8106
J. Phys. Chem. 1992, 96,8106-81 1 1
High-Temperature in Sltu Magic Angle Spinnlng NMR Studies of Chemical Reactions on Catalysts F.Gregory Oliver, Eric J. Munson, and James F. Haw* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: February 25, 1992; In Final Form: April 14, 1992)
We report in situ magic angle spinning (MAS) studies of chemical reactions on zeolite catalysts in the high-temperature range (573-673 K) used in typical catalytic processes. These studies were carried out using a novel high-temperature MAS probe which is also described in this contribution. The chemical studies reported here were selected based on either earlier studies at lower temperatures or the failure to observe any reactions using probes with lower temperature limits. All reactions were carried out in zeolite HZSM-5. Cracking of ethylene oligomers was studied at 623 K. A product distribution consistent with the formation of a pentaamdhted carbonium ion intermediate was observed which was not seen in a previous investigation at a lower temperature. Furthermore, direct observation of three-coordinatecarbenium ion formation during cracking at 523-623 K was achieved. The conversion of methanol to gasoline (MTG) on HZSM-5was studied with a temperaturejump to 623 K. Important differences observed in this study relative to previous investigations at lower temperatures include well-resolved signals for adsorbed vs exogenous (gas phase) methanol and dimethyl ether and a higher yield of aromatics. Two less-reactive methane derivatives were also studied to look for analogies to MTG chemistry. Methyl iodide began reacting on HZSM-5 at ca. 583 K to form light aliphatics and at 623 K was completely consumed to form methane, ethane, and benzene. As was previously observed at lower temperatures, dimethyl sulfide formed trimethylsulfonium ion which proved to be stable even at 673 K in the present contribution.
Introduction In several recent reports, we have described the use of magic angle spinning (MAS) NMR to study the reactions of adsorbates on zeolite catalysts at temperatures up to 523 K.'-3 This temperature reflects the upper limit of the commercial probes in our laboratory. A great many industrial catalytic processes actually take place at 573-673 K$ however, and a number of the systems which we undertook to study at 523 K proved to be unreactive at that temperature. Furthermore, there were differences in the product distributions observed in some of our in situ studies at 523 K1 relative to traditional catalysis studies at higher temperature~.~ For example, methane forms readily in the cracking of aliphatic hydrocarbons by protonation of a methyl group to form a five-coordinate carbonium ion followed by elimination of methane: Yet we observed no methane in a recent cracking study at 523 K,' reflecting the fact that carbonium ion generation is not facile at that temperature. In situ high-temperature MAS NMR is better suited to study these reactions compared to other NMR techniques for observing catalytic reactions. Spectra acquired using high-temperature static NMR suffer from inhomogeneously broadened lines that preclude observation of small quantities of transient species due to overlap of the peaks. MAS NMR of quenched samples may be susceptible to errors due to temperature-dependent equilibria as might be present when the spectroscopy is performed at temperatures greatly different from that of the reaction. We have found the ability to monitor reactions as they are occurring in the NMR rotor useful for observing proposed reactive intermediates in catalyst reactions. In two recent papers, we have reported on the mechanism of the methanol-to-gasoline (MTG) process on zeolite HZSM-5 at temperatures up to 523 K.2,7 This reaction is typically carried out at ca. 623 K, and our lower temperature was reflected in both the kinetics and the product distribution. Attempted studies of several methanol analogues (e.g. methyl iodide) on HZSM-5 were precluded by their failure to react at 523 K. We were thus motivated to develop an MAS probe suitable for in situ NMR at higher temperatures. Stebbins has very recently reviewed NMR at high temperatures.* Although nonspinning experiments at temperatures of over 2000 K have been performed: there were few citations of MAS experiments at temperatures in excess of 473 K,'*'* reflecting the technical difficulty of reliable spinning at high temperatures. Our need for very high reliability even when spinning catalyst samples sealed in glass ampules or
* Author to whom correspondence should be addressed.
