Article pubs.acs.org/EF
Asphaltene Characterization during Hydroprocessing by UltrahighResolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Estrella Rogel*,† and Matthias Witt‡ †
Chevron Energy Technology Company, Richmond, California 94801, United States Bruker Daltonik GmbH, 28359 Bremen, Germany
‡
S Supporting Information *
ABSTRACT: Asphaltenes have a significant impact on the hydroprocessing of residues because they adversely affect the overall rate of hydroprocessing reactions. They also act as coke precursors, leading to catalyst deactivation. Understanding the changes of asphaltenes upon conversion can provide essential clues to the development and optimization of residue conversion processes. In this work, we use ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (positive-ion mode atmospheric pressure photoionization) to follow asphaltene changes during the hydroprocessing of a residue. Asphaltenes were extracted from the feed and from two different products sampled at various times during the run. Results indicate that intensities for hydrocarbon classes and N-containing classes increase for the processed asphaltenes, while the abundance of S classes decreases. Similar results are found when the number of peak assignments is compared. The average double-bond equivalent (DBE) increases as a function of time on stream, which is usually associated with cyclization and aromatization reactions. However, the disappearance of molecules with low DBE as well as molecules with large DBE points to preferential cracking/ hydrogenation of molecules in both regions. A small decrease in average molecular sizes is observed as a function of time on stream. In general, molecular size distributions for different DBEs are narrower for the products than for the feed. The distributions also point to the disappearance of the largest molecules from the products in accordance with the preferential cracking previously reported for large molecules under hydroprocessing conditions. Analyses of the more abundant classes revealed trends that vary depending upon the type of heteroatoms as well as their relative amount within the molecules.
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INTRODUCTION Asphaltenes limit the efficiency of hydroprocessing of heavy oils and residua. They can adversely affect the overall rate of hydroprocessing reactions1 and act as coke precursors, leading to catalyst deactivation.2 Changes in the chemical structure of asphaltenes during upgrading are clearly linked to sediment formation.2,3 The increase in sediment content produces deposition downstream of the reactors in commercial units, reducing on-stream factors and conversion targets.4 Becaus asphaltenes are critical components during the hydroprocessing of the residue,5 the understanding of structural and compositional changes of asphaltenes upon conversion can provide important clues for the development of new technologies as well as the optimization of existing residue conversion processes. Earlier studies of changes in the asphaltene structure during hydroprocessing showed that removal of metals and sulfur decreases the molecular weight6 and H/C ratio7 and, therefore, increases aromaticity.3 These changes are mainly attributed to cracking of alkyl chains.2 Also, an increase in internal carbons in aromatic structures during hydrodemetallization experiments has been reported8 and can be attributed to aromatization of naphthenic ring systems9 as well as the condensation of hydrocarbon free radicals.10 However, polycondensation and dehydrogenation of naphthenes are largely dependent upon the operating conditions and can be minimized by high hydrogen partial pressures and hydrogenation catalysts.11,12 © 2017 American Chemical Society
Recent advances in ultrahigh-resolution mass spectrometry have significantly improved our knowledge of the compositional characteristics of crude-oil-related materials, including asphaltenes.13−17 Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enables the characterization of thousands of compounds present in these complex mixtures, giving the capability to identify compositional differences otherwise undetectable using conventional techniques. While quantification is not yet available using these techniques, this analytical technique can be used to explore differences between samples as well as to follow changes upon conversion.18−23 Asphaltene chemical structural modification during upgrading using high-resolution mass spectrometry has been studied using electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). ESI is best suited for polar components,24 while APPI can efficiently ionize polycyclic aromatic compounds whether they show basic, acidic, or neutral characteristics.16 These ionization techniques have been used to study the effects of thermal conversion20,23 as well as hydroconversion21−23 on asphaltenes. Results of these studies have shown that hydroprocessing decreases the Special Issue: 17th International Conference on Petroleum Phase Behavior and Fouling Received: September 15, 2016 Revised: December 2, 2016 Published: January 19, 2017 3409
