Methanol loading dependent methoxylation in zeolite H-ZSM-5

Room temperature methoxylation is methanol loading dependent: the higher the methanol loading, the faster the methoxylation. Methanol load of ≥2 leads to methoxylation while no methoxylation is observed with ≤1 molecule per Brønsted acidic site.


Introduction
A myriad of industrial catalytic processes, ranging from petroand ne-chemical to environmental processes, depend on crystalline zeolites such as H-ZSM-5 owing to their unique and yet versatile physical and chemical properties, which render not only high catalytic activity and product selectivity but also catalyst stability under harsh reaction conditions. 1,2 Environmental processes like exhaust aer-treatment utilise zeolites to adsorb/convert toxic pollutants, especially at the cold start of engines, which may originate from the unburnt fuel itself (e.g., methanol and/or ethanol blended fuels 3 ) or from the combustion process. 4 The petro-chemical processes include conversion of methanol, which can be derived from renewable resources, into hydrocarbons (MTH) over H-ZSM-5. MTH promises to mitigate the growing global demand for gasoline, polymer grade olens and aromatics through carbon neutral paths. 5 Since its rst development in 1976 (ref. 6 and 7) the potential impact of MTH on the energy and environment landscapes has stimulated interest in both industry and academia, and has triggered extensive research to understand the underlying reaction mechanism using experimental and computational tools. 1,[8][9][10][11][12][13][14][15][16] There is clear evidence that the catalytic activity arises from Brønsted acidic sites 1,8,9,12 and that a hydrocarbon pool formed during the reaction in the zeolite pores plays an important role in the activity, product selectivity and zeolite lifetime. 17 However, there is long standing uncertainty concerning the initial methoxylation, which is not only the rst step in the process but also a key step in hydrocarbon (pool) growth through methylation. 1,8,9,[18][19][20][21][22] Methoxylation occurs on the reaction of methanol with Brønsted acidic sites (H + -O-Si/Al) of H-ZSM-5 giving rise to methoxy species, 8,12,18,19,21,22 which is represented by: (1) Besides methoxy species, water is a by-product whose formation is a clear indication of the methoxylation reaction. The direct role of methoxy species in the rst C-C bond formation in the MTH reaction at 300 C was recently reported by some of us. 23 Moreover, the methylation (which is also known as alkylation) has an impact on a spectrum of petro-and nechemical processes. [18][19][20][21][22][23][24][25][26] Studies based on solid state nuclear magnetic resonance (NMR) and infrared (IR) conclude that methoxylation occurs only at elevated temperatures (>150 C). 22,27,28 In line with these observations, several computational studies, usually examining congurations with only one methanol molecule per unit cell (i.e. one methanol molecule per Brønsted acidic site) suggest that methoxylation could have a signicant activation energy (typically reported values are around 200 (AE20) kJ mol À1 ) and hence require higher temperatures for the reaction. 15,[19][20][21][22] In a marked contrast, our recent inelastic neutron scattering (INS) and operando diffuse reectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS) studies show the occurrence of methoxylation, to some degree, also under ambient conditions on H-ZSM-5 with a saturation level of methanol loading (7 molecules per Brønsted acidic site). [29][30][31] Such a room temperature methoxylation reaction was indeed not ruled out in an earlier study. 32 These observations are at least partially consistent with computational studies that report decreased energy barrier (reported values are varied; 130 (AE30) kJ mole À1 ) for methoxylation with 2 methanol molecules per Brønsted acidic site, as compared to that with one methanol molecule. 20,[33][34][35][36] The calculated energy barrier, although greatly reduced, remains considerable for room temperature methoxylation.
No detailed experimental evidence on the role of methanol loading in methoxylation has been reported so far. The present study, therefore, evaluates the effect of the number of methanol molecules per acidic site on the methoxylation at room temperature by operando DRIFTS and MS. To this end, the methanol loading in H-ZSM-5 (Si/Al z 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively ( Table 1). The results show that the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules. Signicantly, we nd that no methoxylation is observed with #1 molecule per acidic site.

