Radicals in carbonaceous residue deposited on mordenite from methanol

It is shown that control of the degree of coking can lead to the observation of hyperfine structures in the carbonaceous residues deposited from methanol over mordenite (H-MOR) at temperatures relevant to the conversion of methanol to hydrocarbons. EPR measurements of the catalyst samples at various times on stream have been recorded, with a rich hyperfine splitting pattern observed in the early stages of the reaction. Interpretation of the EPR data with the aid of density functional theoretical calculations has afforded the first definitive assignment of the radical cations formed in high temperature coke. The results detail a shortlist of six species: 2,3/2,6/2,7-dimethylnaphthalenium, 2,3,6-trimethylnaphthalenium, 2,3,6,7-tetramethylnaphthalenium, and anthracenium radical cations whose proton hyperfine splitting profiles match the experimental spectra; 2,3,6,7-tetramethylnaphthalenium showed the best agreement. The observation of these particular isomers of polymethylnaphthalene suggest the formation of more highly branched polyaromatic species is less likely within the confines of the H-MOR 12-membered ring channel. These radicals formed when the catalyst is active may constitute key intermediates in the conversion of methanol to light olefins.


Introduction
2][3][4] A number of interrelated processes have been the centre of attention, including methanol dehydration to yield dimethylether, MTO (the conversion of methanol to light olens) and MTH (the conversion of methanol to hydrocarbons).Catalysis of the latter two processes can be achieved with a range of zeolite and zeotype materials but particular interest has centred upon SAPO-34 for MTO and H-ZSM-5 for MTH. 1,2For both these processes, the socalled hydrocarbon pool mechanism has gained widespread acceptance in recent years. 5,6In general, although subject to renements, this mechanism is based upon the formation of an active hydrocarbon pool upon reaction.This hydrocarbon pool is proposed to be comprised of catalytically active alkylated aromatic species. 3,7n concert with the formation of active hydrocarbon species MTH processes is the unavoidable generation of a carbonaceous residue (coke) chiey composed of polyaromatics that coats the surface of the zeolite affecting catalyst deactivation. 8It has long been known this residue harbours radicals that have been identied by EPR spectroscopy, similar to coal and pitch. 9he signal typically manifests as a featureless line whose width is modulated by the radical concentration. 10It is not unsurprising that similar signals were observed in the undesirable formation of carbonaceous residues over acidic zeolite catalysts during MTH processes. 2,11EPR spectroscopy has been sought to identify and elucidate the role of these radicals during methanol conversion.Seminal work by Karge and co-workers utilised in situ and ex situ EPR to monitor hydrocarbon deposited on mordenite from ethene and propene feeds. 12,13Their study revealed short-lived olenic radicals appearing in low temperature coke (<200 C) and abundantly more stable radicals forming at reaction temperatures exceeding 250 C. It was the observation of multiline hyperne splitting in the spectra of the olenic radicals that facilitated their identication, as structural information can only be obtained from hyperne coupling interactions between the unpaired electron and the nuclear spin of protons.][16][17] Within the past few years a degree of attention has centred upon the observation of radicals specic to the conversion of alcohols over zeolites. 18,19Recent attempts have been made to correlate their existence to the mechanism and deactivation of ethanol transformation over HZSM-5, 20 however the absence of hyperne features precluded identication of the radicals formed.Seo and co-workers determined hexamethylbenzenium radicals are generated on H-SAPO-34 and H-SSZ-13 a mere 30 minutes time on stream (TOS) when the catalyst was still active. 21The signal persisted for 3 h, there aer narrowing as larger aromatics formed and the catalyst activity declined. 22ecently, they identied 1,2,4,5-tetramethylbenzenium radicals during MTO over a phosphorus modied H-ZSM-5. 23The modication eliminated many acid sites thereby leaving the radicals sufficiently separated and free from effects of interspin coupling that extinguishes hyperne features.Their assignment was neatly supported by collecting reference spectra of polymethylbenzenium radicals formed on mordenite at 200 C.
Herein, we extend studies of the formation of EPR active radicals as formed over H-mordenite (H-MOR) samples coked using methanol.We concentrate upon establishing whether, or not, there are any relationships between coking prole and radical concentration by varying TOS for the samples generated with a 20% MeOH feed at 260 C. The efficacy of H-MOR for the production of hydrocarbons has been established in previous studies, 22 and investigations undertaken using a variety of techniques demonstrate that features consistent with the formation of a hydrocarbon pool to be formed. 24We observe in spectra of samples with short TOS a rich hyperne splitting prole, and with the aid of theoretical calculations are able to assign the radical species formed in high temperature coke residue.

