Characterization of Heavy Petroleum Fractions by NMR Techniques

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Chapter 4

Characterization of Heavy Petroleum Fractions by NMR Techniques Ajit Pradhan,* Cesar Ovalles, and Michael Moir Petroleum and Materials Characterization, Chevron Energy Technology Company, Richmond, California 94802, United States *E-mail: [email protected].

NMR spectroscopy continues to be a useful tool to characterize heavy petroleum fractions. Over the last few years, advancements in three techniques have opened new avenues to characterize heavy petroleum fractions and catalysts and dramatically improve our understanding of these important feedstocks: 1) incorporation of clay-gel adsorption chromatography for the separation of a heavy petroleum fraction into paraffin, aromatic, and polar fractions has enabled the extension of the Brown-Ladner method to narrower fractions; 2) Diffusion Ordered Spectroscopy (DOSY) continues to be an attractive tool to characterize heavy petroleum fractions and asphaltenes due to dependence on molecular properties such as size, shape, mass, and charge which are not included in conventional spin interactions; 3) availability of advanced instrumentation has enabled extending the application of Dynamic Nuclear Polarization (DNP) towards higher magnetic fields and has opened up new frontiers to characterize heterogeneous catalysts and heavy petroleum fractions.

I. Introduction Understanding the entire composition of a heavy petroleum fraction is of paramount importance to develop better process models, to prevent catalyst fouling, and to avoid the formation of off-spec products. However, knowing full compositions of the heavy petroleum fractions is often very challenging © 2018 American Chemical Society Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

because of: 1) the inherent complexity of these matrices associated with those heavy petroleum fractions, and 2) limitations of conventional measuring techniques. Among all the analytical techniques, Nuclear Magnetic Resonance Spectroscopy (NMR) has been extensively used in the compositional analysis of petroleum (1–4). Several researchers have used high-field NMR Spectroscopy to characterize heavy petroleum fractions (3–7). However, the NMR spectra of crude oils are very complex, making the assignment of different signals difficult. The literature associated with NMR techniques for the characterization of heavy petroleum fractions was comprehensively reviewed and presented as a chapter by Boduszynski and Altgelt in their book “Composition and Analysis of Heavy Petroleum Fractions (2).” More recently Edwards provided an overview of recent developments of NMR technologies in the petroleum industry (3). However, during the last seven years, there has not been any literature review that covers the latest developments in NMR technologies. In this chapter, we cover three NMR techniques that have the potential to significantly improve our understanding of heavy petroleum fractions.

II. An Extended Brown-Ladner Method One methodology commonly used to characterize heavy petroleum fractions is to use the combination of 1H and 13C NMR to determine structural parameters. This method was first reported by Brown and Ladner in 1960 (8). The authors used the combination of 1H NMR and elemental analysis for the purpose of calculating structural parameters, and the original correlation was later extended by various researchers (9–13). In their book, Boduszynski and Altgelt presented a comprehensive review of various approaches used to determine average structural parameters (2). Brown and Ladner’s parameters provided valuable information about the structural data of petroleum fractions and had been widely used by researchers working in the petroleum industry. However, the description of heavy petroleum fractions by average molecular parameters is limited by many assumptions resulting from the inability of NMR to determine the true molecular distribution of the entire petroleum mixture (3). Such a situation happens because 13C NMR cannot differentiate free paraffin molecules and alkyl carbons on substituted aromatic molecules with certainty. Another major hurdle is the determination of naphthenic carbon content and the number of shared aromatic-naphthenic carbon atoms at the boundary between such ring systems. Finally, it is difficult to assess the distribution of sulfur and nitrogen among various functional groups using NMR spectroscopy. In the heavy petroleum fraction, such as the vacuum gas oil (VGO) range, the large diversity of molecular structures, functionality, and the increased alkyl substitutions makes it challenging to separate fractions into compound groups and classes. Representing a mixture of such diverse compounds as a single average molecular structure is unrealistic and unhelpful. As suggested by Boduszynski, to overcome various limitations presented earlier, we extended the Brown-Ladner method to narrower fractions rather than broader crude oil fractions such as VGOs (14). In doing so, we used the 74 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