Methodology
Operando diffuse reectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS) The zeolite H-ZSM-5 with a Si/Al ratio of 25, obtained from Zeolyst International, was calcined in air at 500 C for 24 h. The BET surface area and total pore volume of the calcined zeolite are 390 m 2 g À1 and 0.23 cm 3 g À1 , respectively. Mesopore surface area determined by t-plot method is 34 m 2 g À1 indicating that the total surface area of the zeolite is mainly arising from micropores. Operando DRIFTS and MS experiments were conducted on an Agilent Cary 600 series spectrometer equipped with a Harrick Praying Mantis reaction cell that was connected to a gas dosing system. 30 The reaction cell outlet was connected to a Hiden quantitative gas analysis (QGA) mass spectrometer for analysis of the products. Prior to the spectroscopic measurements, the zeolite was pre-treated in dry N 2 ow (100 ml min À1 ) at 500 C for a few hours and then cooled to room temperature (RT) under the same ow. Methanol pulse experiments were conducted under the same N 2 ow at RT with different loadings that results in around 8, 4, 2 and 1 molecules per Brønsted acidic site. [29][30][31] The evolution of surface adsorbed species and products were monitored by DRIFTS and MS, respectively, for around 30 min under the same N 2 ush.

Computational methods
Quantum mechanical/molecular mechanical (QM/MM) embedded calculations were employed to model zeolite H-ZSM-5. The ChemShell soware 37 was used to optimise zeolite geometries and determine vibrational frequencies of adsorbed species. For H-ZSM-5 models, the tetrahedral T12 site of the siliceous MFI was substituted with, Al which is located at the intersection of the sinusoidal channel. To form the Brønsted acidic site, a charge-compensating proton is bonded to the framework oxygen atom adjacent to T12 site 38 and oriented towards the center of the super cage; this conguration presents the highest deprotonation energy i.e. most stable. 39 For QM/MM calculations, the QM region is the chemically active part of the zeolite model and includes atoms up to the h oxygen atom (i.e. Al-O-Si-O-Si-O) from the central T12 site. The total number of atoms in the cluster model is 2165, including 74 QM atoms and 197 inner MM atoms. Atoms in the inner MM region can move during the geometry optimisation while, the outer MM region is frozen to ensure a bulk-like structure at a far limit from any chemical reactions. The inner and outer MM regions extend from the central T12 site to a radius of 10.58Å (20 a 0 ) and 21.17Å (40 a 0 ), respectively. The QM energy was calculated using spin unpolarised hybrid-DFT with the dispersion-corrected Becke97-3 exchange-correlation (XC) functional, B97-D, as provided in the GAMESS-UK code. [40][41][42] The atomic orbitals were represented by the Ahlrichs and Taylor TZVP Gaussian basis sets. 43 The self-consistent eld (SCF) convergence criteria was set to an energy change of less than 2.72 Â 10 À6 eV (1 Â 10 À7 Hartrees) between SCF iterations. 44,45 The MM energy was calculated using DL_POLY, 46 employing the forceeld of Hill and Sauer, 47,48 with the coordination dependent charges in the original forceeld being replaced with xed 1.2 and À0.6 e point charges for silicon and oxygen, respectively 49 . Geometry optimisations (see ESI †) were performed in a Cartesian coordinate space using the Limited-Memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) algorithm, with a maximum gradient convergence threshold of 0.015 eVÅ À1 . [50][51][52][53] Vibrational frequencies were calculated using ChemShell, with a task-farmed nite-difference approach, 40 allowing us to conrm that geometries correspond to local minima. 54,55 For the vibrational frequency calculations, the adsorbate, active site and second neighbouring framework atoms were displaced; comparison of this approximation against the displacement of all atoms in the QM region shows no differences, as previously reported. 56 Because our simulations aimed to calculate mainly vibrational frequencies of the CH 3 group, a scaling factor for the computed vibrational frequencies was calculated using data for the CH 3 asymmetric stretch of methanol at 93 K, which is the lowest experimental temperature reported. 57 The resulting scaling factors of 0.9306 and 0.9553 were used for normal and deuterated methanol, respectively. The scaling factors are well within the range previously employed of between 0.9 and 0.9614. [58][59][60][61]

Results and discussion
