Oligomerization of Light Olefins to Gasoline: An Advanced NMR

Aug 9, 2016 - Using multiple 1H and 13C NMR spectroscopic techniques, we have investigated the C8 cut from samples of liquid products obtained by the ...
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Oligomerization of Light Olefins to Gasoline: An Advanced NMR Characterization of Liquid Products Christopher P. Nicholas,*,† Linda Laipert,‡ and Sesh Prabhakar*,‡ †

Exploratory Catalysis and Materials Research, ‡Advanced Characterization, Honeywell UOP, 25 East Algonquin Road, Des Plaines, Illinois 60017, United States S Supporting Information *

ABSTRACT: Using multiple 1H and 13C NMR spectroscopic techniques, we have investigated the C8 cut from samples of liquid products obtained by the oligomerization of light olefins over solid phosphoric acid (SPA) and MTW zeolite. The carbon species present, CHn (n = 0−3), were identified by DEPT NMR and quantified using inverse-gated decoupled 13C NMR spectra. The olefinic protons and carbons as well as the types of olefins were quantified. 13C NMR shows that the amount of methyl carbons is similar for both catalysts but that methylene and methyne carbon distributions are different. The product obtained over MTW catalyst showed higher quantities of quaternary olefins, likely from type V tetrasubstituted olefins, compared to that over SPA catalyst. In addition, 2D NMR has also been attempted to understand the proton and carbon connectivities and confirmed the presence of type I and II olefins.

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INTRODUCTION The oligomerization of propene- and butene-containing feedstocks to gasoline is commonly known as catalytic condensation and has been in use for over 80 years since Ipatieff’s discovery.1−4 As first applied, the catalytic condensation process was utilized to make economic use of the light olefinic byproduct gases derived from thermal processing of crude oils and was the first high-octane olefinic motor fuel.5 This basic application was later extended to the processing of propylene and butylenes derived from fluid catalytic cracking operations, a process still in use today.6 Traditionally, the catalyst for this transformation has been the very inexpensive catalyst solid phosphoric acid (SPA),1,4 which is synthesized by mixing clay and phosphoric acid, then extruded.7 SPA is an excellent catalyst for oligomerization, particularly with regard to selectivity shown to gasoline. Zeolites are a class of microporous 4-connected silicate or aluminophosphate networks, of which more than 230 are currently known8 and have proven to be highly successful catalysts for conversion of hydrocarbons to valuable products.9 Due to the difficulty with which SPA is unloaded from commercial reactors, its limited lifetime, and potential corrosion issues caused by H3PO4 released during the reaction, many zeolites have been investigated as potential replacement catalysts for SPA.10−14 These include the well-studied MFI materials,15−18 used in the commercially applied MOGD process, where a key step in the reaction pathway is the oligomerization of ethene and propene to fuels.19,20 We have been working recently to control the oligomer product molecular weight and have discovered promising catalyst compositions21,22 comprising MTW, a one-dimensional 12-membered ring zeolite.23,24 Characterization of the product © 2016 American Chemical Society

formed over these catalysts is essential to determining the suitability of a new catalyst to replace SPA. While boiling point distribution and octane number are relatively easily measured, these methods do not give in-depth molecular information about the desired product. Prior work with GC and GC-MS showed that due to the olefinic nature of the product many peaks coelute in the GC;25,26 therefore, a solution often utilized is to hydrogenate the olefins to the corresponding paraffins.27,28 This approach simplifies the chromatographic analysis, but information regarding the olefin substitution is lost. Recent work with GC×GC methods has shown significant advances in the ability to characterize complex hydrocarbon mixtures,29−31 although method development time frames can be long.31,32 Another powerful analytical tool available to a petroleum chemist is nuclear magnetic resonance spectroscopy. NMR is nuclear-specific and allows the quantification of proton and carbon species. NMR spectroscopy has been routinely employed to partially elucidate the composition of complicated mixtures such as straight run gasoline, kerosene, diesel, and heavy oils.33−35 Paraffins, olefins, oxygenates, and aromatics have distinct chemical shifts, and the understanding of how chemical shifts map to molecular structure in the complex mixtures present in gasoline is continuously being refined. In addition, spectral editing, polarization transfer experiments, and multidimensional NMR techniques in suitable cases allow partial separation of overlapping peaks and allow identification of species not possible in regular 1D NMR.36,37 Here, in this Received: Revised: Accepted: Published: 9140