ASTM D2007 method to separate VGO samples into hydrocarbon type fractions (15). The typical average structural parameters, as determined by 1H and 13C NMR spectroscopy, and the workflow for VGO samples using ASTM D2007 are shown in Table 1 and Figure 1, respectively. This test method is used to classify oil samples with an initial boiling point of at least 200°C into hydrocarbon types of saturates, aromatics and polar compounds and uses clay-gel adsorption chromatography for the separation and the recovery of the fractions. ASTM D2007 is based on preferential adsorption of polar compounds on clay, while aromatics are adsorbed on silica gel. At the same time, as saturated hydrocarbons have a lower affinity for both clay and silica, they are eluted with n-pentane. Aromatic and more polarizable hydrocarbons are recovered from clay and silica adsorbents respectively using appropriate solvents. The separated components are determined gravimetrically after the evaporation of the solvents. Then, the fractions were characterized by NMR spectroscopy and atmospheric pressure photoionization mass spectrometry (APPI-MS) to determine their structural parameters (16). APPI in positive ion mode can efficiently ionize polycyclic aromatic compounds and can be used to determine double bond equivalent (DBE) distributions and the average carbon number of the fractions. This approach not only could be used to characterize heavy petroleum fractions but also be used to monitor hydroprocessing feedstocks. As shown in Table 2, hydroprocessing efficiency for a given catalyst can be evaluated by monitoring the structural parameters of the aromatic and polar fractions of the feed and the products. Surrogate molecules representing feed and products are also shown in the table. The use of surrogate molecules has previously been reported in the literature to generate a molecule that has functionalities similar to the structures that are found in the sample mixture (17).

Figure 1. Workflow to determine average molecular structural parameters for VGO samples. The typical average structural parameters, as determined by NMR spectroscopy and atmospheric pressure photoionization mass spectrometry (APPI-MS). 75 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Table 1. Typical Average Structural Parameters As Determined by 1H and NMR Spectroscopy. (Reproduced with permission from Reference (13). Copyright 2017 American Chemical Society.)

13C

Our study has shown that a combination of the extended Brown and Ladner method on the separated fractions using the D-2007 methodology and use of surrogate molecules to represent average molecular structures of the fractions can effectively be used to monitor structural changes resulting during the hydroprocessing (18). Although, high-field 1D 1H and 13C NMR spectra provide information about the types of proton species, give an average chain length for the hydrocarbon aliphatic chains, and provide a distribution of aromatic carbons, a non-invasive analytical method is needed that can identify molecular components of mixtures and simultaneously characterize the sizes of aggregates and other structures present in complex samples such as heavy petroleum fractions. Commonly used light-scattering experiments cannot resolve such mixtures, and chromatographic methods may disrupt fragile aggregates such as asphaltenes. 76 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Table 2. Monitoring Hydroprocessing of Vacuum Residue feedstock using Average Structural Parameters

77 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

III. DOSY (Diffusion Ordered Spectroscopy) The introduction of a second frequency dimension (2D), through the use of pulse sequences having two independent precession periods, has dramatically improved the resolution of complex NMR spectra. 1H Diffusion Ordered Spectroscopy (DOSY) is one such 2D NMR method that correlates the translational diffusion coefficient, D1, with a resolved 1H NMR spectrum (19). In DOSY NMR, the second NMR dimension depends on molecular properties such as size, shape, mass, and charge that are not included in conventional spin interactions. The DOSY experiment was first reported by Johnson et al. in the early 1990s and the authors obtained the diffusion spectra using a FT pulsed field gradient method (20). The data sets are collected as the gradient pulse areas and are incremented through approximately 50 values to generate diffusion spectra at each chemical shift using a data inversion program. Over the years, DOSY has been extensively used to analyze compositional mixtures based on the differences in the diffusion coefficients of the individual components; however, applications of DOSY reported in the literature to characterize heavy petroleum fractions are limited. In this section, we will review how, due to its sensitivity to both molecular weight and structure, DOSY offers another valuable method for the analysis of complex mixtures such as heavy crude oils and fractions. The application of DOSY to the characterization of diesel fractions was first reported by Kapur et al. (21), however, the study was not conclusive. The first comprehensive study of diesel fractions and asphaltenes was reported by Durand et al. (22) They used DOSY NMR to demonstrate that different types of intermolecular interactions were observed depending upon the sample concentration. In more concentrated samples (more than 10 wt.% in toluene), an aggregation process resulted in the drop of the diffusion coefficients of both the solvent and the solute. The authors used diesel samples to study the potential of the DOSY NMR technique to analyze petroleum samples and concluded that some monoaromatic rings are connected to long alkyl chains, some two aromatic rings are connected to smaller chains, and three aromatic rings in lower proportions are connected to shorter chains. Similarly, the 1H DOSY spectrum of an asphaltene sample reveals the presence of highly substituted polycyclic aromatic hydrocarbons that are connected to long alkyl chains. More recently, Korb et al. did a comparison of 1H DOSY spectra of crude oils with and without asphaltene (23). Although the diffusion coefficients were only measured for the aliphatic protons, the experiments revealed that, in the presence of asphaltenes, two populations of hydrocarbons were identified by distributions of translational diffusional coefficients for CH2 and CH3 groups. These coefficients were separated by a factor 2.64 and 3.0, respectively (see Figure 2). However, in the absence of asphaltenes, the separation factor is reduced to 1.25 for CH2 and 1.5 for CH3 DOSY peaks. The authors concluded that the slowest distribution could be due to a small portion of hydrocarbons interacting with the asphaltene nanoaggregates. Similarly, the fastest diffusion distribution could be due to hydrocarbons moving in between the asphaltene nanoaggregates. 78 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