Sample preparation
Approximately 0.28 of H-mordenite (H-MOR; Si/Al ¼ 10) was held in a quartz xed-bed reactor tube connected to a stainless steel reactor.All reactor lines were trace heated to 150 C. The sample was heated under an Ar ow (22 mL min À1 ; GHSV ¼ 3000 h À1 ) and heated to the desired reaction temperature.Once at temperature, the feed was switched to bypass mode and purged with liquid methanol (0.01 mL min À1 ) pumped via Knauer HPLC pump in addition to the previous Ar feed.Once a constant feed of methanol was observed using a GC-TCD detector at the rear end of the rig, the methanol/Ar ow was directed towards the H-MOR for the desired length of time.Following the reaction period, the methanol feed was stopped, the heater switched off and the H-MOR was allowed to cool until <50 C in an Ar ow (22 mL min À1 ).Once cooled the reactor tube was immediately removed, plugged with rubber bungs and transferred into an inert atmosphere glove box.

EPR spectroscopy
Powder samples of H-MOR following carbon deposition from methanol were loaded to a height of 20 mm in a 4 mm (o.d.) quartz EPR tube.The tube was sealed with a J. Young Teon stopcock under an atmosphere of Ar inside a glovebox.X-band EPR spectra were taken at ambient temperature (293 K) using a Bruker ELEXSYS E500 spectrometer equipped with a super high-Q cavity (ER 4122 SHQE).The experimental parameters for EPR spectra were as follows: microwave frequency ¼ 9.786 GHz; microwave power ¼ 0.63 mW; modulation amplitude ¼ 0.05 mT; modulation frequency ¼ 100 kHz; time constant ¼ 10.24 ms; conversion time ¼ 20.48 ms; number of scans ¼ 20.Spectral simulations were generated using XSophe distributed by Bruker Biospin GmbH. 25 Other physical measurements Carbon analyses were undertaken using an Exeter Analytical CE-440 elemental analyser.TGA proles were measured using a TA Instruments Q500 instrument under a ow of air.The temperature was ramped from 20 to 1000 C at a rate of 10 C min À1 .Surface areas were determined by applying the Brunauer-Emmett-Teller (BET) method to the nitrogen physisorption isotherm obtained at À196 C in a Micromeritics Gemini instrument.The sample was rst heated up to 250 C for 2 h under vacuum and degassed at 250 C for 2 h prior to analysis. 13C MAS NMR data were acquired by the EPSRC NMR Service at the University of Durham using a Varian VNMRS spectrometer equipped with a 9.40 T magnet operating at 100.56 MHz.A 6 mm (rotor o.d.) magic angle spinning probe was utilised.Crosspolarisation spectra were obtained using a 1 s delay, a 1 ms contact time, a sample spin rate of 6.8 kHz and "TOSS" spinning side band suppression.

Calculations
The program package ORCA was used for all calculations. 26The input geometry for the monocationic radicals was derived from their parent neutral molecule generated in ArgusLab.The geometries of all molecules were fully optimized by a spinunrestricted DFT method employing the B3LYP functional. 27riple-x-quality basis sets with one set of polarization functions (def2-TZVP) were used all atoms. 28The RIJCOSX approximation 29 combined with the appropriate Ahlrichs auxiliary basis set 30 was used to speed up the calculations.The self-consistent eld calculations were tightly converged (1 Â 10 À8 E h in energy, 1 Â 10 À7 E h in the density charge, and 1 Â 10 À7 in the maximum element of the DIIS 31 error vector).The geometry was converged with the following convergence criteria: change in energy < 10 À5 E h , average force < 5 Â 10 À4 E h Bohr À1 , and the maximum force 10 À4 E h Bohr À1 .The coordinates of the atoms are included in the ESI.† Spin density maps were visualised with the program Molekel. 32