April 25, 2016 July 26, 2016 August 9, 2016 August 9, 2016 DOI: 10.1021/acs.iecr.6b01591 Ind. Eng. Chem. Res. 2016, 55, 9140−9146

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Industrial & Engineering Chemistry Research

respectively. The samples were dissolved in CDCl3, and spectra were obtained using a 5 mm broad band probe, and TMS was used as an internal reference. Proton spin−lattice relaxation times (t1) were measured experimentally using an inversion recovery sequence. The longest t1 value found was 0.3 s. The 1 H NMR spectrum was acquired by using 13.5 μs pulses and 15 s delay between scans, and the total number of scans acquired was 128. Quantitative 13C NMR spectra were obtained by adding small amounts (0.05 mmol) of chromium acetylacetonate to the sample and using an inverse-gated pulse sequence.39 All 13C spectra were acquired by using 11.3 μs pulses and 15 s delay between acquisitions. A delay of 15 s is sufficient to provide quantitative intensities, and intensities did not change with increases in delay time. The number of scans was 4096. Identification of the number of protons attached to carbon was done by employing DEPT (distortionless enhancement by polarization transfer).40,41 In addition, standard 2D NMR techniques such as homonuclear correlation (1H−1H correlation spectroscopy (COSY)) and 1H−13C heteronuclear correlation experiments (heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond coherence (HMBC)) in inverse mode were also carried out for both the samples.40,41 All the 2D experiments were carried out using gradient enhanced sequences. Typically 256 t1 points, 32 scans, and a delay of 2 s were used. The number of t2 points was 4096. The data were zero-filled to 8 K × 1 K before Fourier transformation. The spectrum was presented as magnitude mode in the indirect dimension.

paper, we investigate the C8 cuts of the oligomerization products obtained using SPA and MTW catalysts by quantitative 1H, 13C, and spectral editing techniques to better characterize and understand the oligomer products produced by the two catalysts.

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EXPERIMENTAL SECTION

Generation of Liquid Products. Catalytic experiments were carried out in the liquid phase by introducing C3/C4 feed into a 7/8 in. ID steel reactor held at 500 psig with an online dual column Agilent 6890GC. Carbon number ranges were set with an n-paraffin standard. Conditions were chosen to compare with standard feed rates to oligomerization units, which run from 0.5 to 5 WHSV at temperatures of about 150− 220 °C. The test was a 3-block test: 12 h at 2.5 WHSV and 150 °C, 12 h at 2.5 WHSV and 170 °C, and 16 h at 1.0 WHSV and 170 °C. Conversion ranged between 50 and 70 wt %. After a 40 h run, the downstream product charger was emptied, feed alkanes carefully evaporated, and the liquid product then fractionated by spinning band distillation. A cut of the liquid product was taken from a boiling point of 99 °C (n-heptane) to 125 °C (n-octane) to separate the C8 product. The C3/C4 feedstock was supplied as a synthetic blend containing 25 mol % propane, 28.6% propene, 10% isobutene, 18.7% isobutane, 3.2% 1-butene, 8.8% n-butane, and 5.6% 2-butenes. Typically, the feed to oligomerization units comes from the C3/C4 stream off of a FCC unit. While an FCC “wet gas” stream rarely contains the exact concentrations used herein, the ∼50% olefin concentration and C3/C4 ratio are in the range of those found commercially. In addition, the exact concentrations in the stream at each refinery tend to vary as the feedstock to the FCC unit and operating conditions vary. Therefore, the blend was made to simulate this typical refinery “wet gas” stream. One run was performed with SPA-2 obtained from UOP as the catalyst. MTW zeolite was synthesized by the literature procedure to yield a phase pure sample at SiO2/Al2O3 = 36.38 The MTW zeolite was then extruded to yield 1/16 in. cylinders of 5 wt %/95 wt % MTW/Al2O3, dried, and calcined at 550 °C. The surface area was 235m2/g, with a micropore volume of 0.009 mL/g and total pore volume of 0.214 mL/g by N2 BET. NMR Characterization of Liquid Products. The C8 cut samples of the liquid products obtained using MTW and SPA catalysts were characterized by NMR spectroscopy. These two product samples were designated as MTW-C8 and SPA-C8, respectively. An overview of the techniques utilized and the information obtained is shown in Table 1. 1H and 13C liquidstate NMR spectra of the samples were collected by employing a Bruker Avance Spectrometer operating at 11.74 T. The 1H and 13C NMR frequencies were 500.1317 and 125.7715 MHz,