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and the Paracell analyzer cell. An Apollo II dual ESI/matrix-assisted laser desorption/ionization (MALDI) ion source was used. Samples were analyzed using positive-ion mode APPI. The transient length of the mass spectrometric measurements was 3.3 s. Sine apodization was applied, and spectra were processed in absorption mode, resulting in a resolving power of 1 300 000 at m/z 400. The spectra were externally calibrated with arginine clusters in ESI in positive-ion mode. Spectra were single-point-calibrated during acquisition with a known mass (lock mass calibration). The final spectrum was additionally internally calibrated in DataAnalysis 4.2 (Bruker Daltonics) with a known homologous series using quadratic calibration. All root-mean-square (RMS) mass errors of the internal calibration were below 110 ppb. The RMS mass errors of the internal calibration of the measurements were on average 55 and 92 ppb. Samples were prepared by diluting them 1:100 in toluene as stock solution. The stock solution was diluted 1:200 in 50:50 CH3OH/ toluene for final spray solution (50 ppm). This solution was directly injected to the APPI source with a syringe pump at a flow rate of 10 μL/min. The ion accumulation time was 30 ms, and 200 single scans were added for the final mass spectrum. Internal mass calibration, spectral interpretation, and export of mass lists were performed using DataAnalysis 4.2 (Bruker Daltonics). The analysis of the data, including calculation of molecular formulas and relative abundances of compound classes, was performed using Composer 1.0.6 (Sierra Analytics). Elemental composition assignment was based on Kendrick mass defect sorting. A maximum mass error of 0.5 ppm and a maximum number of heteroatoms of N = 3, O = 3, and S = 3 were allowed for molecular formula calculation. Double-bond equivalence (DBE) were calculated using the standard equation.26 The isotopic peaks (13C, 34S, etc.) were calculated in the algorithm of the Composer software. Protonated species and radical cation compound classes were calculated separately in the Composer software. Weighted average intensities were calculated using both protonated species and radical cations.
complexity of asphaltenes and increases their aromaticity dramatically at high severities.21 It has also been demonstrated that nitrogen-containing species are more resistant to hydroprocessing than sulfur species.22 Additionally, data obtained on the basis of the comparative study of maltenes and asphaltenes before and after conversion have also indicated that virgin asphaltenes are composed of a mixture of island- and archipelago-type molecules.23 All of these studies have shown the great potential of high-resolution mass spectrometry to help in the understanding of the conversion of the heaviest materials in petroleum. This technique is particularly relevant to understand the composition of fractions where conventional analytical techniques are of limited use and reactivity is influenced by mutual inhibition and other effects associated with the high complexity of these materials. In this paper, the analyses of asphaltenes extracted from a feed and their hydroprocessed products are carried out using APPI. The goal of this work is to study the evolution of asphaltene chemical composition as a function of time on stream (length of time that the catalyst is in use). In particular, our main interest is to provide data that can help to better understand the complex reactivity of asphaltenes.
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EXPERIMENTAL SECTION
Materials. Asphaltenes were extracted from a feed, and its two products were obtained during hydroprocessing using a modified ASTM D6560 test.25 In this version, a 1:20 sample/n-heptane ratio was used, and the blend was filtered at 80 °C. The precipitated material was washed using hot heptane before drying and weighing. The last traces of precipitate were removed from the digestion beaker using chloroform and then recovered. Comparisons of this modified method and the original method have shown results that are within the stated reproducibility of ASTM D6560. Table 1 shows the elemental
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RESULTS AND DISCUSSION Class Distributions of Asphaltenes as a Function of Time on Stream. Class distribution plots are shown in Figure 1 based on weighted average intensities considering radical
Table 1. Elemental Characterization of Asphaltenes feed product 1 product 2
time (h)
hydrogen (wt %)
carbon (wt %)
0 368 968
8.03 5.92 5.98
83.01 88.08 87.89
analysis of the extracted asphaltenes. Elemental analysis was determined by a standard combustion method using a Carlo Erba model 1108 analyzer. In this method, the sample is combusted with oxygen in helium carrier gas to produce nitrogen oxides, carbon dioxide, and water as the combustion products of nitrogen, carbon, and hydrogen. Nitrogen oxides are reduced to nitrogen gas, and excess oxygen is consumed in a copper reduction tube, leaving only N2, CO2, and H2O in the carrier gas stream. These combustion products are separated and quantitatively determined with a thermal conductivity detector. Standard deviation depends upon the element: C, 0.11; H, 0.15; and N, 0.15. Sulfur was not determined for these samples. The products were obtained as follows: a vacuum residue was hydroprocessed in a continuous-flow pilot-plant unit composed of a series of two mini ebullating bed units. An alumina-supported catalyst was loaded in each reactor prior to the test. The conversion level was kept in the range of 38 + 4% (1000 °F+ fraction) by increasing the reactor temperature from 405 °C at the begin of the run to 410 °C at the end, compensating for catalyst deactivation. Liquid hourly space velocity (LHSV) and total pressure were kept constant during the test (LHSV, 0.2 h−1; total pressure, 2400 psi). Characteristics of the feed and products can be found in the Supporting Information. FT-ICR MS Analysis. The samples were analyzed using a solariX XR FT-ICR mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 12 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France)
Figure 1. Relative abundances of the classes as a function of the time on stream. Values were calculated on the basis of weighted intensities.
