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Solid State NMR Spectroscopy and Refinery Catalysts

Mohammad A. Khadim∗ and Mansour A. Al-Shafei

P. O. Box 62, Research & Development Center Saudi Aramco, Dhahran, 31311, Saudi Arabia

ABSTRACT During the last two decades, high-resolution solid-state NMR spectroscopy has emerged as a powerful tool for the investigation of zeolitic structures. This is so because NMR is sensitive to local orderings and geometries of zeolite structures. In this paper we will present applications of the solid state NMR spectroscopy as applied to zeolites and other catalysts used in a refinery. Some of the examples include changes in structures of zeolites during a catalytic process, status and structural changes in de-activation and re-generation of catalysts, characterization of coke and organic deposits on spent catalysts, thermal and hydrothermal stability of zeolites, dealumination of catalysts, quantitative determination of aluminum in zeolites and other catalysts, etc. Where appropriate, supporting data from the scanning electron microscopy will also be presented. INTRODUCTION Many petroleum-refining processes are critically dependent on the use of solid catalysts to achieve desirable selectivities and rate of product formation. In order to understand the function of such heterogeneous catalysts and to optimize their performance, it is essential to characterize the structure of catalytic materials and surfaces, to understand the interactions between molecules and catalysts, and to follow the dynamics of diffusion and chemical reactions of molecules during the course of catalytic processes. In recent years, NMR has emerged as a powerful tool, often a unique one, for the study of solid materials. It was natural that the heterogeneous catalysts should be strong candidates for such NMR applications. Many factors are responsible for the recent applications of solid-state NMR to the catalytic reactions. Among those are the advances in resolution, sensitivity, multiple pulse techniques, cross polarization, and magic angle spinning. The aim of the present communication is to select certain topics of general interest to illustrate the scope of NMR applications, and to encourage those working in the field of refinery related catalysts to apply NMR spectroscopy to better understand their feed-catalyst-product system.

∗ Corresponding author. Tel: +966 (3) 872-0830, Fax: +966-3-876-2811. E-Mail address: mohammad.khadim@aramco.com. & khadimma@yahoo.com

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EXPERIMENTAL Solid state CP/MS experiments were performed on a Varian Unity-400 MHz NMR spectrometer operating at the frequency of 400 MHz for proton, 100.6 MHz for 13C, 79.46 MHz for 29Si, and 104.21 MHz for 27Al nuclei. Data were collected using 7-mm sample holder and spinning rates of 3000 to 6000 kHz. Generally, 5000 to 10,000 scans were collected and averaged with a delay time of 2 sec. Some 29Si NMR spectra were measured using Si3N4 of the sample holder material which showed 29Si resonance at 48.51 ppm with respect to TMS (Tetramethylsilane). Catalyst samples were analyzed employing a Leica 360 Scanning Electron Microscope (SEM) attached to a light element Noran Voyager II X-ray 2100/2110 Microanalysis system. Prior to mounting in the SEM, the samples were prepared by gold coating instead of carbon coating to prevent specimen charging and possible interference from carbon X-ray emission. Catalysts were scanned by a 20 kV electron beam and the generated X-rays from the scanned areas were analyzed to identify various elements present in the samples. Using the air oxidative method, spent Claus catalyst samples were regenerated by a carbon burn-off treatment in a ventilated muffle furnace. The samples were heated in a programmable furnace at 375 oC for 8 hours and removed from the furnace after cooling down to room temperature. Samples of spent Clause catalysts were also regenerated by chemical treatment. Samples were treated with 5% by weight malonic acid with aluminum nitrate under ambient conditions for 6 hours and then filtered to separate the wet regenerated catalyst from the leachate. The wet regenerated catalyst was then dried at 100 oC under vacuum for 12 hours followed by calcinations in a muffle furnace at about 375 oC for 5 hours. The calcined regenerated catalyst was cooled down to room temperature in a vacuum desicator. RESULTS AND DISCUSSION NMR Consideration for Solid Catalysts: In a catalyst powder, the NMR signals can be very broad for some nuclei. There are at least three sources of line broadening when considering catalysts:

(a) orientation effect: The frequency of an NMR signal, the chemical shift, depends on orientation of molecules in a solid. In a solid catalyst powder, there will be all possible molecular orientations. Because of this anisotropy in chemical shift, the signal will be broad. The orientational effects are overcome by spinning a solid sample at 54.7o relative to the applied magnetic field.