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RESULTS AND DISCUSSION

Conversion and selectivity to product by carbon number during oligomerization over SPA and MTW under the three conditions utilized are shown in Figure 1. Conversion over SPA and MTW is essentially equivalent at each of the three conditions, ranging from 55 to 67 wt %, with conversion increasing as temperature or contact time increases. At equivalent conversion levels, SPA always produces a higher yield of codimer C7 product than MTW, with MTW producing higher yields of heavier C12+ products. To obtain a better understanding of speciation within specific cuts, the oligomerization products obtained over SPA or MTW were fractionated by spinning band distillation and characterized by simulated distillation42 as well as octane number. Of the total product, 97% produced over SPA boiled prior to 225

Table 1. NMR Techniques Utilized in This Paper and the Information Obtained Regarding the Oligomerized Products technique

information obtained

H 1D NMR 13 C 1D NMR DEPT 45 DEPT 90 DEPT 135 COSY HSQC HMBC

quantitation of all H species speciation by olefin type quantitation of all C species identification of quaternary olefin species by absence identification of CH species identification of CH2 species by inversion type I, II olefins, methyl branching 1 H−C−C−1H bonding alkyl branching, substituted olefins

1

Figure 1. Conversion of C3/C4 feed and selectivities to product carbon number ranges over SPA and MTW under the three conditions used in generating the liquid product. 9141

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Figure 2. Boiling point distribution as a function of the mass of the gasoline fraction (BP = 50−225 °C) of the oligomerization product produced over SPA (black squares) and MTW (blue diamonds) catalysts. The inset shows the boiling point distribution as a function of the total mass of the C8 fractionated product (BP = 99−125 °C) over both catalysts.

Figure 3. 1H NMR spectrum of (a) MTW-C8 and (b) SPA-C8 oligomerization products. Olefinic region is shown in the inset.

°C, a typical end point for gasoline,43 whereas 84% of the total product produced over MTW catalyst was gasoline. The boiling point as a function of mass percent of the gasoline fraction of the samples is shown in Figure 2. Similar boiling point distributions are observed over SPA and MTW catalysts within the 50−225 °C range, though it is clear that MTW catalyst produces a slightly higher molecular weight product than SPA. Analysis of the gasoline product showed equivalent high octane numbers (96RON and 83MON for SPA product; 97RON and 82MON for MTW).44 The oligomerization products were additionally fractionated by spinning band distillation to simplify the more detailed product speciation analyses. The product boiling between 99 and 125 °C will contain primarily C8 compounds. The C8 compounds include not only primary oligomerization products, such as 3,4-dimethylhexene from oligomerization of 1-butene or 2-butene with 2-butene, but also isomerized (both double bond and skeletal) C8s and cracked heavy product. SPA-C8 and MTW-C8 contain equivalent boiling point distributions as shown in the inset of Figure 2 and are derived from essentially equivalent conversion (Figure 1); hence, product distribution differences are primarily due to catalyst differences. The SPAC8 and MTW-C8 samples were then further investigated by both 1H and 13C NMR. 1 H NMR Spectra. 1H NMR spectra of the two samples, MTW-C8 and SPA-C8, are shown in Figure 3. The chemical shift ranges for aliphatics, olefins, and aromatics are wellestablished in the literature.45,46 The aliphatic region appears at 0.8−3 ppm. We observe peaks in the methyl, methylene, methyne, and olefinic regions. The inset in each spectrum corresponds to the olefinic region (6.5−4.5 ppm). We do not observe any signals in the aromatic region (6.5−9 ppm). The absence of aromatics in the sample has also been confirmed with GC×GC experiments, which show that branched olefins are the only product; the absence of paraffins is also noted. Methyl groups appear from 0.8 to 1 ppm, whereas methylene and methyne protons have overlapping regions between 1 and 2.02 ppm.45−47 As is clearly evident from the number of peaks