Operando DRIFTS and MS studies DRIFTS difference spectra of zeolite H-ZSM-5 with a methanol loading of 8 molecules per acidic site are shown in Fig. 1. It is evident that the spectra are dominated by hydrogen bonded methanol species that are unambiguously characterised by the triplet arising from the Fermi resonance caused by n(O-H) (of both methanol and zeolite OH groups) with 2d and 2g overtones, which falls between 1500 and 3500 cm À1 . 62,63 Within this region, the C-H stretching modes of the hydrogen bonded methanol (protonated methanol geometry and is discussed later) between 3100 and 2800 cm À1 are evident; however, no other bands attributable to methoxy species could be distinguished. Therefore, wavenumbers below 1500 and above 3500 cm À1 are probed for evidence of methoxy and O-H stretching modes, respectively. 30 A band at 938 that contains a shoulder at 990 is present along with another band at 1180 cm À1 (Fig. 1B). Typical P-Q-R bands of gas phase methanol appear at around 1008, 1032 and 1056 cm À1 (Fig. 1A); however, these bands disappear within the rst few minutes of the reaction under the N 2 ow. In the n(O-H) region, at least four negative bands appear between 3600 and 3750 cm À1 (Fig. 1C and Table 1). The band at 938 cm À1 is attributed to C-O stretch of the methoxy species 30,64 and the corresponding methyl rock band appears at 1180 cm À1 (ref. 65) indicating the occurrence of methoxylation at RT. 30 The shoulder at 990 cm À1 could also arise from the C-O stretch of a second type of methoxy species or of the hydrogen bonded methanol. 30 In line with the low frequency methoxy bands, consumption of different hydroxyls in the reaction (either in hydrogen bonding or methoxylation) is evident from negative bands above 3600 cm À1 ( Fig. 1A and C). The negative bands at 3610 and 3744 cm À1 are due to consumption of Brønsted acidic and silanol groups, respectively. 30,62,63 The band at 3665 cm À1 was previously assigned to hydrolysed extra-framework Al(Al-OH). 30,63 The assignment of the band at around 3725 cm À1 is not straight forward and hence it is tentatively attributed to the involvement of differently coordinated hydroxyl groups. 30 Similar spectroscopic features are observed for zeolites with methanol loadings of 4 and 2 molecules per Brønsted acidic site, although the overall intensity of the spectra is reduced by decreasing methanol loading from 8 to 2 molecules per acidic site. However, striking differences in the consumption of hydroxyls (above 3600 cm À1 ) and low frequency methoxy bands (below 1500 cm À1 ) are evident from the zeolite with the lowest methanol loading of 1 molecule per acidic site (Fig. 2) as compared to that with the highest methanol loaded zeolite (Fig. 1). Examining the DRIFTS spectra of the lowest methanol loading in more detail, we note rst that the signature infrared prole of the lowest methanol loaded zeolite in Fig. 2 matches precisely that of hydrogen bonded methanol with a neutral geometry, 32,62 which is different from the highest methanol loaded zeolite prole (Fig. 1) that reect the protonated methanol geometry. 30,32,62 The C-H stretching modes of the neutral geometry appear at 2958 and 2850 cm À1 which can be attributed to n as (C-H) and n s (C-H) modes, respectively, of hydrogen bonded methanol with the neutral geometry. 62,63 The position of the bands is at slightly higher wavenumbers as compared to that of the protonated geometry which shows up at 2950 and 2843 cm À1 (Fig. 1), in line with previous studies. 30,63 The band at 3550 cm À1 is also characteristic of the hydrogen bonded methanol with a neutral geometry 62 and the band intensity increases with increasing hydrogen bonded methanol triplet and with a growing negative band at 3610 cm À1 , which is indicative of the involvement of Brønsted acidic sites. Moreover, the absence of multiple hydrogen bonded methanol molecules in the form of dimers or oligomers is evident when we compare Fig. 1 and 2 from the missing absorbance centred at 3260 cm À1 . 62,63 Even more signicantly, only one negative band at 3611 cm À1 is present, unlike four negative bands for the highest methanol loaded zeolite, and bands assignable to low frequency methoxy bands are completely missing. Instead, a broad band between 1000 and 850 cm À1 is observed, which could be a combination of different bands. For example, a contribution from the Si-O-Si asymmetric stretch at around 880 cm À1 (Fig. 2B and Table 1) and hydrogen bonded methanol bands (assignable to C-O stretch and methyl rock) could also contribute between 900 and 1100 cm À1 . Accordingly, the negative band at 3610 cm À1 is attributed to involvement of Brønsted acidic hydroxyls in hydrogen bonding with methanol indicating that the Brønsted acidic hydroxyls are more reactive than other hydroxyls such as hydrolysed extra-framework Al-OH and silanol groups (see Fig. 1). 30 It is noteworthy that the methyl rock of methoxy band at around 1180 cm À1 is not present and hence a contribution from the C-O stretch of the methoxy species to the broad band between 1100 and 850 cm À1 can be ruled out, which is further indicated by the signature C-H stretching modes of the methoxy and hydrogen bonded methanol. To this end, the C-H stretching region is magnied and four different methanol loading experiments are compared in Fig. 3.