(b) The second source of broadening in the solid catalysts is the dipolar interaction.

Each molecule interacts with the neighboring nuclei. Since a nucleus behaves like

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a small magnet, nearby nuclei contribute to changes in the local magnetic field. This degree of interaction depends on relative orientation of the interacting nuclei with respect to the field. The broadening effect due to dipolar interactions is overcome by the application of high power decoupling. The dipolar interaction is a factor only for neighboring nuclei. It is not important in the case of low natural abundance isotopes such as 29Si.

(c) The third source of line broadening is the result of quadrupolar interaction

between a nucleus of spin > ½ and a non-spherically symmetric distribution of valence electrons.

The refinery catalysts such as zeolytes contain approximately 20-35 % of Si and 20-10 % Al. The spent catalyst may contain deposited coke and other organic or inorganic material. The solid state NMR technique can be used to observe Si, Al, C-13, proton, P, and Xe nuclei, among many others. Next, we will look at some of these nuclei. Application of 29Si NMR: The usefulness of the 29Si NMR depends on certain characteristics of the zeolite structures. These structures consist of silicon tetrahedral and the aluminum tetrahedral linked through oxygen atoms. The silicon chemical shift in alumino-silicates (zeolites) depends on the number of neighboring aluminum atoms. There are five types of environments in which silicon atoms can be identified [1]. They correspond to silicons bonded through oxygen bridges to other silicon atoms, or one, two, three, and four aluminum atoms. These five types of silicon atoms are observed in the chemical shift ranges shown in Table. 1. ---------------------------------------------------------- Table 1 ---------------------------------------------------------- The tetrahedral aluminum has a formal negative charge neutralized by a cation, generally sodium or hydrogen. The hydrogen cation renders the zeolite as acidic. It has been proposed that aluminum tetrahedral will not link together because of the repulsion of two adjacent negative charges. This rule is generally known as Lowenstein’s rule. Consequently, an aluminum atom will always be surrounded by four silicon tetrahedra. The information presented in the preceding two paragraphs may be used to probe the local Si/Al distribution in simple zeolite lattices. The 29Si NMR spectrum leads us to determine the following three quantities:

1. Si / Al Ratio ( R ) in the Zeolite Lattice: Since an aluminum atom will always be surrounded by four silicon tetrahedra, the total number of aluminum atoms in the structure will be one-fourth of the total number of Si-Al bonds. The intensity of a silicon resonance is proportional to the number of associated silicon atoms. The Si/Al ratio, R, is then given by the following equation, [2]:

4

4

Σ ISi (n Al)

Si n = 0

R = ----------- = ---------------------------- Equation 1.

Al 4

Σ 0.25 n ISi (n Al)

n = 0

where I is the intensity of a particular Silicon resonance and n indicates the number of coordinated Al atoms for that resonance. Since the silicon resonances are somewhat broad and partially overlapped, the intensity of each resonance is accurately determined by deconvolution techniques. For example, the spectrum in Figure 1 shows the Si / Al ratio of about 2.5. ------------------------------------------------------ Figure 1. 29Si NMR Spectrum of Zeolite Y with File: Zeolite_Fig1P -------------------------------------------------------------

2. No. of Al Atoms Per Unit Cell of Lattice: The number of aluminum atoms per unit cell in a zeolite lattice, NAl , is given by the formula [3,4] shown in Equation 2:

NAl = 192 / (R+1) Equation 2.

where R is the Si / Al ratio as determined by the Equation 1 above. For example, the spectrum in Figure 1 yields about 55 Al atoms per unit cell of the lattice. The distribution of aluminium is important in zeolites as many important issues are related to this property such as the tendency of coke deposition, the hydrogen transfer activity, and the acid site strength. The NMR method of calculating the Si / Al ratio is particularly useful since it calculates the aluminium atoms in the zeolite framework as opposed to the chemical analysis which also determines the Si / Al but it includes all aluminum including the framework as well as aluminum occluded in the cavities or present as impurity which is not part of the zeolite lattice.