Results and discussion
The coking proles of the H-MOR sample aer 5 h TOS run under four different partial pressures of methanol (20%, 50%, 60%, 90% with the balance in all cases being argon) are presented in Fig. 1.
A pronounced step is evident in the proles of the 20%, 50% and 60% MeOH partial pressures demonstrating that the samples progress from a regime of a relatively light degree of coke formation to one where much more severe coke formation occurs over a relatively narrow temperature range.This step occurs $20 C lower for the 90% MeOH/Ar mixture than for the other reaction mixtures.Indeed, the sensitivity of the process is further underlined by the relatively large error bars associated with the degree of coking for temperatures which are within the range of the step in the proles.The error bars were determined from replicate runs and they reect the degree of sensitivity within these coking regimes.The step in coking behaviour is consistent with a change in selectivity as a function of temperature passing from a regime wherein dimethylether is formed by dehydration, associated with a low degree of coking, to one where the methanol to hydrocarbons reaction occurs which may be associated with the formation of an active hydrocarbon pool and the formation of waxes which deposit in the downstream cold spot of the reactor.Nitrogen physisorption analysis shows that the more heavily coked materials exhibit a type II isotherm which is indicative of micropore lling by carbonaceous species as indicated by the reduction in BET surface area (which strictly is not applicable to the type I isotherms of the fresh materials but which is used in comparative terms; Fig. S1 †).In comparing the various proles presented in Fig. 1 it can be seen that the degree of coking beyond the step is relatively independent of methanol partial pressure.
In terms of our aim to monitor the relationship between coking prole and the formation of EPR active radicals, we have selected the 20% methanol feed at 260 C for further investigation.This temperature is close to the top of the step for this partial pressure as illustrated in Fig. 1.Accordingly, since the degree of coking could be controlled over a relatively wide range by variation of TOS for this temperature, it was felt that this would offer an opportunity to speciate the nature of the coke and to draw comparisons with the EPR behaviour.

EPR spectroscopy
Unused catalyst produced no signal aside from an almost undiscernible peak at g $ 4.3 ascribed to Fe 3+ impurities. 33,34his resolution of this peak could have been improved with higher power and cryogenic temperatures, however, it had previously be determined that these transition metal impurities are not involved in coke radical formation. 12Furthermore, this signal is extinguished during the course of the reaction at 260 C. 35 The EPR signal evident for coked samples present relatively featureless spectra centred at a g-value of 2.0026 consistent with carbon-based radicals. 9The signal rst emerges for a catalyst sample aer 1 h on the stream.The spectrum is very weak indicating minimal coking at this time and commensurate with the lack of colour (light tan) in the sample (Fig. S2 †).A more pronounced signal is observed aer 2 h TOS.The "fresh" catalyst sample was taken directly from the reactor and loaded into the EPR tube under an inert atmosphere.The longevity of the radicals was explored by exposing the sample to atmosphere for 24 h and recording its effect on the EPR signal (labelled "exposed").These spectra are presented in Fig. 2. The signal for the air contacted samples survived several weeks, though there was no further increase in intensity aer 1-2 days stored in air.A wide sweep spectrum shows that in addition to the dominant carbon radical at g ¼ 2.0026, a very broad feature is observed with g $ 2.17 (width 40 mT; Fig. S3 †).This is signal most likely stems from amorphous iron oxide/hydroxide where the coking alters the geometry and relaxation properties of the Fe 3+ centres. 33he spectrum of the fresh catalyst aer 2 h exhibits some ne structure attributed to hyperne coupling through the crossing point.This spectrum is reminiscent of the signal of high temperature coke measured by Karge and co-workers over the temperature range 250-450 C. 12,13 Most interesting is its nearly identical appearance to the signal from coke residue coating the external surface of H-UZM-12 zeolites under a methanol feed. 21The H-UZM-12 framework possesses ERI topology with large cages 13.0 Å long and 6.3 Å in diameter. 36he hyperne features are slightly attenuated but still visible in the air exposed sample, a consequence of the noticeable increase in the signal intensity.Further catalyst aliquots taken at 3, 4 and 5 h produced an almost identical prole to the spectra shown in Fig. 2. Extending the TOS afforded EPR spectral that were almost featureless (Fig. S2 †), though some hyperne structure is visible in the second derivative (Fig. S4 †).Importantly, all signals have the same spectral width ($0.6 mT), which suggests these radicals are of the same or similar chemical composition irrespective of their TOS.The signal is consistent with high temperature coke described by Karge and co-workers when carbonising ethylene and propylene over H-MOR at temperatures exceeding 200 C. 12,13 The effects of exposure to air are displayed in Fig. 3 with a comparison of the absorption spectrum at different TOS intervals; the signal intensity given by the area under the absorption peak.Overall, the general form of the EPR prole is maintained, however the intensity of the air contacted spectra is considerably greater than their fresh catalyst counterparts, indicating further evolution of radicals.This is in contrast to carbon radicals adsorbed on a silica-alumina where the introduction of oxygen reduced the signal intensity by approximately one-third and decreased the spin lattice relaxation times by an order of magnitude. 37he effects of TOS upon both post-reaction carbon content and EPR signal intensity are displayed in Fig. 4. The list includes samples collected at 75, 90 and 105 min TOS in order to determine the approximate time frame for the evolution of coke radicals.Interestingly, EPR intensity data peak at 90 minutes, which then fall sharply with longer TOS.Aer 90 minutes the H-MOR still exhibits a high surface area (Fig. S1 †), such that the radical content indicates high coverage of the acid sites (Si/Al ¼ 10).For TOS beyond 2 h, the coking prole and surface area analysis is characteristic of pore blockage, which is mode of deactivation in monodimensional zeolites like H-MOR.38 This has the effect of increasing coking of the catalyst surface, leading to clustering of radicals that would be fewer in number but more prone to the effects of spin-spin interactions on their EPR lineshape, such as the loss of hyperne structure (Fig. S2 †).We posit this pore blockage reduces the exposure of radicals in the channels from contact with oxygen, and the aforementioned impact of the EPR spectra.Therefore the increase in signal intensity for TOS >2 h arises from aerial oxidation of surface adsorbed coke, 39 and is proportional to the acid site content of the zeolite.38 This conclusion is also consistent with the drop in signal intensity for the air-contacted samples at 90 and 105 min when oxygen has unimpeded access to the catalyst interior.
As anticipated, the degree of coking varies over a wide range.Inspection of the trend evident demonstrates that EPR signal intensity is not a simple function of carbon content, although with the exception of the 180 min TOS data, there does seem to be such a relationship, which is perhaps not surprising.At 180 min, there appears to be a reduction in the amount of coke deposited with respect to the two data points to either side and this is also associated with a signicant enhancement in EPR signal intensity for this data point.This effect, which is outside that expected taking into experimental uncertainty, has been shown to be reproducible and could relate a change in the nature of coking at this reaction time.
Speciation of the coke deposited upon the samples as a function of TOS at 260 C was undertaken.TGA proles undertaken in air are presented in Fig. 5.The mass losses  occurring <130 C correspond to the of water and the progressive diminution of this feature as a function of extended reaction time, and hence degree of coking, as would be expected can be seen.Furthermore, mass losses corresponding to species such as adsorbed methanol and/or dimethylether occur in the range ca.150-230 C. Upon extended reaction times, species which are lost in the 250-350 C become evidentprevious work has shown the losses in this range to be associated with desorption of carbonaceous species rather than oxidative processes. 24The broad features in the range ca.400-700 C are associated with hard coke being lost through oxidation. 24