present, both spectra are complex, but the NMR spectrum of the SPA-C8 sample appears to have a significantly greater number of peaks than that of the MTW-C8 sample, indicating a much wider species distribution. A commonly used method of determining branching in complex products such as this polygasoline is to determine the CH2/CH3 (Hβ/Hγ) ratio from the proton spectra.47 This ratio is 1 for MTW and 0.8 for SPA, using the following chemical shift regions: Hα (3.8−1.8 ppm), Hβ (1.8−1.03 ppm), and Hγ (1.03−0.4 ppm). The calculated ratio can be questioned due to severe overlap and broadening of peaks in the proton spectra because of the complexity of the sample and its olefinic nature (vide inf ra). Both samples contain primarily aliphatic protons; therefore, the amounts of olefinic protons obtained from the integrated intensities are low. The olefinic proton region is 4.5% of the total 1H signal for MTW-derived product and 4.8% for the SPA-C8 samples by integration, intermediate between the 0 and 18.8% theoretically possible for a mono-olefinic C8 hydrocarbon. A tetrasubstituted olefin such as 3,4-dimethyl-3hexene would have 0% olefinic protons, whereas a 1-olefin such as 3,3,4-trimethyl-1-pentene would have 18.8% olefinic protons. While a tremendous number of species are obviously present, it is already clear that highly substituted olefins are preferred products as the average species has between 3 and 4 substituents on the double bond. The aliphatic region is more complicated due to severe overlap and multiplicities, so quantitation of different CHn species is not attempted. Assignments in the olefinic region of the 1H spectrum are better established and have a greater diversity of chemical shifts, so we have quantified five types of olefins and their total amounts present in both the samples. Table 2 shows these results. The chemical shift ranges for the five types are taken from the literature48 and are also calculated using ACD/Labs software.49 For both the MTW-C8 and SPA-C8 samples, symmetrically disubstituted type II olefins are disfavored, while trisubstituted type IV olefins are favored. In total, there are a 9142

DOI: 10.1021/acs.iecr.6b01591 Ind. Eng. Chem. Res. 2016, 55, 9140−9146

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Industrial & Engineering Chemistry Research Table 2. Types of Olefins, Chemical Shift Ranges, and Integrals from 1H NMR

percent olefinsa olefin type I II III IV V a

monosubstituted double bond disubstituted double bond disubstituted double bond trisubstituted double bond tetrasubstituted double bond

formula

chemical shift range (ppm)

MTW-C8

SPA-C8

R−CHCH2 R−CHCHR′ RR′CCH2 RR′CCHR″ RR′CCR″R‴

4.8−5.05 5.3−5.55 4.5−4.8 5.05−5.3

0.9 0.6 1.5 1.5

1.1 0.7 0.8 2.2

The remaining proton intensity is due to aliphatic protons.

Figure 4. Inverse-gated 13C NMR spectrum along with DEPT spectra of oligomerization (a) MTW-C8 products and (b) SPA-C8. From bottom to top: quantitative 13C, DEPT-45, DEPT-90, and DEPT-135 NMR spectra. Solvent peak (d-chloroform at 77.4 ppm) has been digitally removed.

identified by comparing DEPT-45 spectra with the inversegated 13C spectrum. DEPT-90 shows only CH groups. In the DEPT-135 spectrum, CH2 groups were identified by their sign change, while CH3 signal sign remains the same. Individual subspectra due to CH, CH2, and CH3 can also be obtained using appropriate scaling and subtraction techniques such as ADEPT,40 but here, we present separately the DEPT-45, -90, and -135 spectra along with the quantitative 13C spectrum as we use the DEPT data only for identification of carbon species. Using the peaks identified by the DEPT experiment, the distribution of C, CH, CH2, and CH3 species were quantified using the inverse-gated 13C NMR spectrum. The proportions of different carbon species found for the two samples investigated are listed in Table 3. As shown in Table 3, the number of quaternary carbons is significantly higher in the MTW-C8 product than in SPA-C8,