3. Silanol Surface Defects: Water adsorbed on silica participates in forming the

surface hydroxyl groups, Si (OR)3 OH, called silanol. Both physisorbed and

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chemisorbed water can be removed by dehydration with evacuation at room temperature and high temperature, respectively. In addition to removing adsorbed water, dehydration is accompanied by dehydroxylation (or condensation) of geminal hydroxyl groups resulting in single silanol groups [5,6] (referred to as vicinal groups). The silicon NMR spectrum in Figure 2 represents such a dehydration process.

---------------------------------------------------- Figure 2. 29Si NMR Spectrum of a zeolites catalyst: Removal File: Zeolite_Fig2P --------------------------------------------------------

The silanol groups, [Si (OR)3 OH], occur as defects on the zeolite surfaces and are of interest because of their potential involvement in catalysis. The detection of silanol groups is accomplished by the cross-polarization technique in which magnetization is transferred from proton of hydroxyl group to the nearby silicon atom. It results in an increase in the intensity of the corresponding silicon atom. Unfortunately, the chemical shift of a Si – OH bond is similar to that of Si – O – Al. Therefore, the cross-polarization technique helps only in the detection of silanol groups and not in their quantitative estimation [7].

Application of 27Al NMR: The primary information obtained from the 27Al-NMR spectra is related to its state of coordination. There are two major coordination states for aluminum: (a) octahedral which gives a peak at 0 ppm with respect to the aqueous [{Al(H2O)6}

+3.aq], and (b) tetrahedral which gives a peak at about 55-65 ppm [7]. Table 2 shows the 27Al NMR chemical shifts in some coordination states of Al with some other ------------------------------------------------ Table 2 ------------------------------------------------- elements. The spectrum in Figure 3 is characteristic of aluminium in zeolites. The ------------------------------------------------------ Figure 3: Solid State 27Al NMR spectrum of an FCC catalyst File : Zeolite_Fig3 --------------------------------------------------------------

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sensitivity of 27Al NMR spectra to coordination makes them ideal probes of reactions involving the zeolite lattices. For example, dealumination of the zeolite lattice during certain reactions such as hydrocracking / hydrotreating can be followed by observing changes in the 27Al NMR spectra. Figure 4 compares 29Si NMR spectra of a fresh sample of FCC catalyst with its regenerated form to be discussed later. Figures 5-7 compare the SEM micrographs and the EDS spectra of fresh, spent, and regenerated alumina catalyst samples. ---------------------------------------------------------------- Figure 4: Solid State 27Al NMR spectra (104.2 MHz) of fresh (sharp) File: zeolites_Fig4 --------------------------------------------------------------------- Figure 5: Photograph, EDS spectrum, and SEM micrograph of fresh File: zeolites_Fig5 ----------------------------------------------------------------------- Figure 6: Photograph, EDS spectrum, and SEM micrograph of spent File: zeolites_Fig6 --------------------------------------------------------------------------- Figure 7: Photograph, EDS spectrum, and SEM micrograph of regenerated File: zeolites_Fig7 ------------------------------------------------------------------------------- The EDS spectrum of a fresh catalyst (Figure 5) shows only two peaks at 0.52 and 1.48 kev for oxygen and aluminum, respectively. In Figure 6, the spent Clause catalyst shows the presence of additional elements which are carbon, sulfur, oxygen, and iron. Figure 7 shows the EDS spectrum and the SEM micrographs of the same catalyst sample regenerated by air oxidation. The EDS spectrum shows the removal of nearly 90% of contaminants. The SEM micrographs and the EDS spectra indicate that the regeneration of the spent catalyst is about 90% successful, the technique does not reveal the details of the chemical structure. Figure 4 shows that, in the fresh catalyst, the tetrahedral aluminum gives a relatively sharp peak at about 58 ppm because all aluminum atoms in that coordinated state have the same environment, i.e. Al (- O – Si)4. Similarly, all aluminum atoms in an octahedral coordination have similar chemical environment, that is, Al (-O– Si)6 which result in a relatively sharp peak seen at 0 ppm in the fresh catalyst. As the spent catalyst is regenerated locally by the refinery, the regenerated catalyst does not really “regenerate” the original state of catalyst. The aluminum which had occupied the symmetrical tetrahedral and octahedral lattice positions in the fresh catalyst, is now present in asymmetric and / or amorphous environment.