DFT calculations
The concentration of cationic radicals was sufficiently low aer 75 min TOS that the resulting spectrum exhibited resolved hyperne splitting (Fig. 6).A similar spectral prole was measured for the 105 min sample.At these two time intervals, the radicals are sparsely deposited throughout the pores of H-MOR, minimising the spin-spin interaction between them.This allows their hyperne features to remain visible.The impact of the spin-spin interaction is evidenced at higher radical concentration (longer reaction times) where the spectra are distinctly isotropic and featureless.However, despite the low radical concentration, the spin-spin interaction still manifests in the 75 min spectrum.Unpaired electrons interact with each in two ways: (i) exchange coupling which occurs through bonds, both covalent and non-covalent; and (ii) dipolar coupling which is a through space interaction. 40These two couplings have an opposing effect in the EPR spectrum: the exchange interaction causes the hyperne lines to collapse and the signal becomes narrower, whereas the dipolar interaction causes additional features to appear from a phenomenon known as zero-eld splitting, which overall broadens the EPR signal. 41For the 75 min spectrum, $17 hyperne lines are visible; the most prominent ones are in the centre of the spectrum as the line crosses the zero point best visualised in the second derivative plot (Fig. 6).The wings of the spectrum are less resolved which suggests dipolar broadening is dominant over exchange narrowing, commensurate with the low radical concentration.This is not unsurprising as the through bond interaction is attenuated by the lack of bona de covalent bonds between radical cations, instead utilising less efficient pstacking and other non-covalent pathways for neighbouring spins to communicate.
The spectrum recorded aer 75 min is reminiscent of polymethylaromatic radicals, [14][15][16][17]21,23,[42][43][44][45][46][47] and therefore a survey of potential candidates was undertaken to determine the likely species that were coating the pores of H-MOR. The survey as conducted as follows: the radical cation of a polymethylaromatic molecule was geometry optimised at the B3LYP level of DFT, and then the hyperne coupling from each proton was calculated from the Mulliken spin population analysis.Proton hyperne coupling constants (a H ) were calculated from the Mulliken spin density (r H ) distribution using the relationship a H ¼ r H Â 1419 (in MHz) derived from the McConnell equation.48 Our theoretical estimate was compared with experimental data for authentic radical cations that were included in the survey, namely 1,2,4,5-tetramethylbenzenium, hexamethylbenzenium, anthracenium, 9,10-dimethylanthracenium, and pyrenium.The experimental are contrasted with the calculated a H values in Table 1.Overall there is good agreement, especially for the anthracenium radical cation adsorbed on a silica-alumina surface.16,47 It was noted that the a H were only 4% larger for the surface absorbed anthracenium radical compared with chemically generated variant.
This journal is © The Royal Society of Chemistry 2016 position, and the same two sets of four identied in naphthalenium, which is the minimum required to match the experiment.Secondly, the coupling of methyl protons is typically larger than the aromatic ones.The resonance structures for the radical cations of polymethylbenzenes deposit a large amount of spin density on the methyl carbon. 15,23,43,44,49The position of the substituent is also critical; the hyperne coupling of methyl protons is larger in p-xylene than m-xylene because of the stability of the resonance structures in the former. 23Based on these observations the monocationic radicals of all polymethylbenzenes, indene, 1-methylnaphthalene, 1,2-/1,4-dimethylnaphthalene, 1-/9-methylanthracene, and 1,2-/9,10-dimethylanthracene radicals were eliminated from the list as they were too broad due to the large coupling of the methyl protons.Three other contenders, 2-methylnaphthalenium, 2,3,6-trimethylanthracenium and 2,3,6,7-tetramethylanthracenium produced too many hyperne lines and were similarly excluded.The outcome was a shortlist of six radical cations: 2,3-/2,6-/2,7-dimethylnaphthalenium, 2,3,6-trimethylnaphthalenium, 2,3,6,7-tetramethylnaphthalenium, and anthracenium.Phenanthrenium and pyrenium radicals, despite having four and three equivalent sets of protons, respectively, yielded an insufficient number of lines to match the spectrum prole (Fig. 7).