The zeolite with the highest methanol loading of 8 molecules per acidic site clearly shows bands at 2980 and 2876 cm À1 , attributable to n as (C-H) and n s (C-H) of methoxy species (Fig. 3A). 27,30,62,63,65 However, these bands are somewhat obscured by the intense triplet arising from the protonated hydrogen bonded methanol species that present C-H stretching  bands at 2950, 2922 and 2843 cm À1 . 30,62,63,65 It appears that the intensity of the methoxy bands at 2980 and 2876 cm À1 diminishes on decreasing the methanol loading from 8 to 2 molecules per acidic site, and with no indication of such bands for the lowest methanol loading of 1 molecule per acidic site. To verify this, the difference spectra derived from the earliest measurement and at a later stage of the reaction are compared in Fig. 3B. It is clear that the bands assignable to methoxy species emerge at 2980 and 2876 cm À1 , and 2973 and 2868 cm À1 , 8,23,27,30 which imply the occurrence of at least two types of methoxy species. This observation is consistent with our earlier INS and DRIFTS data that show two types of methoxy species 29,30 and is in line with earlier infrared and NMR reports. 27,28 Clearly, the intensity of the methoxy bands at 2980 and 2875 cm À1 , and 2973 and 2865 cm À1 diminishes on decreasing the methanol loading from 8 molecules per acidic site to 2 and no bands attributable methoxy species are present for the lowest methanol load of 1 molecule per acidic site (Fig. 3B), suggesting the occurrence of loading dependent room temperature methoxylation. This conclusion is further corroborated by the evolution of the methyl rock band of methoxy species at 1180 cm À1 (Fig. 4).
It is evident from Fig. 4A that the methyl rock band grows rapidly for the highest methanol loading zeolite and thereaer decreases gradually suggesting the occurrence of partial hydrolysis of methoxy species due to the presence of water, which is a result of the methoxylation reaction (eqn (1)), in the zeolite pores. 30 A similar observation is also reported by NMR and, signicantly those methoxy species on ZSM-5 are not completely eroded on hydrolysis with water at room temperature unlike on Y and SAPO-34, 28 implying the unique intrinsic nature of active acidic sites in ZSM-5. The evolution of the band is clearly hampered on decreasing the methanol loading from 8 to 2 molecules per acidic site and crucially no intensity gain of the band is observed for the lowest methanol loading of 1 molecule per acidic site (Fig. 4A). Based on this observation, the rate of methoxylation as a function of methanol loading is derived by following the rate of intensity gain of the methyl rock band at 1180 cm À1 (Fig. 4B), for which the evolution of the band during the rst 100 seconds is considered. The rate of methoxylation is 20 and 40 times faster for the zeolite with the highest methanol loading of 8 molecules per acidic site than that for the loadings of 4 and 2 molecules per acidic site, respectively. The occurrence of room temperature methoxylation is further corroborated with MS data (Fig. 4C) which show the evolution of water during the reaction. It is evident that the relative amount of water formed during the reaction decreases on decreasing the methanol loading from 8 to 2 molecules per acidic site and no water formation is observed for the lowest methanol loading of 1 molecule per acidic site, which is conrmed by the area under the MS prole as a function of methanol loading in the inset of Fig. 4C. The rate of water formation is derived from the rst 100 seconds of the reaction (Fig. 4D), which is similar to that considered for the rate of methyl rock band evolution (Fig. 4B). As expected, the rate of water formation is 2 and 12 times faster for the highest methanol loading zeolite than that for the 4 and 2 molecules per acidic sites, respectively. The relative rates derived from DRIFTS and MS are at least an order of magnitude faster for the highest methanol loading zeolite than that for the zeolite with a loading of 2 molecules per acidic site and conrm the loading dependent room temperature methoxylation in zeolite H-ZSM-5.