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The simple two-peak spectrum of 27Al-NMR has been put to many useful applications. Some examples will be mentioned: ------------------------------------------------------------ Figure 8: Solid state 79.5 MHz 29Si NMR spectrum of FCC catalyst File : Zeolite_Fig8 -----------------------------------------------------------------

• Most of the alumina in the FCC catalyst as shown in Figure 8 is in the Si[O-Al]3 and Si[O-Al]2 state. Uopn regeneration, the coordination spheres of aluminum change and now become Si[O-Al]1 and Si[O-Al]0, i.e. each silicon atom is now surrounded by either one or none aluminum atom as compared to 2 or 3 aluminum atoms in the fresh sample.

• In a zeolite system, there are two types of aluminum atoms: those in the lattice

(octahedral and tetrahedral) and in the bulk of the catalyst which are not part of the lattice. By modifying the NMR experiment, it is possible to determine the quantity of aluminum in the lattice and in the bulk by Al NMR nutation experiments [8].

• In a related application to molecular sieves, the 27Al spectra provide a different

type of information. When molecular sieves become hydrated the coordination number of Al changes from four to six, i.e. it changes from tetrahedral to octahedral forms. Al-27 NMR clearly and easily shows such a transformation of topology [9].

• Hydrothermal de-alumination is an important procedure to produce highly

siliceous zeolites. One example is shown in Figure 9 [10]. As you can see, the highly siliceous zeolites show resonances corresponding to the only group Si (4Si) such that silicon atoms are not surrounded by the aluminum functions.

----------------------------------------------------------------------

Figure 9: Solid state 27Al NMR spectra of highly siliceous and the File: Zeolite_Fig9 -----------------------------------------------------------------------------

• The relative amounts of aluminium in framework and non-framework sites provides helpful information to understand alkane isomerization and conversion reactions by catalysts. In the isomerization and conversion processes of alkanes the carbonium ion generation is an important step. It has been shown that the number of acid sites capable of initiating carbonium ion pathways depends on a

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balance between aluminium in the framework and dislodged or non-framework aluminium [11].

13C-NMR and Coke related problems In a fluid catalytic cracking process, the catalysts are reversibly deactivated by coke deposits. A detailed understanding of coke composition and the impact of catalyst properties, feed composition, and feed-catalyst interactions on coke formation is critical to the development of FCC catalysts and commercial FCC operations. The enormous complexity of FCC coke demand the application of multiple analytical techniques for a detailed description of coke composition and properties. These include solid state NMR, XPS (X-ray photoelectron spectroscopy) among the few major techniques. Carbon-13 solid state NMR techniques have been used to obtain information about aromatic and aliphatic carbon concentrations in FCC cokes. For example, in Figure 10, a -------------------------------------------------------- Figure 10: 100 MHz solid state 13C NMR spectrum of FCC coke File: Zeolite_Fig10 --------------------------------------------------------------- large aromatic peak appears at about 130 ppm and a small aliphatic peak appears at about 20 ppm. The region 140-150 ppm is characteristic for nitrogen heterocycles but it is seen masked under the broad peak. When desired, the XPS [12] (X-ray Photoelectron spectroscopy) may be utilized to determine the nature of nitrogen compounds such as nitrogen in a polar or non-polar compound. When suitable samples of coked catalyst are withdrawn from an experimental setup, one can determine how the aromaticity of coke is changed as the coking or catalytic process is increased by temperature or pressure. Such experiments can suggest when the de-alkylation of coke’s precursor molecules takes place. Solid state C-13 NMR can be used to deduce a variety of molecular parameters about the structure of coke on the catalysts. The examples are:

1. Fraction of aromatic carbons in the coke: It is not as simple as it sounds. In a standard carbon-13 NMR spectrum of coke, about half of the carbons produce resonance signals and labeled as “visible” carbons. The other half of the carbons

e”, and do not produce any signal. Therefore, one needs to see the “visible” and “invisible” carbons to estimate the fraction of aromatic carbons. The “invisible” carbons are estimated by the application of specialized solid state carbon-13 NMR techniques.

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2. Fraction of ‘graphite-like’ carbons in the coke: This is the ‘invisible’ portion of carbon atoms in the coke mentioned above and specialized techniques are needed to estimate it.

3. Fraction of ‘protonated’ aromatic carbons: This quantity requires the knowledge of fraction of “visible” protonated aromatic carbons, fraction of “visible” aromatic carbons, and the fraction of “invisible graphite-like” carbons.

4. The density of carbonaceous deposits: It is obtained as a sum of the contributions to density by the condensed aliphatic hydrocarbons, ‘visible’ aromatic hydrocarbons, and the ‘invisible’ graphite-like carbons.

5. Proton/carbon atomic ratio. 6. Fraction of “visible” protonated aromatic carbons: This quantity is obtained from

the specialized solid state carbon NMR experiments. 7. Coke volume: This is generally obtained from the solid state 27Al-NMR. The

coke volume is the volume occupied by the carbonaceous coke in the pores of one gram of coked catalyst. From the coke volume and the pore volume of the coke catalyst, it is possible to calculate the degree of pore filling by the coke and the metal sulfides.

There are many other applications of solid state carbon-13 NMR, P-31 NMR, 129Xe-NMR, etc. which will not be described here. Acknowledgement The authors wish to acknowledge the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabia Oil Company (Saudi Aramco) for granting permission to present and publish this paper. References.

1. C. A. Fyfe, J. M. Thomas, J. Klinowski, and C. G. Gobbi, Angew. Chem., 1983, 95, p. 257.

2. J. Klinowski, S. Ramdas, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Farad. Trans. 1983, 78, p. 1025.

3. D. W. Breck and E. M. Flannigan, in Molecular Sieves, Soc. Chem Ind., London, 1968, p. 47.

4. J. R. Sohn, S. J. DeCanio, J. H. Lunsford, and D. J. O’Donnel, Zeolites, 1986, 6, p. 225.

5. K. Unger, Porous Silica; Elsevier: New York, 1979. 6. D. W. Sindorf and G. E. Maciel, j> Amer. Chem. Soc., 1983, 105, 1487. 7. G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak, and E. Lippmaa,

Zeolites, 1985, 2, 59. 8. P. P. Man and J. Klinowski, J. Chem. Soc., Chem. Commun., 1988, 1291. 9. P. J. Grobet, J. A. Martens, I. Balakrishnan, M. Martens, and P. A. Jacobs, Appl.

Catal., 1989, 56, L21.

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10. C. A. Fyfe, G. C. Gobbi, W. J. Murphy, R. S. Ozubko, and D. A. Slack, J. Amer. Chem. Soc., 1984, 106, 4435.

11. R. A. Beyerlein, G. B. McVicker, L. N. Yacullo, and J. J. Ziemiak, J. Phys. Chem., 1988, 92, 1967.

12. K. Qian, D. C. Tomczak, E. F. Rakiewicz, R. H. Harding, G. Yaluris, Wu. Cheng, X. Zhao, and A. W. Peters, Energy & Fuels, 1997, 11, 596.