greater number of protons associated with olefins in the SPAC8 sample than in the MTW-C8 sample. 13 C NMR and DEPT Techniques. We then carried out quantitative 13C NMR of these two samples because larger chemical shift dispersion of 13C allows better identification of species and also allows us to detect quaternary carbons. Additionally, 13C NMR is a better option than 1H NMR because we can identify methyl and methylene groups readily using spectral-editing techniques. 1D 13C NMR spectra of MTW-C8 and SPA-C8 are shown in the bottom spectra of Figure 4a and b, respectively. We observe signals in the olefinic (100−160 ppm) and aliphatic regions (0−50 ppm) regions in both samples. In MTW-C8 samples, there are additionally small peaks around 50−60 ppm, which may be due to C−O bonds not present in SPA-C8 samples. This may be due to the presence of small amounts of oxidation products. The amounts of olefinic carbons observed by 13C NMR are 22 and 19% for the MTW-C8 and SPA-C8 products, respectively, close to the expected 2/8 carbons or 25% for mono-olefinic C8 species. Because we observe more than 200 peaks and also because of overlapping chemical shift regions for different carbon species, we carried out DEPT NMR to assign signals corresponding to protons connected to carbons (CH3, CH2, CH, and C). DEPT spectra for MTW-C8 and SPA-C8 are shown in Figure 4a and b (top three spectra), respectively. The spectra are shown in the following order (from bottom to top): quantitative 13C NMR, DEPT-45, DEPT-90, and DEPT-135. In DEPT experiments, peaks due to quaternary carbon are absent as there are no directly attached protons. Quaternary carbons were thus easily

Table 3. Quantitation of Species by Carbon Typea percent carbon species type

SPA-C8

MTW-C8

C CH CH2 CH3 olefinic carbon quaternary olefin/total olefin

0.8 34.8 15.6 48.8 18.9 24.8

13.4 27.5 8.1 51.0 21.6 49.0

a

Carbon types were identified via the three DEPT experiments and quantified in the inverse-gated 13C NMR spectrum.

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Industrial & Engineering Chemistry Research and the ratio of olefinic quaternary carbon to total olefin is twice that of SPA. Types III, IV, and V olefins each contain quaternary olefin carbons and are then expected to be in much higher abundance in MTW-catalyzed oligomerization product than in SPA catalyzed. 1H NMR (Table 2) showed equivalent amounts of type III and IV olefins on a combined basis for MTW-C8 and SPA-C8, so the difference observed here in 13C NMR should come from type V tetrasubstituted olefins. The CH2/CH3 ratios calculated from 13C NMR are 0.16 and 0.32 for MTW-C8 and SPA-C8, respectively. This ratio calculated from the 13C NMR spectra is significantly lower than that calculated from 1H NMR spectra. This may be due to severe overlap and broadening of the peaks in the 1H NMR and the shifting of branched methyl group peaks out of the Hγ region. Though there is considerable overlap in 13C NMR, the peaks due to methyl (both terminal and branched), methylene, and methyne were identified using DEPT NMR; hence, the ratios calculated from 13C NMR are considerably more reliable. These results clearly show that the olefins in product catalyzed by MTW are more branched than those of SPA and are also supported by the 1H NMR results, which show the percentage of olefinic protons in MTW-C8 is lower than that of SPA-C8. As evident, the acidity and shape selectivity of zeolites plays an important role in controlling the product distribution. 2D NMR. Spectral-editing and multidimensional NMR techniques have been demonstrated to be useful in identifying connectivities for different species. Due to the complex nature of the system under study, we expect severe overlap in the 2D spectrum. Nevertheless, 2D NMR has been employed to partially identify the different species present in complex systems, and experiments like COSY, HSQC, and HMBC have been previously employed for partial structural elucidation of fossil fuels, gasoline, diesel, and kerosene.50−52 These experiments are typically utilized in qualitative fashion because while progress is being made to improve quantitation, even in simple systems polarization transfer is imperfectly uniform; hence, peak heights are not directly proportional to species concentration.32 We recorded COSY spectra of both C8 cuts in an attempt to identify neighboring protons. Gradient-enhanced 1H−1H correlation spectra of SPA-C8 are shown in Figure 5. The COSY spectrum of MTW-C8 shown in Figure S1 appears quite similar and hence is not discussed further. Peaks along the diagonal are due to self-referencing of peaks in the 1D proton NMR spectrum, while cross-peaks occurring symmetrically off the diagonal represent protons attached to neighboring carbons via H−C−C−H bonds. In spite of the complexity of the spectrum, we can identify certain features. For example, we observe cross peaks designated as peaks a (5 and 5.8 ppm) and b (2 ppm, 5.8 ppm). Cross peak a is due to olefinic protons from the two carbons of a double bond (type I or II olefin), while cross peak b is due to coupling between protons in olefins of the types I, II, and IV, respectively. Protons at 1.6, 1.96, and 2.04 ppm all have correlation peaks in the range of 4.5−5.6 ppm. The correlation observed between peaks at 1.96 and 5.42 ppm allows us to identify the presence of type II olefins. Also, the peak at 2.52 ppm has correlation peaks with protons in the range of 4.82−5.03 ppm. Methyl groups at 0.9 ppm have correlation peaks in the range of 3−1.1 ppm indicating alkyl branching, confirming a possible source of error in the 1H NMR determined CH2/CH3 ratio as discussed previously. We also obtained HSQC spectra for both samples. The HSQC technique allows observation of directly bonded proton