The Mulliken spin density analysis for each radical is shown in Fig. S6, † with the majority of the spin density is conned to the aromatic carbon atoms.The majority shareholders of the unpaired spin are the 1,4,5,8 carbons of naphthalene unit, and 1,4,5,8,9,10 carbons of anthracene unit; methyl groups attached at any of these positions would give large coupling constants.The proton splitting pattern for each of the shortlisted radicals was used to generate a simulation for the spectrum recorded aer 75 min (Fig. 6).The spin-Hamiltonian, Ĥ ¼ gm B BS + P aSI, was used, where the rst term is the electron Zeeman splitting where the parameter g is the Landé g-factor (g ¼ 2.0026), m B the Bohr magneton, B the magnetic eld, S the total electron spin (S ¼ 1/2 for these radical cations). 40The second term is the hyperne interaction as the sum of all protons (I ¼ 1/2) in the system parameterised by the coupling constant a.The linewidth was xed at 2 MHz being the smallest peak-to-peak distance in the spectrum.
For the dimethylnaphthalenium radical cations, a splitting pattern of 2 > 2 > 6 > 2 was used.In the case of 2,3-dimethyl substitution, the 1,4 protons have the largest coupling constants, then the 5,8, the six methyl protons and nally the 3,7 aromatic protons.The computed coupling constant for each set of protons is presented in Fig. 7.The 2,7-dimethylnaphthalenium radical gave the same magnitude of hyperne splitting, whereas the 2,6 variant gave a signicantly larger coupling of the methyl protons and concomitantly a miniscule one for the 3,7 protons.In generating these simulations, two simplications have been employed.Firstly, an isotropic spin-Hamiltonian has been used even though the radicals are xed to the surface of the H-MOR and unable to freely rotate. 13,14,56This ignores the anisotropy of the proton hyperne coupling, however the observed difference between parallel and perpendicular components, depending on the specic aromatic radical, can be rather small. 46,57Moreover the motional averaging of both anisotropic hyperne coupling and the g-matrix is improved at higher temperatures, i.e. room temperature. 17,45econdly, contributions to the lineshape from exchange narrowing and dipolar broadening have been ignored as these are not easily included in the simulation.The effect is that dipolar broadening may generate additional splitting from zero-eld interaction, 40 such that not all the lines result from hyperne coupling.The exchange interaction will compress the spacing between the hyperne lines making these couplings appear smaller than they actually are. 41Nevertheless, the close agreement between simulated and computed proton coupling constants would seem to validate these assumptions.However, despite the closer agreement of the coupling constants for the 2,6-dimethylnaphthalenium radical compared with the other two isomers, we are unable to provide a more denitive assignment of the radical species in the sample.It is quite possible that these radicals are present in varying amounts and thus the EPR signal is a superposition of several spectra.
The lower symmetry of 2,3,6-trimethylnaphthalenium radical is expressed in the 1 > 2 > 1 > 6 > 3 > 1 splitting pattern (Fig. 7).The weakest coupled proton was ignored as this value of 1.6 MHz is smaller than the linewidth (2 MHz for each simulation).Despite the aforementioned requirement of a three sets of protons, the 4 > 12 pattern for 2,3,6,7-tetramethylnaphthalenium does produce the correct number of hyperne lines, and in excellent agreement with the calculated coupling constants (Fig. 7).The nal contender, the anthracenium radical cation, showed the largest disparity between experiment and theory.This species could probably be excluded as is it plausible that there is signicantly less anthracene formed than naphthalene (and its derivatives) this stage of the reaction.
The observation of larger polymethylaromatic radical cations compared to those proposed for H-ZSM-5 is interesting.Structurally, the two zeolites differ in the size of their channel structures with larger 12-membered ring channels being present in H-MOR compared to the 10-membered ring channels in H-ZSM-5.Whilst 4-and 8-membered ring channels are present in the H-MOR structure, it is improbable that these could contain the aromatic cations evident in the EPR spectra presented within this manuscript which suggests that, as expected, the active hydrocarbon pool is restricted to the large channels.However, the connes of the pore-channels in these zeolites seemingly restrict the formation of branched aromatics, such as hexamethylbenzene, phenanthrene and pyrene that are frequently registered in H-SAPO-34 and related cage-like frameworks. 36,50,52Although the results reported here do not preclude the formation of branched aromatics, it could be construed that the dimensions of H-MOR pores accelerate disproportion of these intermediates to the desired olenic products. 22,58