performing fairly rapid temperatures jumps led to the rejection of several prototypes which, although capable of much higher temperatures, were insufficiently reliable for routine use. The probe design reported here is, in our experience, highly reliable at temperatures of up to 673 K. It is possible that this is a lower temperature limit, as we have not yet had reason to test the spinning module to destruction at higher temperatures. High-temperature in situ MAS NMR, Le., studies at temperatures comparable to those in actual catalytic reactions, is illustrated with several important reactions on zeolite HZSM-5. Ethylene oligomers were cracked at 623 K to a product distribution, including methane, that was quite different from that seen previous1y.I Interestingly, relatively stable three-coordinate carbonium ions were observed while the reaction was in progress. The kinetics of the MTG reaction was much faster at 623 K than at 523 K, and the product distribution included more aromatics. The first 13C MAS spectrum at 523 K showed well-resolved resonances for adsorbed vs exogenous (gas phase) methanol and dimethyl ether. Methyl iodide began reacting at ca. 583 K to form light aliphatic hydrocarbons and was converted completely to primarily methane, ethane, and benzene at 623 K. We recently showed that dimethyl sulfide forms trimethylsulfonium on HZSM-5 which is stable to at least 523 K.I3 In the present investigation we extend its limit of stability to 673 K. Experimental Section Vespel/eorOa Nitride sphming System. Our commercial pro& are supplied by Chemagnetics, Inc., and spin zirconia rotors that are 40 mm in length and have 7.5 mm 0.d. We have previously developed methods for preparing and sealing catalyst/adsorbate samples in such rotors, so it was desirable to design the higher temperature spinning modules to accommodate them. A key feature of the standard Chemagnetics pencil spinning module is the use of a central variable-temperature (VT) chamber which is heated or cooled by a nitrogen gas stream. Separate streams are used for rotor spinning (drive gas) and stability (bearing gas), which are kept near ambient temperatures regardless of the temperature of the VT flow, allowing plastics with limited temperature range to be used to cap the rotor on both ends. The commercial spinning module is fabricated from Vespel which limits the upper temperature to ca. 523 K. Our high-temperature spinning module is depicted in Figure 1. The largest difference between this design and the commercial one is that the module is constructed from a greater number of components to provide for the use of ceramic parts in the regions
0022-365419212096-8106%03.00/0 0 1992 American Chemical Society
MAS NMR Studies of Chemical Reactions on Catalysts
Figure 1. Assembly diagram showing the components of the high-temperature magic angle spinning module. A description of the components of the module is found in the text.
exposed to the highest temperatures. The parts near the ends, which require the most intricate machinery and which are subjected to the greatest shocks in rotor crashes, are fabricated from Vespel. This choice of hybrid materials provides, in our experience, the best compromise between high-temperature performance, ease of machinery, and ruggedness. A number of prototypes were constructed using an all-ceramic (Macor) design, but they tended to crack during rotor crashes. The Vespel end pieces in the present design act as shock absorbers, and the module in Figure 1 has never failed catastrophically. The NMR transmitter-receiver coil (1) is mounted onto the coil support (2) which is fit into the variable-temperaturechamber housing (3). The latter two components are machined from boron nitride ceramic (HP grade, Carborundum Corp.). It is important to note that some grades of boron nitride are very fragile and are not well suited to this application. The upper bearing cartridge/cooling system manifold (4), upper air bearing ( 5 ) , and upper retainer (6) are constructed from Vespel SP-1 (Dupont Corp.). The lower bearing and stator housing (7), lower bearing (8), and bearing and stator cartridge (9) are also made of Vespel. The brass stator (lo), Viton O-ring seals (1 l), and lower retainer (12, Delrin) complete the lower section of the module. The entire assembly is secured by four brass screws (13). Four separate air streams are delivered to the spinning system during variabletemperature operations. Two air channels deliver compressed gas through hollow ceramic axles (14 and 15) midway along the VT chamber housing. These are the air suppliers for the air bearings and stator, respectively. A third gas stream cools the wall of the VT chamber housing to minimize thermal expansion and/or heating of the adjacent Vespel components. This gas stream (not shown for clarity) enters the module through a fitting, circulates through channels in the VT chamber housing, and exists through a hole. The fourth air channel delivers the heated variable-temperature gas (usually nitrogen) to the module through the 13/9 Pyrex socket joint (16) and exits through a ceramic tube (17). Standard zirconia pencil rotors with Vespel drive tips and Vespel and/or Macor spaces and caps were used in all experiments. Heater and Probe. The high-temperature spinning module was mounted on a commercial double-resonance probe of coaxial