Figure 5. Gradient enhanced 1H−1H COSY NMR spectrum of SPAC8 product. Cross peaks labeled a and b are those discussed in the text. The spectra on top of horizontal and vertical axes are the regular 1D 1H NMR spectra.

and carbon through H−C bonds. As expected, we observe strong correlations between protons identified by 1H chemical shift as olefins with carbons identified by 13C chemical shift as olefins. The same is true in the aliphatic region of the spectrum. Figure 6 shows the HSQC spectrum of the SPA-C8 sample, while Figure S2 shows that for MTW-C8.

Figure 6. Gradient-enhanced inverse-detected HSQC NMR spectrum of SPA-C8 product. Regular 1H and 13C NMR spectra are plotted on horizontal and vertical axes, respectively. For 13C, solvent peak (dchloroform at 77.4 ppm) has been digitally removed.

To identify proton and carbon long-range connectivities present in the products, we acquired HMBC spectra of these two samples. Figure 7 shows the HMBC spectrum of the SPAC8 sample. The MTW-C8 spectrum is quite similar and shown in Figure S3. In the HMBC experiment, one bond couplings between carbon and hydrogen are suppressed and C−(C)x−H couplings (x = 1−3) are visible. We observe long-range 9144

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COSY, HSQC and HMBC 2D NMR of MTW-C8 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Joyce Skrocki for performing the spinning band distillation, Margo Steward for extruding the MTW catalyst, and UOP for funding and permission to publish.