Conclusions
This manuscript demonstrates that it is possible to observe hyperne structures within the hydrocarbonaceous residue EPR spectra for H-MOR by careful control of the degree of coking with a low partial pressure of methanol.Through the application of experimentally calibrated DFT calculation of hyperne coupling constants and sampling a large selection of potential polymethylaromatic species, we have been able to denitively assign the radical cations that constitute the EPR signal of high temperature coke. 12,13In the case of H-MOR it has been shown that larger aromatic radical cations are formed compared to those expected in, for example, H-ZSM-5, however, both materials appear to disfavour the formation of the highly branched aromatics associated with MTO in H-SAPO-34. 36,52The EPR signal is invariant over the course of the coking reaction, which demonstrates these methylated aromatic radicals are present in the active catalyst.Therefore it is conceivable these radicals, analogous to tetramethyl-and hexamethyl-benzene, 21,23 are also reaction intermediates in the formation of light olens.

Fig. 1 Fig. 2
Fig. 1 Coking profiles, as C wt%, of post reaction samples produced at different reaction temperatures for different partial pressures of MeOH.

Fig. 3
Fig. 3 Comparison of the absorption spectra collected at ambient temperature for the fresh and air exposed catalyst at hourly time intervals (reactor conditions: 20% MeOH/Ar at 260 C).

Fig. 4
Fig. 4 Comparison of the radical content in the air deprived (red trace) and exposed (blue trace) samples derived from EPR signal intensity and the total C wt% (20% MeOH/Ar at 260 C; gray trace) at various time intervals.

Fig. 6 X
Fig. 6 X-band EPR first derivative spectrum (red line) overlaid with the second derivative spectrum (black line) of the fresh H-MOR catalyst sample after 75 min TOS.

Table 1
Comparison of experimental and calculated proton hyperfine coupling constants (in MHz) for polyaromatic radical cations