cations as well as protonated species generated by positive APPI. Figure 1 shows that the hydrocarbon (HC) class relative abundance increases for the processed products, while the abundance of S classes decreases. Similar results are found when the number of peak assignments is used in the comparison. For N-containing classes, increases and decreases 3410
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Figure 2. Relative carbon number distributions as a function of time on stream for the molecular formulas with (a) DBE = 10, (b) DBE = 26, and (c) DBE = 34. Values were calculated on the basis of weighted intensities.
are found depending upon the number and type of other heteroatoms (O and S) present in the molecules. The decline of S classes is the result of the relatively easy removal of sulfur during catalytic upgrading.27 In contrast, the low reactivity of nitrogen compounds27 might lead to the accumulation of them into heptane-insoluble material. In fact, hydrodenitrogenation (HDN) is considered the most difficult hydrotreatment reaction.28 Calculations of total nitrogen and sulfur contents in the asphaltenes using the mass spectrometry (MS) data indicate an increase in the nitrogen content as the reaction proceeds and a decrease in the sulfur content. These calculations also show that the average number of nitrogen atoms per molecule increased slightly from 0.78 to 0.82, indicating that nitrogen in rings basically remains unaltered and the contribution of nitrogen species coming from the processing of maltenes is small. Elemental analysis of asphaltenes from hydrotreated products obtained from diverse crude oils showed that nitrogen increases as hydrotreating proceeds.28−31 However, other data7 pointed to a stable nitrogen concentration. This discrepancy has been attributed to different feedstocks.32 Calculations based on MS data also indicate that the average number of sulfur atoms per molecule decreased around 50% (from 1.01 to 0.53). According to the MS data, average oxygen atoms per molecule also decreased, although less (from 0.88 to 0.67). Also, it is interesting to notice that the initial rate of increase/ decrease of these heteroatoms is larger than the rate at longer times (see the Supporting Information). These results are similar to those found during the analysis of asphaltenes as a
function of time on stream during hydroprocessing of Maya crude oil in a fixed bed.9 The average DBE of the asphaltenes increases based on weighted average intensities (from 20.1 to 24.2 at 968 h). In principle, this change can be attributed to cyclization/ aromatization reactions.9 However, there is a decrease in low DBE species that can be produced by hydrogenation reactions of those species with the smallest DBE (see the Supporting Information). Hydrogenation makes these molecules more soluble in the product, and as a consequence, they become part of the maltenes and disappear from the asphaltene fraction. The disappearance of molecules in this region (low DBE) can shift the maximum of the distribution toward larger DBE values without the need for cyclization/aromatization reactions. In general, a small decrease in average molecular sizes is observed as a function of time on stream. With regard to molecular weights, values decrease from 559 to 519 Da (368 h) and 527 Da (968 h) when calculated on the basis of response. This finding is in agreement with conventional characterization of asphaltenes focused on the effects of processing.6,9 A better idea of the effect of processing in molecular sizes can be found in Figure 2. In this figure, size distributions (on the basis of weighted intensities) are compared for different DBEs. In these examples, distributions shift to the left (lower C values) and are narrower in the products than in the feed. The narrowing of the distributions has been observed in previous works using highresolution mass spectrometry.23 These distributions also indicate that the largest molecules present in the feed disappear from the plots for the products. This is in agreement with the previous work that points out the preferential cracking of the 3411
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Figure 3. Comparison of the compositional space occupied by molecules that disappeared from the feed during processing to those that appeared in the products. The plot includes the carbon number and DBE distributions for both kinds of molecules.