Furthermore, we have veried the occurrence of loading dependent room temperature methoxylation by isotopic methanol experiments using CD 3 OH. Zeolites with loadings of 8 (the highest) and 1 (the lowest) methanol molecules per acidic site are compared in Fig. 5. The hydrogen bonded methanol C-D stretching bands at 2217 and 2082 cm À1 attributable to n as (C-D) and n s (C-D) modes are present for both the zeolites (Fig. 5A). However, no bands assignable to methoxy C-D stretching modes are visible from Fig. 5A and hence difference spectra derived from the earliest measurement and at a later stage of the reaction are compared in Fig. 5B, which is similar to the one reported in Fig. 3B. It is evident that the highest methanol loading zeolite exhibits the bands that are attributable to methoxy species at around 2242, 2176 and 2066 cm À1 , 12 while these bands are completely missing for the lowest methanol loading zeolite. The bands at 2242 and 2176 cm À1 are assigned to methoxy n as (C-D) and the corresponding n s (C-D) stretching modes are at 2072 and 2066 cm À1 , which appear to encompass two types of methoxy species. The band at 2072 cm À1 should be treated with some caution because the hydrogen bonded methanol with neutral (lowest methanol load) geometry exhibits a blue shi in the C-D stretching modes as compared with the protonated geometry (highest methanol load), as discussed for the normal methanol loading experiments in Fig. 1  and 2. Thus, the band at 2072 cm À1 could also reect the hydrogen bonded methanol of protonated geometry, whilst the counterpart band at 2082 cm À1 is observed for the neutral geometry. Nonetheless, the isotopic methanol experiments conrm the occurrence of loading dependent room temperature methoxylation, in agreement with normal methanol loading experiments reported in Fig. 3B and also in agreement with our earlier INS and DRIFTS studies. 29,30 In line with these observations, water formation is observed only for the highest methanol loading zeolite by MS (not shown), and no water formation is detected for the lowest methanol loading zeolite, consistent with the above results reported with normal methanol experiments in Fig. 4C. In line with methanol loading dependent room temperature methoxylation experiments, recent simulations of methoxylation show that the energy barriers decrease from 160 to 119 kJ mol À1 when the loading is increased from 1 to 4 methanol molecules per Brønsted acidic site, respectively. 66 The energy barriers calculated are in line with the previous literature. 20,[33][34][35][36] However, the energy barrier of 119 kJ mol À1 for 4 methanol molecules per acidic site remains signicant in the context of a process observed at room temperature. Also, we recognise the importance of Si/Al ratio of the zeolite in the reaction which is a topic for future study.

Computational studies
To complete our analysis, the assignment of the DRIFTS bands to surface methoxy and hydrogen bonded methanol species is veried by quantum mechanical/molecular mechanical (QM/ MM) calculations (Table 2 and 3). Calculated vibrational frequencies of methoxy species at the Brønsted acidic site are 925, 1157/1165, 2880 and 2988 cm À1 . The band at 925 cm À1 is arising from the n(C-O) stretch (structure A in Table 2) 21 and the pair of bands at 1157/1165 cm À1 are due to perpendicular and parallel methyl rock modes (the parallel mode is represented by structure B in Table 2), which themselves are not resolved in the DRIFTS data (see Fig. 1 and ref. 30). The bands at 2880 and 2988 cm À1 result from n s (C-H) and n as (C-H) stretching modes of the methoxy species, respectively (structures C and D in Table 2). By comparison with the DRIFTS data, the calculated vibrational frequencies of methoxy n(C-O) stretch and methyl rock modes are slightly red shied, while n(C-H) stretching modes are slightly blue shied. Also, n(C-O) stretch and methyl rock modes are slightly red shied as compared to those reported in our earlier study, which can be attributed to the two different methods employed. 