Isothermal cyclic conversion of methane to methanol using copper-exchanged ZSM-5 zeolite materials under mild conditions

Applied Catalysis A, General 587 (2019) 117272

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Isothermal cyclic conversion of methane to methanol using copperexchanged ZSM-5 zeolite materials under mild conditions L. Burnetta, M. Rysakovaa, K. Wanga, J. González-Carballob, R.P. Toozeb, F.R. García-Garcíaa, a b

T ⁎

School of Engineering, Institute of Materials and Processes, University of Edinburgh, Robert Stevenson Road, Edinburgh EH9 3FB, UK Drochaid Research Services, Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: Direct conversion of methane to methanol Isothermal cyclic Copper-exchanged ZSM-5 zeolites In situ UV–vis spectroscopy

Direct conversion of methane to methanol (DMTM) has become a particularly attractive route for the functionalisation of natural gas. Here the proven capability of copper-exchanged ZSM-5 zeolites to carry out DMTM at mild conditions has been extended by also demonstrating its capability to perform repeated reactions in a concept known as cycling. A series of five copper-exchanged ZSM-5 zeolites with different Si:Al ratio were prepared via aqueous ion exchange. The materials characterisation was carried out using a combination of transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption isotherms at −196 °C (BET and BJH methods), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and in-situ ultraviolet-visible spectroscopy (UV–vis) techniques. Under mild isothermal conditions (air activation; T = 200 °C; P = 1 atm), methanol productions of 2.8 ± 0.1 μmol·gcat−1, 26.6 ± 0.1 μmol·gcat−1, 12.5 ± 0.1 μmol·gcat−1, 10.8 ± 0.1 μmol·gcat−1 and 3.2 ± 0.1 μmol·gcat−1 were achieved for copper-exchanged ZSM-5 containing 20:1, 30:1, 50:1, 80:1, and 200-400:1 Si:Al ratio, respectively. Comparing the results of cycling Cu-ZSM-5 (Si:Al = 200-400:1) with those obtained for Cu-ZSM-5 (Si:Al = 30:1) has led to the conclusion that hydrophobicity plays a decisive role in the capability to cycle, with materials containing a higher Si:Al being better suited to cycling. A total of five repeated cycles were achieved using CuZSM-5 (Si:Al = 200-400:1). The methanol production was higher, the lower the Si:Al ratio with an optimal Si:Al = 30:1. However, TEM analysis suggests that precipitation of copper nanoparticles on the catalyst support structure could account for the reduced activity found within the material containing the lowest Si:Al. in situ UVvis spectroscopy characterisation of the copper-exchanged ZSM-5 zeolite materials under similar DMTM reaction conditions suggested that active copper complexes were being created and then destroyed during the DMTM reaction. A speculative discussion of the copper complexes present within these materials has also been provided.

1. Introduction The “Methanol Economy” is entirely underpinned by the capability to transform methane into methanol in an efficient and industrially economically attractive way: a problem that continues to have an elusive solution. The current method of methanol production is plagued with high energy demands and poor overall efficiency at small scales. The favoured industrial technique involves a two-stage energy-intensive process. Firstly, methane is reacted with steam to produce syngas across a Ni/Al2O3 catalyst. Requiring highly elevated temperatures and pressures, approximately 25% of the methane feedstock is combusted to maintain these extreme conditions [1]. In the second step, syngas is catalytically converted to methanol across a Cu/ZnO/Al2O3 catalyst at temperatures of approximately 250 °C and pressures up to ⁎

100 atm [2,3]. These high energy demands make the production of methanol a prime example of the economy of scale, with large quantities needing to be produced to make the process economically feasible [4]. For this reason new and innovative processes are being sought that would allow methanol to be produced in smaller quantities and more energy efficiently [5]. The direct conversion of methane to methanol (DMTM) has been seen as a dream reaction within chemistry for decades [6]. However, the symmetry of the methane molecule and the very high strength of the C–H bond (438.8 kJ/mol) make methane a difficult molecule to activate and partially oxidise without over oxidizing to carbon dioxide and water [2]. Methanotrophic bacteria are known to use the enzyme methane monooxygenase (MMO) to directly convert methane to methanol at

Corresponding author. E-mail address: [email protected] (F.R. García-García).

https://doi.org/10.1016/j.apcata.2019.117272 Received 25 July 2019; Received in revised form 20 September 2019; Accepted 21 September 2019 Available online 25 September 2019 0926-860X/ © 2019 Published by Elsevier B.V.