larger molecules.33 A reduction in size and an increase in aromaticity can be consequences of breaking aliphatic chains.32 The reduction in size can be roughly estimated using the plot DBE versus C. For these estimations, each molecular species is considered to be part of a homologous series, and in each series, the species closer to the planar limit is assumed to be the beginning of the series and composed exclusively of rings.23 On the basis of these assumptions, calculations of the number of carbon atoms in alkyl chains per molecule indicated an average reduction of around five carbons during processing: from 19.4 (feed) to 14.2 (368 h) and 14.3 (968 h). These values showed agreement with previous research of hydroconversion on asphaltenes using high-resolution mass spectrometry, indicating reduction in the size of alkyl chains.23 Nevertheless, analyzing the appearance and disappearance of species in the compositional space can help with the understanding of the different reaction trends. Figure 3 shows a comparison of the unique species present in asphaltenes from the feed and in asphaltenes from the first product. These species are the species that disappear from the feed and the species that were not in the feed but appear in the product, respectively. In this plot, the species that disappeared from the feed cover the whole compositional space, whereas the species that appeared on the product cover a narrower area of the compositional space. Significant differences are found in the DBE distribution as well as in the carbon distribution. First of all, distributions are narrower for the product than for the feed. In the case of DBE values, the shift of the product distribution up indicates that the new species appearing in the product are more aromatic than those that disappear from the feed. Average DBE values for disappearing species and new species are 20.8 and 28.9, respectively. In the case of the carbon distribution, this result indicates that species with relatively small sizes as well as with large sizes disappear from the feed, but new molecules of similar small or large size do not appear in the product. This result is consistent with the tendencies already
shown in Figure 4 for different DBE values considering all of the species in the feed and products.
Figure 4. Relative abundances of species with different heteroatom contents as a function of time on stream. Values were calculated on the basis of weighted intensities.
As expected, the largest molecules also contain the largest amount of heteroatoms. In the feed, molecules with just one heteroatom have an average of 34 carbon atoms, molecules containing three heteroatoms have an average of 42 carbon atoms, and molecules with six heteroatoms have an average of 47 carbon atoms. Figure 4 shows the effect of processing clustered different species based on their heteroatom content. Molecules with a heteroatom content larger than one disappear during processing, while those with one or no heteroatoms increase. This finding is in agreement with previous reports that indicate that reaction rates of larger asphaltenes are more rapid than those of smaller asphaltenes.33,34 Class Changes. A comparison of the principal classes of the feed and products can provide information about individual 3412
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other feeds have also shown that, during thermal cracking processes below 400 °C, the cracking of alkyl chains is the predominant mechanism.8 On the basis of this evidence and the low temperature used in the experiments, the most likely explanation is that the produced molecules are mainly the result of the cracking of large HC class molecules or transformations of molecules from other classes through cracking and/or elimination of heteroatomic functionalities. However, it is important to point out that, even though the average molecule in the HC class in the products is larger than in the feed, the average number of carbon atoms in alkyl chains per molecule decreases around three carbons during processing: from 18.0 (feed) to 15.0 (368 h) and 14.1 (968 h). On the basis of these results, it can be estimated that the ring systems of the HC class molecules are larger in the products than in the feed. N and N2 Classes. The processing of heavier crude oils with a high nitrogen content has increased the interest to obtain a better understanding of HDN, which is the most difficult hydrotreatment reaction.37 In particular, it is essential to gain insight into which nitrogen-containing compounds are the most difficult to treat.38 It has been reported in the literature that all nitrogen in asphaltenes is aromatic,39 and because of this, the nitrogen content remains almost unchanged after hydrotreating. Therefore, nitrogen is likely to concentrate on asphaltenes. In fact, it has been found that the rate of the direct removal of nitrogen from asphaltenes is rather low.1 Evolution of the N class can help to understand the reaction pathways of these molecules. For N class compounds, weighted average response values for DBE and C indicate that DBE slightly increases as a function of time on stream: from 22.7 to 23.9 (368 h) and 24.0 (968 h), while the average number of carbons decreases: from 41.9 to 38.8 (368 h) and 39.1 (968 h). For N2 class compounds, the average DBE decreases slightly (from 25.5 to 24.6 and 24.4) and the average number of carbons also decreases (from 46.4 to 40.2 and 40.7). Similar increases/ decreases are also found when the unweighted averages are used. It was also found that DBE distributions for N and N2 class compounds are wider for the products, indicating a larger diversity of DBE values in comparison to the feed. The widening of the DBE distributions