts, relative to the mass of catalyst. We have also seen that the concentration of high mass material in the feed has a direct bearing on the amount of deposition on catalyst surfaces.4,9 However, deposition of carbonaceous material on catalyst particles cannot be an endless sink of valuable coal extract. In steady-state systems, levels of deposition appear to stabilize within a matter of hourss or tens of hours at most. Not surprisingly, model

compound studies (cited above) as well as hydrocracking experiments with coal-derived materials show that CoMo or NiMo/γ-Al2O3 catalysts show low levels of activity during hydrocracking, after an initial short period of “grace”. Our problem, however, concerns levels of conversion (to smaller mass material) that may be expected for the larger molecular mass components of the extract sample. Table 2 (column V) shows that the hydrocracking of the TI fraction led to an increase (“negative conversion) of >450 °C bp material. The result was experimentally reproducible and points directly to the existence of active dehydrogenation processes, although the reason this result should have been so different from that of the PI fraction requires explanation. It is possible that past a certain level of loading, the catalyst ceases to function altogether; although further investigation is clearly warranted. As an aside, conversion of heavy material during the hydrocracking of the original coal extract was the highest of all samples. The differencesadmittedly only marginally greater than experimental scatterssuggests that the original “whole” coal extract in tetralin may have acted as a more effective H donor for the larger molecules of “asphaltenes” and “preasphaltenes” than tetralin alone. Table 2 also shows that with the exception of the TI fraction, the levels of conversion to lighter fractions of >450 °C bp material were fairly low, ranging between 20 and 34%. As explained above, the Point of Ayr extract

1164


used in the present study has been rather extensively characterized in this laboratory.39 The pyrolysis-GCMS of the heaviest fractions (immobile in pyridine in planar chromatography) showed only aliphatic and light alkyl aromatic fragments, strongly suggesting that PCA ring systems within this fraction were embedded in structures that did not volatilize (and charred directly) or were too large (larger than, say, coronene) to pass through the chromatographic column. This ties in with our earlier SEC chromatograms of the residual >450 °C bp material isolated after catalytic hydrocracking experiments. The structural features of the material were clearly different from the >450 °C bp fraction of the starting materials. We could clearly observe that the material was much reduced in MM range [Figure 10 of ref 15] and was chemically stable, i.e., very slow conversion was observed in longer hydrocracking experiments. Significantly, extracts from three different liquefaction procedures (and different SEC chromatograms) all gave >450 °C bp residues after hydrocracking that looked very similar by SEC.15 It would appear that ref 42, which held that preasphaltenes were much more difficult to hydrocrack compared to asphaltenes, did not go far enough. It is difficult to escape the conclusion that in the hydrocracking of the largest molecules: it is the bridging structures between PCA ring systems that may break down; the PCA ring systems themselves appear to remain largely unaltered under catalytic hydrocracking conditions. A quantitative estimation of the extent of the stability of PCA ring systems, requiring detailed NMR analyses of each stage of the process is currently being prepared for publication. Summary and Conclusions The study has attempted to probe links between structural features of bituminous coal extracts and (a) catalyst fouling levels, and (b) levels of conversion that could be expected for the larger molecular mass components. A coal extract has been fractionated into two pairs of fractions: pentane-soluble (PS) and -insoluble (PI); toluene-soluble (TS) and -insoluble (TI). Changes in the properties of hydrocracked products and levels of carbon-deposition on a commercial presulfided NiMo/ γ-Al2O3 catalyst have been examined. Our findings may be summarized as follows. 1. Comparison of the performance of two size exclusion chromatography (SEC) columns of different porosities, under otherwise identical test conditions, has led to the identification of fine column porosity as a factor in partial loss of large molecular mass (MM) material in SEC columns. Apart from showing the limitation of a chromatographic column, this finding provides independent confirmation of the presence of very large molecules, beyond the void volume of the Mixed-E column, in these samples. 2. Structural Features of the Sample Fractions. SEC chromatograms showed MM-distributions decreasing in the order: TI > PI > TS > PS. The trend was consistent with that given by TGA-derived boiling point distributions. UV-fluorescence spectral shifts closely paralleled (42) Sakanishi, K.; Zhao, X. Z.; Korai, Y. Q.; Fujitsu, H.; Mochida, I. Fuel Process. Technol. 1988, 20, 233.

Suelves et al.

the same order as that of the TGA derived boiling point distributions and that of the SEC-derived MM distributions. Probe-mass spectra of the PS showed a highly complex spectrum with evidence for the well-known range of polycyclic aromatic hydrocarbons and hydrogenated aromatics. Analogous spectra of the PI and TI fractions gave significantly lower signal intensities, in line with the expected relative scarcity of smaller molecules in these fractions. 3. Structural Features of Hydrocracked Products. Within the range where probe mass spectrometry is capable of observation (up to ∼600 u), the hydrocracked products of all the fractions studied appear to contain similar ranges of molecular species. However, the SEC chromatograms and UV-fluorescence spectra, as well as TGA derived boiling point distributions have shown differences in structural features and in MM distributions. Differences between hydrocracked products from different fractions appear to be confined to higher ranges of masses, above the mass range of detection by probe mass spectrometry (probably corresponding to involatile residue on the MS probe). 4. Catalyst Fouling Levels. A reliable correspondence was found between catalyst fouling levels and the concentration of >450 °C bp material in the feed. During catalytic hydrocracking, heavier fractions leave larger carbonaceous deposits on catalyst surfaces. Under these depositional conditions, activities of CoMo or NiMo/γ-Al2O3 catalysts are inevitably low and possibly confined to shuttling hydrogen from catalyst surfaces to the bulk fluid. The cracking process itself appears to be mostly pyrolytic in nature. 5. Conversion of Larger Molecular Mass Components. The hydrocracking of the toluene-insoluble fraction led to an increase (“negative conversion) of >450 °C bp material. The result was experimentally reproducible and points to the existence of active dehydrogenation processes, in the presence of high concentrations of preasphaltenes. For all other samples, the levels of conversion of >450 °C bp material ranged between 20 and 34%. Earlier SEC chromatograms of the residual >450 °C bp material have shown structural features that were clearly different from the >450 °C bp fraction of the starting materials: residue materials were much reduced in MM range and was chemically quite stable.20 These results are consistent with a model of large extract molecules, where large (> 300 u) PCA ring systems are embedded within a matrix held together by several different structural types of bridges (etheric, aliphatic, hydroaromatic, etc.). During hydrocracking of these largest molecules, bridging structures between PCA ring systems may break down although the PCA ring systems themselves remain largely unaltered. Our results suggest that larger PCA groups, which are liberated by the hydrocracking process, are more likely to deposit on catalyst surfaces. The work shows the analysis of carbon laydown in any given context requires careful prior evaluation of the structural features of the substrate itself. The implications for the processing of petroleum resids and the necessity for the reevaluation of assumptions built


into the assessment of petroleum asphaltene structures are obvious. Acknowledgment. The authors thank the British Coal Utilization Research Association (BCURA) and the UK Department of Trade and Industry for financial support under project B32a, the European Union for grants to I.S. and M-J.L. (Marie Curie Research


Fellowships) and the University of London Intercollegiate Research Service (ULIRS) for provision of mass spectrometry services at Kings College. The authors also thank Mr S.A. Moore, formerly of British Coal, Point of Ayr Liquefaction Plant, for supplying the pilot plant materials and for helpful discussions. EF010013D

Fractionation of Coal Extracts Prior to Hydrocracking: An Attempt to

Recommend Documents