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(1) Ipatieff, V. N.; Corson, B. B.; Egloff, G. Polymerization, A New Source of Gasoline. Ind. Eng. Chem. 1935, 27, 1077−81. (2) Ipatieff, V. N.; Schaad, R. E. Heptenes and Heptanes from Propylene and Butylenes. Ind. Eng. Chem. 1945, 37, 362−4. (3) Ipatieff, V. N.; Pines, H. Propylene Polymerization under High Pressure and Temperature with and without Phosphoric Acid. Ind. Eng. Chem. 1936, 28, 684−6. (4) de Klerk, A.; Engelbrecht, D. J.; Boikanyo, H. Oligomerization of Fischer−Tropsch Olefins: Effect of Feed and Operating Conditions on Hydrogenated Motor-Gasoline Quality. Ind. Eng. Chem. Res. 2004, 43, 7449−55. (5) Yergin, D. The Allies’ War. in The Prize: The Epic Quest for Oil, Money and Power; Free Press: New York, 1991. (6) Pujado, P. R.; Ward, D. J. Catalytic Olefin Condensation. In Handbook of Petroleum Processing; Jones, D. S. J., Pujado, P., Eds.; Springer: Dordrecht, 2006. (7) Prinsloo, N. M. Preparation of a Solid Phosphoric Acid Catalyst from Low-Quality Kieselguhr: Parameters Controlling Catalyst Quality and Performance. Ind. Eng. Chem. Res. 2007, 46, 7838−43. (8) Baerlocher, Ch.; McCusker, L. B. International Zeolite Association Database of Zeolite Structures. http://www.iza-structure.org/databases/. (9) Nicholas, C. P. Overview and Recent Development in Catalytic Applications of Zeolites. In Zeolites in Industrial Separation and Catalysis; Kulprathipanja, S., Ed.; Wiley-VCH: Weinheim, Germany, 2010. (10) Pater, J. P. G.; Jacobs, P. A.; Martens, J. A. 1-Hexene Oligomerization in Liquid, Vapor, and Supercritical Phases over Beidellite and Ultrastable Y Zeolite Catalysts. J. Catal. 1998, 179, 477− 82. (11) Schmidt, R.; Welch, M. B.; Randolph, B. B. Oligomerization of C5 Olefins in Light Catalytic Naphtha. Energy Fuels 2008, 22, 1148− 55. (12) Martens, J. A.; Verrelst, W. H.; Mathys, G. M.; Brown, S. H.; Jacobs, P. A. Tailored catalytic propene trimerization over acidic zeolites with tubular pores. Angew. Chem., Int. Ed. 2005, 44, 5687−90. (13) Martinez, C.; Doskocil, E. J.; Corma, A. Improved THETA-1 for Light Olefins Oligomerization to Diesel: Influence of Textural and Acidic Properties. Top. Catal. 2014, 57, 668−82. (14) Kojima, M.; Rautenbach, M. W.; O’Connor, C. T. Butene oligomerization over ion-exchanged mordenite. Ind. Eng. Chem. Res. 1988, 27, 248−52. (15) van den Berg, J. P.; Wolthuizen, J. P.; Clague, A. D. H.; Hays, G. R.; Huis, R.; van Hoof, J. H. C. Low-temperature oligomerization of small olefins on zeolite H-ZSM-5. An investigation with highresolution solid-state 13C-NMR. J. Catal. 1983, 80, 130−138. (16) Corma, A.; Martinez, C.; Doskocil, E. Designing MFI-based catalysts with improved catalyst life for C3= and C5= oligomerization to high quality liquid fuels. J. Catal. 2013, 300, 183−96. (17) deKlerk, A. Properties of Synthetic Fuels from H-ZSM-5 Oligomerization of Fischer−Tropsch Type Feed Materials. Energy Fuels 2007, 21, 3084−9.

Figure 7. Gradient-enhanced inverse-detected HMBC NMR spectrum of SPA-C8 product. One bond couplings are suppressed in this experiment and only long-range couplings are observed. Regular 1H and 13C NMR spectra are plotted on horizontal and vertical axes, respectively. For 13C, solvent peak (d-chloroform at 77.4 ppm) has been digitally removed.

connectivities between protons and carbons present in methyl, methylene, and methyne groups, indicating alkyl branching. We also observe correlation peaks between methyl, methylene, and methyne groups with olefinic protons, indicating substituted olefins. These peaks are not very intense as the number of olefinic protons is low in both the samples. In contrast to the olefinic region of HSQC, which is very intense due to strong one-bond C−H couplings, correlations in HMBC are due to much weaker multiple bond coupling, so the region is less intense.

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CONCLUSIONS We have oligomerized C3 and C4 olefins into gasoline products and compared the C8 cuts of the light olefin oligomerization products obtained from two catalysts: conventional SPA catalyst and a zeolitic catalyst (MTW). We demonstrated for samples with identical octane numbers and boiling point distributions from equivalent conversion levels that 1H and 13C NMR spectroscopy along with spectral-editing techniques allow us to distinguish hydrocarbon species present in these samples. Though the amount of olefinic carbons is similar for both the catalysts, 13C NMR showed that the use of MTW catalyst results in significantly more branched olefins (particularly “quat” carbon) than does the use of SPA. This conclusion was supported by 1H NMR of MTW-C8, which shows lower quantities of olefinic protons than that obtained with SPA. Quantitation of carbon species shows that the amount of methyl groups is similar for both catalysts, but the distribution of methylene and methyne groups are different. We have also confirmed the presence of type I and II olefins by 2D NMR techniques.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01591. 9145

DOI: 10.1021/acs.iecr.6b01591 Ind. Eng. Chem. Res. 2016, 55, 9140−9146

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.6b01591 Ind. Eng. Chem. Res. 2016, 55, 9140−9146