The Journal of Physical Chemistry, Vol. 96, No. 20, I992 8107 transmission line design that had been modified to replace plastics subjected to higher temperatures. The most important modification was the replacement of a Teflon bulkhead on which the spinning module is mounted with one machined from a ceramic foam (Cotronics Corp.). The heater unit loads into the top of the magnet bore and ~ ~ ~ etocthe t spinning s module via a Pyrex glass tube with a 13/9 ball joint that mates with the inlet tube (Figure 1, part 16). Two 500-W heating elements (ARI Corp.) are positioned in a Pyrex tube through which the VT gas flows. The magnet bore is protected from excessive heating by a layer of ceramic foam insulation and an external water jacket. Sample Preparation. We customarily seal catalyst/adsorbate samples with grooved plugs made of Kel-F and Macor, but this method proved unsatisfactory for studies above 523 K. We therefore flame-sealed all samples in glass amp~1es.l~ The melting point standards used for temperature calibration were hexamethylbenzene (Aldrich), caffeine (MCB Reagents), and isophthalic acid (Sigma). zeolite HZSM-5 was obtained from Union Carbide Corp. and was activated using a previously described proced~re.'~Methanol-13C, methyl-13C iodide, and ethylene-' 3C2were obtained from Cambridge Isotopes. Adsorptions were carried out using standard vacuum line techniques, and typical loadings were ca. 2.0 mmol/g. NMR Spectroscopy. All 13CMAS NMR spectra were acquired at 50.06 MHz on a modified Chemagnetics CMC-200 spectrometer. All spectra shown were acquired using single pulse excitation with decoupling (90° pulse = 4.5 ps, 1- to 2-s pulse delay, 200-400 scans), though spectra were also acquired without decoupling or using cross polarization (2-ms contact time, 0.5to 1-s pulse delay, 200-400 scans). Typically, spectra were acquired in 3-7 min. In cases in which fast kinetics were anticipated, only one type of spectrum was acquired in order to improve the time resolution of the in situ experiment. Results and Discussion The high-temperature MAS module depicted in Figure 1 readily achieves a spinning speed of 4 kHz and has reached 7 kHz in tests. The electronic characteristics of the probe (e.g. signal-to-noise, efficiency) were essentially identical to those obtained on an analogous commercial probe. In practice, however, the signal to noise at any given temperature is reduced by a factor of ca. 2 relative to our previous investigations by the payload reduction necessitated by the glass ampule. The probe has been in use for several months without serious malfunction. It has logged greater than 25 h at temperatures in excess of 573 K, and we have used it for over 45 in situ experiments thus far. The most frequent problem has been breakage of the glass ampule with escape of volatile components, but the occurrence of this problem is minimized by the use of spinning speeds below 3.5 kHz and the use of boron nitride dust to help seat the glass ampule in the rotor. The copper transmitter-receiver coil tends to fail after multiple experiments at 623 K or higher, probably due to annealing. This problem could be corrected by changing materials, but periodic replacement of the copper coil has not been an inconvenience. Accurate temperature measurement in VT MAS experiments is frequently a challenge because of the need to isolate the sensor element from the coil.I6 Temperature measurements were performed using a thermistor located upstream in the VT gas flow 5 cm from the coil. Little or no discrepancy between the actual and indicated temperatures was observed provided that the VT gas flow rate was at least 200 scf/h. This was verified by melting standards such as hexamethylbenzene (mp 440 K) and caffeine (mp 510 K). The melting of isophthalic acid (mp 614 K) is shown in Figure 2 as a demonstration of temperature accuracy in the region used in the catalytic studies reported below. 13CMAS spectra obtained with a 90° pulse and a delay of 2 s emphasized signals from the more quickly relaxing molten phase. Melting began at ca.615 K and was complete at ca. 625 K. We conclude that the temperature measurement error was ca. 5 K and the temperature gradient across the sample was ca. 10 K. The actual
8108 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 603
Oliver et al.
h
K 298
K
615 K
I 621 K
573
628
K
K
1
I
I
200
175
150
I 125
I 100
PPm Figure 2. I3C MAS NMR spectra showing the melting of isophthalic acid between 615 and 625 K. No peaks were observed below the melting
point due to prohibitively long "C spin-lattice relaxation times for the solid. Isophthalic acid has a reported melting point of 614 K. gradient in the catalysis studies reported below was probably smaller than 10 K due to eficient conduction by gas-phase species. The catalytic cracking of crude oil into gasolinerange hydrccarbons on zeolite catalysts is responsible for a large portion of world petroleum prod~ction.4.~ We have previously investigated the mechanisms responsible for catalytic cracking in zeolites using in situ I3C MAS NMR.' In that study propene oligomers on zeolite HY cracked at 503 K to form c3