30 The calculated vibrational frequencies of the methoxy species are slightly affected by co-adsorption of either methanol and/or water molecules in the unit cell, representing the experimental conditions, which indicates the sensitivity of vibrational frequencies of adsorbed species to the local environment of the zeolite unit cell. Based on these observations, we conclude that the calculated vibrational frequencies are in line with the experimental DRIFTS data as is evident from Table 1 and 2. The vibrational frequencies of deuterated methoxy species are calculated to be 753, 865/900, 2078 and 2253 cm À1 , which are arising from n(C-O) stretch, r(CD 3 ) rock, n s (C-D) and n as (C-D) modes, respectively ( Table 2). The calculated n(C-D) stretching modes represent an ideal case of only one kind of methoxy species as opposed to the experimental data that reect a complex combination of reactants and product molecules in a unit cell, and hence are blue shied slightly as compared to the DRIFTS results (see Table 1 and 2). The vibrational frequencies of hydrogen bonded methanol with neutral and protonated geometries are also calculated and compared with the experimental data in Table 3. Addition of one methanol molecule at the Brønsted acidic site gives rise to hydrogen bonded structure with the neutral geometry as depicted with structure A in Table 3. The geometry forms a six membered ring structure at the Brønsted acidic site and is similar to the one reported in the literature. 14,32,62 The calculated vibrational frequencies for the n s (C-H) and n as (C-H) stretching modes of the neutral geometry are 2848 and 2962 cm À1 , respectively, and are in excellent agreement with the experimental data reported in Table 3 and, Fig. 3  corresponding deuterated methanol geometry gives rise to vibrational frequencies of 2093 and 2262 cm À1 for n s (C-D) and n as (C-D) stretching modes, which are in line with DRIFTS data that show the n s (C-D) mode at 2082 cm À1 and the broad band that envelops the n as (C-D) region with an indication at 2263 cm À1 . Addition of a second methanol molecule in the unit cell results in an eight membered ring structure at the Brønsted acid site and that the acidic proton shuttles between the framework oxygen and methanol hydroxyl, forming a protonated methanol geometry as depicted with structure B in Table 3.

and 5. The
The vibrational frequencies calculated for the n s (C-H) and n as (C-H) stretching modes of the protonated methanol geometry are calculated to have frequencies of 2844 and 2944 cm À1 , respectively. These values are consistent with the experimental observations (Table 3), including the observed relative redshi in the vibrational frequencies of n(C-H) modes of the neutral geometry as compared with the protonated one. 63 The experimental results show a z8 cm À1 redshi for the neutral geometry as compared to protonated geometry, while simulations show 10 (AE5) cm À1 . The corresponding deuterated methanol geometry yields vibrational frequencies of 2083 and 2272 cm À1 for n s (C-D) and n as (C-D) modes. The vibrational frequency of 2083 cm À1 for n s (C-D) is in excellent agreement with the observed DRIFTS band, whereas the calculated 2272 cm À1 frequency for n as (C-D) appeared as a shoulder at 2267 cm À1 to the prominent band at 2217 cm À1 observed by DRIFTS. This once again suggests the occurrence of a combination of different bands in the n as (C-D) region under experimental conditions, which might be difficult to be captured completely by simulations. 67 Nonetheless, the calculated vibrational frequencies do reect our assignment of the DRIFTS bands as is evident from Table 1, 2 and 3.

Summary and conclusions
Our study of the methanol loading dependent methoxylation in zeolite H-ZSM-5 (Si/Al z 25) pores under ambient conditions has enabled us to probe simultaneously methoxy species and reaction products. The assignments of infrared vibrational frequencies of methoxy and hydrogen bonded methanol have been supported by QM/MM simulations, which are consistent with the experimental DRIFTS data. Both experiment and simulation show that the methoxy bands at around 940, 1180, 2868-2876 and 2980-2973 cm À1 correspond to n(C-O), r(CH 3 ), n s (C-H) and n as (C-H), respectively. From our results, it is evident that the room temperature methoxylation in H-ZSM-5 is methanol loading dependent: the higher the loading, the faster the methoxylation. The reaction is more than an order of magnitude faster with 8 molecules per Brønsted acidic site than that with 2 molecules. As well as methoxylation, hydrogen bonded methanol with protonated geometries are also formed. Signicantly, no methoxylation is observed with methanol loading of #1 molecule per acidic site, but only hydrogen bonded methanol with neutral geometry is detected. Thus, the structure of hydrogen bonded methanol is also loading dependent. These ndings will have signicant implications for reactions involving zeolite H-ZSM-5 and methanol.

Conflicts of interest
There are no conicts to declare.