The residence time determined by the DOSY experiment suggests that the average radius of exploration for the 2D hydrocarbon diffusion, r2D is about 3.9 nm, which is in the same order of magnitude as reported in the literature using SAXS and SANS techniques.

Figure 2. Two-dimensional DOSY NMR spectra that correlate the 1H NMR spectrum at 300 MHz and the distributions of translational diffusion coefficients obtained through a pulsed field gradient spin echo method with bipolar pulsed gradients. Results obtained on sample 1 at (a) 294 K and (b) 313 K and at (c) 294 K for sample 2. (Reproduced with permission from Reference (23). Copyright 2013 American Chemical Society.) 79 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Characterization of heavy petroleum fractions and especially asphaltenes is still a challenge and further research is needed to determine size, configuration and flocculation of asphaltenes in their native crude oils. Due to the optical opacity of the samples that limits use of many analytical techniques, NMR spectroscopy and especially DOSY has the potential to help us better understand these phenomena in future.

IV. Dynamic Nuclear Polarization (DNP) Although NMR has been extensively used to characterize petroleum feedstocks, products, and catalysts, a major limitation of NMR is its intrinsically low sensitivity, especially for nuclei with low natural abundance and/or low γ value. To a certain extent, operation at higher magnetic fields has helped to address this issue. However, a more effective and commonly used technique is polarization transfer from high- γ nuclei with high natural abundance to low- γ heteronuclei (24). Owing to its larger gyromagnetic moment, unpaired electrons reach high polarization at modest field strengths and temperatures below 10K. Dynamic Nuclear Polarization (DNP) achieves 102 to 103-fold improvements in sensitivity by transferring the large polarization of unpaired electrons to nearby nuclei via microwave irradiation of electron-nuclear transition (25, 26). The first DNP experiment was performed in the early 1950s (27), but until recent years, the technique had limited availability due to formidable engineering challenge in developing suitable hardware (28). However, over the past two decades, availability of advanced instrumentation has enabled extending application of DNP towards higher magnetic fields. In this section, we will present several studies reported in recent times carried out using DNP to characterize catalysts and heavy petroleum fractions. All mechanisms of DNP proceed by transferring polarization from electrons to nuclei following irradiation at or near the frequency of electron-nuclear transitions. However, it is carried out differently in liquid and solid samples. In the solid state, polarization is commonly transported to other nuclei via spin diffusion. Alternatively, in the liquid state, translational diffusion is commonly used for polarization transfer (29). In this sense, DNP measurements are carried out as 1) driven by the Overhauser Effect (OE) where the entire process is carried out in the liquid state; 2) in the solid-state, driven by the cross effect using high-power microwave sources in conjunction with magic-angle spinning carried out at low temperatures; 3) performed by polarization ex-situ in the liquid state with subsequent transfer to the NMR magnet to acquire the spectrum. In this section, we provide an overview of recent efforts to explore DNP for possible applications in the petroleum industry, especially in the characterization of heterogeneous catalysts and heavy petroleum fractions. The development of new heterogeneous catalysts requires detailed characterization of active sites, and solid-state NMR has emerged as a powerful tool to provide key molecular level information about the structure and dynamics of the catalyst itself (30–32). However, hours or even days of signal averaging time is often needed to obtain a spectrum with an adequate signal-to-noise ratio. 80 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Dramatic increases in the signal intensity of surface species have been achieved by recording solid-state NMR spectra at high magnetic fields (5.0-18.8 T) combined with the DNP carried out at low temperatures (100 K or lower). Griffin et al. were the first to demonstrate high field DNP with MAS experiment to obtain a 23-fold signal enhancement (33). A DNP-enhanced 1H-driven spin- diffusion pulse sequence used by them is presented in Figure 3. They also used a gyrotron microwave source operating at 140 GHz for a DNP application to carry out 1H-13C CP-MAS and two-dimensional 13C-13C correlation spectroscopy experiments. Over the years, enhancement of signals of heteronuclei such as 13C, 15N, 27Al, 29Si, 111Cd, etc. has been reported.

