Solid State NMR Spectroscopy and Refinery Catalysts

Transcription

1 Solid State NMR Spectroscopy and Refinery Catalysts Mohammad A. Khadim and Mansour A. -Shafei P.. 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 regeneration 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. INTRDUCTIN 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) , Fax: address: mohammad.khadim@aramco.com. & khadimma@yahoo.com 1

2 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, MHz for 13 C, MHz for 29, and MHz for 27 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 29 NMR spectra were measured using 3 N 4 of the sample holder material which showed 29 resonance at 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 o C 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 o C under vacuum for 12 hours followed by calcinations in a muffle furnace at about 375 o C for 5 hours. The calcined regenerated catalyst was cooled down to room temperature in a vacuum desicator. RESULTS AND DISCUSSIN 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 54.7 o 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. nce a nucleus behaves like 2

3 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 29. (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 % of and %. The spent catalyst may contain deposited coke and other organic or inorganic material. The solid state NMR technique can be used to observe,, C-13, proton, P, and Xe nuclei, among many others. Next, we will look at some of these nuclei. Application of 29 NMR: The usefulness of the 29 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 Table 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 / distribution in simple zeolite lattices. The 29 NMR spectrum leads us to determine the following three quantities: 1. / Ratio ( R ) in the Zeolite Lattice: nce 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 - bonds. The intensity of a silicon resonance is proportional to the number of associated silicon atoms. The / ratio, R, is then given by the following equation, [2]: 3

4 n = 0 4 Σ I (n ) R = = Equation 1. 4 Σ 0.25 n I (n ) n = 0 where I is the intensity of a particular licon resonance and n indicates the number of coordinated atoms for that resonance. nce 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 / ratio of about Figure NMR Spectrum of Zeolite Y with File: Zeolite_Fig1P No. of Atoms Per Unit Cell of Lattice: The number of aluminum atoms per unit cell in a zeolite lattice, N, is given by the formula [3,4] shown in Equation 2: N = 192 / (R+1) Equation 2. where R is the / ratio as determined by the Equation 1 above. For example, the spectrum in Figure 1 yields about 55 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 / ratio is particularly useful since it calculates the aluminium atoms in the zeolite framework as opposed to the chemical analysis which also determines the / 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. lanol Surface Defects: Water adsorbed on silica participates in forming the surface hydroxyl groups, (R) 3 H, called silanol. Both physisorbed and 4

5 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 NMR Spectrum of a zeolites catalyst: Removal File: Zeolite_Fig2P The silanol groups, [ (R) 3 H], 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 H bond is similar to that of. Therefore, the cross-polarization technique helps only in the detection of silanol groups and not in their quantitative estimation [7]. Application of 27 NMR: The primary information obtained from the 27 -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 [{(H 2 ) 6 } +3.aq], and (b) tetrahedral which gives a peak at about ppm [7]. Table 2 shows the 27 NMR chemical shifts in some coordination states of with some other Table elements. The spectrum in Figure 3 is characteristic of aluminium in zeolites. The Figure 3: Solid State 27 NMR spectrum of an FCC catalyst File : Zeolite_Fig

6 sensitivity of 27 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 27 NMR spectra. Figure 4 compares 29 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 27 NMR spectra (104.2 MHz) of fresh (sharp) File: zeolites_fig Figure 5: Photograph, EDS spectrum, and SEM micrograph of fresh File: zeolites_fig Figure 6: Photograph, EDS spectrum, and SEM micrograph of spent File: zeolites_fig Figure 7: Photograph, EDS spectrum, and SEM micrograph of regenerated File: zeolites_fig 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. (- ) 4. milarly, all aluminum atoms in an octahedral coordination have similar chemical environment, that is, (- ) 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. 6

7 The simple two-peak spectrum of 27 -NMR has been put to many useful applications. Some examples will be mentioned: Figure 8: Solid state 79.5 MHz 29 NMR spectrum of FCC catalyst File : Zeolite_Fig Most of the alumina in the FCC catalyst as shown in Figure 8 is in the [-] 3 and [-] 2 state. Uopn regeneration, the coordination spheres of aluminum change and now become [-] 1 and [-] 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 NMR nutation experiments [8]. In a related application to molecular sieves, the 27 spectra provide a different type of information. When molecular sieves become hydrated the coordination number of changes from four to six, i.e. it changes from tetrahedral to octahedral forms. -27 NMR clearly and easily shows such a transformation of topology [9]. Hydrothermal de-alumination is an important procedure to produce highly siliceous zeolites. ne example is shown in Figure 9 [10]. As you can see, the highly siliceous zeolites show resonances corresponding to the only group (4) such that silicon atoms are not surrounded by the aluminum functions Figure 9: Solid state 27 NMR spectra of highly siliceous and the File: Zeolite_Fig 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 7