and the decrease in molecular size are indications of the difficulties of the denitrogenation as well as cracking of larger molecules. For an example of these findings, see the Supporting Information. For the N and N2 classes, we compared the molecules that appeared in the products at 368 and 968 h to those that disappeared from the feed. The comparison of the identified species indicates that more new molecules in these classes appear than disappear (1812 versus 648). The comparison of new molecules and the molecules disappearing reveals that the average DBE of the molecules that disappeared is larger than the average DBE of the molecules that appear for the N class (feed, 30.2; product, 28.8) and N2 class (feed, 35.9; product, 27.5), while the carbon numbers are lower for both classes (N class, 73.7 versus 51.5; N2 class, 74.1 versus 46.3). These values also lead to a larger aromaticity [DBE/(C + N)] in the new molecules in comparison to the disappearing molecules (0.55 versus 0.40 for the N class and 0.58 versus 0.48 for the N2 class). Because these molecules have lower DBE and smaller sizes, one possible source is the breaking of large molecules with more than one aromatic core. Figure 6 shows an example of this for the N class. The comparison on Figure 6 of the newly generated
changes that occur in different classes and how those changes might compare to the behavior of the total of molecules described in the previous section. HC Class. Weighted average response values for DBE and C indicate that DBE and the number of carbons increase as a function of time on stream: from 18.2 to 24.6 (368) and 25.7 (968 h) and from 35.8 to 38.8 (368 h) and 39.9 (968 h), respectively. Similar increases are also found when the unweighted averages are used, although differences between averages are smaller than in the weighted calculation. The comparison of the C number distributions (see the Supporting Information) shows that the changes in average size are largely due to the disappearance of small and large molecules, while the mode of the distribution remains almost unchanged. With regard to DBE changes, molecules with low and large DBE preferentially disappeared. Low DBE molecules are probably easily hydrogenated and become maltenes. However, in contrast, large molecules with large DBE probably disappeared as a result of cracking reactions. The number of new HC species that were not present in the feed is more than double the number of HC species that disappear from the feed (957 versus 436) (Figure 5). On
Figure 5. Comparison of the compositional space occupied by HC class species that disappeared from the feed during processing to those that appeared in the products.
average, the appearing species have larger DBEs (30.2 versus 21.0), and they are more aromatic with larger DBE/C ratios (0.33 versus 0.58). In fact, the limit slope for new molecules is 0.75 in contrast to 0.51 for the molecules that disappeared. This limit is defined as a straight line that represents the upper boundary for the molecules in the plot DBE versus C.35 The maximum value of this slope is 0.90 for hydrocarbons,36 corresponding to peri-condensed aromatic structures. For catacondensed structures, this value is around 0.75, while for saturated cyclic compounds, the value is 0.25.13 Figure 5 shows a comparison of the compositional spaces occupied by the new produced molecules and those that disappear from the feed. The differences observed between the new molecules and the molecules originally in the feed can be explained as the result of cracking of alkyl chains that left the aromatic core exposed or the product of cyclization and aromatic condensation that leads to coke formation.9 In a previous study, only dealkylation of side alkyl chains for Maya asphaltenes has been reported for temperatures lower than 420 °C, while aromatic condensation was observed only at higher temperatures.28 Experiments using 3413
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compounds indicates that hydrotreatment of asphaltene compounds are dominated by hydrogenation and cracking reactions and not by direct HDN of the nitrogen ring.43 However, because there is a significant difference in reactivity between pyridinic and pyrrolic asphaltene molecules, it can be assumed that the presence of pyridinic moieties somehow affects the reactivity in a different fashion than pyrrolic moieties. S Class. Sulfur in petroleum residues is present mostly in thiophenic form as part of the complex aromatic structures.1 Figure 1 shows that the relative response of this class decreases significantly during processing. Weighted average response values for DBE and C indicate that DBE increases as a function of time on stream: from 20.5 to 25.2 (368 h) and 24.3 (968 h), while the average number of carbons remains practically constant: from 40.8 to 39.5 (368 h) and 40.3 (968 h). These results indicate that the S class became more aromatic and that the distribution of the species remained centered at the same size after processing (both effects can be appreciated in the Supporting Information). It was found that the 1244 species that were present in the feed were not present in the products, while 255 new peaks appear in the products for this class. This finding supports the already known fact that sulfur compounds have a high reactivity.27 Examination of these different species shows that the species that disappear from the feed occupy the whole compositional space, while the new species occupy just the center, as shown in Figure 7. The position of the new species in
Figure 6. Comparison of the compositional space occupied by N class species that disappeared from the feed during processing to those that appeared in the products.