Figure 3. DNP-enhanced 1H-driven spin- diffusion pulse sequence. In this example, microwave irradiation (139.66 GHz from the gyrotron) polarizes 1H (211.62 MHz), and the enhanced polarization is transferred to 13C via cross polarization, CP. (Reproduced with permission from Reference (33). Copyright 2002 American Chemical Society.)

Coperet et al. recently summarized how DNP could be used to improve the sensitivity of NMR, and make various challenging 1D/2D NMR experiments feasible to help determine the structures of surface sites (34). For example, DNP allows for the acquisition of 15N NMR spectra at natural isotopic abundance, distinguishing organic ligands on the surface from the ligands on the wall in the periodic mesoporous organosilicates (Figure 4A). Additionally, a 27Al-29Si heteronuclear correlation experiment 2D INEPT was carried out to reveal the connectivity between tetrahedral Si and Al atoms and the existence of pseudo-bridging silanols in the aluminosilicate sample (Figure 4B). As illustrated in the third example, the 1H-X (29Si or 13C) correlation spectroscopy could be effectively used to study surface-ligand interactions in hybrid materials (Figure 4C). In another example, Cd atoms on the surface were differentiated from those in the bulk Cd atoms of CdSe quantum dots using 111Cd-13C correlation (Figure 4D). It is also possible to determine the coordination environment of Sn-beta zeolite using 119Sn NMR and DFT calculations (Figure 4E). Similarly, DNP made the implementation of a J-based 2D INADEQUATE experiment feasible (Figure 4F). Thus, as illustrated by these examples, DNP has made solid-state NMR an ideal tool to determine the structures of active sites in heterogeneous catalysts. 81 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Figure 4. DNP-enhanced spectra of various samples; (A) 1D 15N CP-MAS spectra of IrCp*-ppy-PMO and ppy-PMO, (B) 2D 27Al-29Si scalar and dipolar based refocused INEPT spectra of aluminosilicate with varied Al/Si ratios, (C) 2D 1H−29Si HETCOR spectra of Mat-ImR and Mat-ImF, and (D) 2D 111Cd-13C D-HMQC spectrum of CdSe quantum dots capped with13C-1-oleate ligands. (E) DFT-calculated T6 Sn closed-site structure, and experimental and simulated 119Sn CSA. (F) DNP-enhanced 2D refocused INADEQUATE spectrum of silica-supported tungsten TBP metallacyclobutane. (Reproduced with permission from reference (34). Copyright 2017 American Chemical Society.) (see color insert)

The only method available to carry out polarization transfer in the liquid state is by Overhauser DNP. Although crude oil is a suitable substance for attempting selective enhancement of spins in either maltenes or aromatic tracer molecules, application of DNP reported in the literature to characterize crude oil or heavy petroleum fractions is still very limited. Hurlimann et al. recently reported DNP equilibrium enhancement of a crude oil with 13% asphaltene content on the 1H NMR signal of the sample and similarly on the 19F NMR signal of tracer molecules (35). He concluded that the strong signal enhancement in DNP experiments of crude oil doped with tracer molecules is a consequence of either or both the nuclear-electron interaction with the asphaltenes’ radical component or trapping of maltenes within asphaltene aggregates in crude oils. DNP has also been used to study intermolecular spin-spin interactions between nuclear spins of hydrogen in the solvent medium and free electron spins in the asphaltene micelles. 82 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Similarly, Kirimli et al. studied interactions between unpaired electrons of the asphaltene micelles and the hydrogen nuclei of the solvent molecules and concluded that the interaction is dipolar in character (36). The estimated DNP parameters reveal the role solvents play in the aggregation of asphaltenes. As the availability of the DNP experimental set-up increases, we expect a dramatic increase in the use of DNP for the characterization of asphaltenes and heavy petroleum fractions in the near future.

Summary Characterization of heavy petroleum fractions, and especially asphaltenes, is still a challenge; however, recent advancements in NMR technologies provide new avenues to improve our understanding of these complex hydrocarbon mixtures. A combination of chromatographic column separation into polars, aromatics, and saturates, and average molecular structures derived using NMR Spectroscopy provides an excellent tool to characterize heavy petroleum fractions and monitor refining processes such as VGO Hydroprocessing. Due to the optical opacity of the heavy petroleum fraction samples that limits the use of many analytical techniques, DOSY has a potential to provide valuable information on molecular properties such as size, shape, mass, and charge that are not included in conventional spin interactions. Without any doubt, DNP applications have started to open new frontiers to characterize surface sites in heterogeneous catalysts with atomic-level precision. However, applications reported to characterize heavy petroleum fractions are very limited.

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