8 balance between aluminium in the framework and dislodged or non-framework aluminium [11]. 13 C-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 13 C NMR spectrum of FCC coke File: Zeolite_Fig large aromatic peak appears at about 130 ppm and a small aliphatic peak appears at about 20 ppm. The region 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. 8

9 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 27 -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, 129 Xe- 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 il 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 J. Klinowski, S. Ramdas, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Farad. Trans. 1983, 78, p D. W. Breck and E. M. Flannigan, in Molecular eves, Soc. Chem Ind., London, 1968, p J. R. Sohn, S. J. DeCanio, J. H. Lunsford, and D. J. Donnel, Zeolites, 1986, 6, p K. Unger, Porous lica; Elsevier: New York, D. W. ndorf and G. E. Maciel, j> Amer. Chem. Soc., 1983, 105, G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak, and E. Lippmaa, Zeolites, 1985, 2, P. P. Man and J. Klinowski, J. Chem. Soc., Chem. Commun., 1988, P. J. Grobet, J. A. Martens, I. Balakrishnan, M. Martens, and P. A. Jacobs, Appl. Catal., 1989, 56, L21. 9

10 10. C. A. Fyfe, G. C. Gobbi, W. J. Murphy, R. S. zubko, and D. A. Slack, J. Amer. Chem. Soc., 1984, 106, R. A. Beyerlein, G. B. McVicker, L. N. Yacullo, and J. J. Ziemiak, J. Phys. Chem., 1988, 92, 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,

11 Table 1 Characteristic 29 NMR chemical shift ranges (ppm a ) of silicon environments, (-) n, in zeolites n = PPM (-105) (-107) (-95)-(-105) (-88)-(-95) (-86) (-92) (-80)-(-86) a PPM from TMS, Tetramethylsilane Table 2 Characteristic 27 NMR chemical shift ranges (ppm a ) of uminium environments in zeolites Coordination Environment of - -P Tetrahedral Pentahedral ctahedral (-5) 10 (-20) a PPM from 3+ (octahedral) in solution 11

12 ppm Figure NMR Spectrum of Zeolite A with / =

13 After 80 o C, 48 hours 3 N 4 Before heating PPM Figure NMR Spectrum of a zeolite catalyst: Removal of adsorbed water. 13

14 H H 60 0 ppm Figure 3. Solid state 27 AL NMR spectrum of an FCC catalyst, MHz 14

15 Fresh Regen Hz Figure 4: Solid state 27 NMR spectra (104.2 MHz) of fresh (sharp peaks at 0 and 60 ppm) and regenerated alumina catalysts. 15

16 C o u o Energy Figure 5. Photograph, EDS spectrum and SEM migrograph of fresh catalyst Sample A showing, as a major elements present in the scan area at 818X. 16 8

17 Au Au Figure 6. Photograph, EDS spectrum and SEM migrograph of spent catalyst Sample B showing,, and Fe elements present in the scan area at 818X. Au element comes from gold coating material. 17

18 Figure 7. Photograph, EDS spectrum and SEM migrograph of chemical regenerated catalyst Sample D showing, and Au elements present in the scan area at 818X. 18

19 PPM Regenerated Fresh 3 N 4 (48.5 ppm from TMS) Figure 8. Solid state 79.5 MHz 29 NMR spectrum of FCC catalyst. Fresh (lower) and regenerated (upper) catalyst. 19

20 Zeolite Y Mordenite ffretite mega Figure 9. Solid state 27 NMR spectra of highly siliceous and the corresponding low / ratio forms of some compounds. [ref. 10 ] 20

21 Aromatic carbons N-C * * iphati c carbons PPM Figure MHz solid state 13 C NMR spectrum of FCC coke. (*) spinning side band. 21