molecules and the molecules that disappear shows clearly that the disappearing molecules are preferentially the largest. These molecules likely cracked during processing. It is unlikely that they went under denitrogenation directly. In this figure, the appearance of a number of species with low DBE ( HDO > HDN.47 For the O and O2 classes, molecular sizes increased as well as DBE during processing. The average C number increased for the O class from 21 to 35 (368 h) and 36 (968 h), respectively. For the O2 class, the increase is smaller: from 30 to 34 (368 h) and 35 (968 h). Average DBE changed from 11 to 22 (368 h) and 23 (968 h) for the O class. For the O2 class, average DBE increased from 15 to 19 (368 h) and 22 (968 h). We found that species that disappeared from both classes in the feed are mainly low-size−low-DBE molecules, as shown in Figures 8 and 9, in contrast with the appearing molecules with
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CONCLUSION Detailed characterization use of the asphaltenes using ultrahighresolution FT-ICR MS during hydroprocessing indicates the following under the conditions of this process: Asphaltenes become more aromatic likely from the breaking of alkyl chains, showing an estimated reduction of five carbon atoms in alkyl chains per molecule. In general, molecules with heteroatom content larger than one tend to disappear during processing, while those with one or no heteroatoms increase. Different classes showed distinctive patterns regarding reactivity. N, N2, and HC large molecules showed larger reactivity than smaller molecules. In contrast, small molecules show the largest reactivity for O and O2 classes. S class molecules react somewhat equally independent of size. All of the studied classes showed a significant increase in aromaticity. Nitrogen- and oxygen-containing species are the most resistant to processing, and sulfur species are the most reactive species. Pyridinic species present in the N class in the asphaltenes of the feed are more easily converted than pyrrolic species.
Figure 8. Comparison of the compositional space occupied by O class species that disappeared from the feed during processing to those that appeared in the products.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02363. Feed and product characterization (Table 1), reconstructed APPI FT-ICR mass spectra of asphaltenes in the feed (Figure 1), product at 368 h (Figure 2), and product at 968 h (Figure 3) of radical cation species and feed (Figure 4), product at 368 h (Figure 5), and product at 968 h (Figure 6) of protonated species, sulfur and nitrogen contents in asphaltenes as a function of time on stream (Figure 7), asphaltene DBE distribution as a
Figure 9. Comparison of the compositional space occupied by O2 class species that disappeared from the feed during processing to those that appeared in the products.
larger sizes and larger DBEs. These changes produced the increases in size and DBE mentioned in the previous paragraph. Additionally, for the O class, 238 species disappeared (around 20% of detected O species) from the feed and 726 new species appeared in the products. In balance, these numbers indicate that the HDO of the O class species is low because of the 3415
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function of time on stream (Figure 8), carbon number and DBE distributions for the HC class (Figure 9), N2 class (Figure 10), and S class (Figure 11) as a function of time on stream, and class distribution for protonated species (Figure 12) and radical cation species (Figure 13) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Estrella Rogel: 0000-0003-0834-2526 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Estrella Rogel thanks Dr. C. Ovalles and Dr. Michael Moir for their valuable comments.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b02363 Energy Fuels 2017, 31, 3409−3416