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Table 1

Characteristic 29Si NMR chemical shift ranges (ppma) of silicon

environments, Si(O-Al)n , in zeolites _____________________________________________________________n = 0 1 2 3 4

Si

O

OO

O

Si

Si

SiSi

Si

O

OO

O

Si

Al

SiSi

Si

O

OO

O

Al

Al

SiSi

Si

O

OO

O

Al

Al

SiAl

Si

O

OO

O

Al

Al

AlAl

PPM (-105)–(-107) (-95)-(-105) (-88)-(-95) (-86) – (-92) (-80)-(-86) ________________________________________________________________________aPPM from TMS, Tetramethylsilane

Table 2

Characteristic 27Al NMR chemical shift ranges (ppma) of Aluminium

environments in zeolites _____________________________________________________________ Coordination Environment of Al

O-Si O-P Tetrahedral 50 – 70 35 – 45 Pentahedral 20 – 35 Octahedral (-5) – 10 (-20) _____________________________________________________________aPPM from Al3+ (octahedral) in solution

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-80 -90 -100 -110

ppm

Figure 1. 29Si NMR Spectrum of Zeolite A with Si / Al = 2.5

SiO

OOO

Si

Si

SiSi

SiO

OOO

Si

Al

SiSi

SiO

OOO

Al

Al

SiSi

SiO

OOO

Al

Al

SiAl

SiO

OOO

Al

Al

AlAl

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Figure 2. 29Si NMR Spectrum of a zeolite catalyst: Removal of adsorbed water.

Si3N4

After 80 oC, 48 hours

Before heating

91.3 PPM 48.5 95.1

101.6

106.5

Al 4 3 2 1 0

14

060 ppm

Al

OH

O

OO

O

O

AlOH

O

OO

Figure 3. Solid state 27AL NMR spectrum of an FCC catalyst, 104.2 MHz

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Figure 4: Solid state 27NMR spectra (104.2 MHz) of fresh (sharp peaks

at 0 and 60 ppm) and regenerated alumina catalysts.

0500025000 -25000

Hz

Fresh Regen

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FFiigguurree 55.. PPhhoottooggrraapphh,, EEDDSS ssppeeccttrruumm aanndd SSEEMM mmiiggrrooggrraapphh ooff ffrreesshh ccaattaallyysstt SSaammppllee AA sshhoowwiinngg AAll,, OO aass aa mmaajjoorr eelleemmeennttss pprreesseenntt iinn tthhee ssccaann aarreeaa aatt 881188XX..

Energy

Cou

Al

o

800

700

600

500

400

300

200

100

0 1 2 3 4 5 6 7 8

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FFiigguurree 66.. PPhhoottooggrraapphh,, EEDDSS ssppeeccttrruumm aanndd SSEEMM mmiiggrrooggrraapphh ooff ssppeenntt ccaattaallyysstt SSaammppllee BB sshhoowwiinngg AAll,, OO,, aanndd FFee eelleemmeennttss pprreesseenntt iinn tthhee ssccaann aarreeaa aatt 881188XX.. AAuu eelleemmeenntt ccoommeess ffrroomm ggoolldd ccooaattiinngg mmaatteerriiaall..

Au

Au

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FFiigguurree 77.. PPhhoottooggrraapphh,, EEDDSS ssppeeccttrruumm aanndd SSEEMM mmiiggrrooggrraapphh ooff cchheemmiiccaall rreeggeenneerraatteedd ccaattaallyysstt SSaammppllee DD sshhoowwiinngg OO,, AAll aanndd AAuu eelleemmeennttss pprreesseenntt iinn tthhee ssccaann aarreeaa aatt 881188XX..

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Al 4 3 2 0

Si3N4

48.5

1 0

(48.5 ppm from TMS)

91

95102

107

Fresh

Regenerated

106

111

1

Figure 8. Solid state 79.5 MHz 29Si NMR spectrum of FCC catalyst. Fresh (lower) and regenerated (upper) catalyst.

PPM

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Figure 9. Solid state 27Al NMR spectra of highly siliceous and the corresponding low

Si/Al ratio forms of some compounds. [ref. 10 ]

Zeolite Y Mordenite

Offretite Omega

21

PPM

300 400 100 200 0 -100

Figure 10. 100 MHz solid state 13C NMR spectrum of FCC coke. (*) spinning side band.

Aromatic carbons

Aliphatic carbons

N-C

* *