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Fig. 1. Schematic representation of some of the copper complexes that can be formed inside the ZSM-5 zeolite.

out with ex situ washing. In order to improve the methanol production, the DMTM reaction can be repeated in a concept known as cycling. Similarly to the threestage procedure suggested by Alayon et al. [15] the material is activated, reacted and then extracted. However, in the process of cycling, initially carried out by Bozbag et al. [16], the three-stage reaction procedure known as one ‘cycle’ is repeated and methanol extracted ‘online’ using steam. Using this method Bozbag et al. [16] reported up to eight cycles using a copper-exchanged MOR zeolite material. An initial methanol production of 13 μmol gcat−1 was achieved with a 30% increase being found for cycle two, which remained steady thereafter. Although methanol production was improved, the procedure still involved high-temperature activation at 450 °C at the start of every cycle. Likewise, Grundner et al. [17] reported up to eight cycles using a copper-exchanged MOR zeolite material. Although Grundner et al. [17] methanol production was significantly larger than that observed by Bozbag et al. [16] (i.e. 120 μmol gcat−1 vs 13 μmol gcat−1), the activation of the material in oxygen was still carried out at high-temperature (i.e. 500 °C). Pappas et al. [18] also observed that methanol production and selectivity gradually increased when cycling a copper-exchanged CHA zeolite material during the DMTM reaction. Hence, after four cycles the methanol production reached 125 μmol gcat−1, a 17% increase compared with the first cycle. Despite, being the highest methanol production reported so far for copper-exchanged CHA zeolite based materials, the activation of the material was still carried up at 500 °C using 1000 mbar of oxygen. Within the cage-like structure of the ZSM-5 zeolite there are multiple sites that copper cations can deposit to form a spectrum of copper complexes, some of which are shown in Fig.1 [19]. A great deal of work has been carried out to determine the precise structures responsible for DMTM within copper-exchanged ZSM-5 materials and how the oxygen potential during activation affects copper complexes formed during activation. This project seeks to further develop the proven capability of copper-exchanged ZSM-5 materials to carry out DMTM by also demonstrating its capability to repeat the reaction in multiple cycles. In this work a series of five copper-exchanged ZSM-5 zeolites materials have been prepared via ion exchange with a distribution of Si:Al contents. The combination of low temperature (200 °C) activation in air, low pressure reaction, and isothermal conditions mean that the DMTM reaction will be studied at mild conditions. By attempting to carry out as many cycles as possible before deactivation the activity of the materials will be studied in a hope to further demonstrate the process of

room temperature and pressure with a very high degree of selectivity [1]. Depending upon its location within the bacteria, MMO can be split into two forms: soluble MMO (sMMO) and particulate MMO (pMMO). Within pMMO, a di-copper core has been found to be the active site for DMTM whereas a di-iron core was identified within sMMO [7]. Due to the lucrative prospect of DMTM at mild conditions, attempts are being made to duplicate the activity of MMO in conditions as close to those in nature as possible. More recently, a great deal of work has been carried out into the use of zeolites as the host structures of iron and copper cations [8]. The structure of zeolites acts as a framework for metal cations to deposit and potentially form structures that are active for DMTM. In this respect, Sobolev et al. [9,10] studied the biomimetictype activity of iron exchanged zeolites in the hope of achieving DMTM at mild conditions. In their work, iron-exchanged ZSM-5 materials were found to be capable of DMTM at room temperature after an initial activation period with nitrous oxide at 240 °C. It is important to highlight that Sobolev et al. [9,10] were the first to develop the three-stage process of activation, reaction and extraction for DMTM. A 0.30 wt% Fe2O3 ZSM-5 (SiO2/Al2O3 = 100) catalyst was initially activated at 240 °C in a stream of nitrous oxide. From there, the sample was reacted with methane at room temperature. After reaction methanol was removed either using water or a water solution of acetonitrile as a solvent. Using this method they reported methanol productions of up to 6.5 μmol gcat−1. A decade later, Groothaert et al. [11–13] described as stepped conversion where the catalyst was initially activated at high temperature, typically 450 °C, in a stream of nitrous oxide or oxygen. Then the sample was reacted with methane at 200 °C. After reaction, methanol was removed either using a stream of steam or liquid water. Using this method and zeolites such as ZSM-5 and mordenite (MOR) they reported methanol productions up to 12 μmol gcat−1. Building on the work of Groothaert et al. [11–13], Sheppard et al. [14] were the first to propose the use of an isothermal (150 °C) reaction procedure along with in situ monitoring of the reaction products [14]. By carrying out the process isothermally the high cost associated with the activation of the materials at high temperatures, typically 450–500 °C, can be avoided. Therefore, the processes of reaction and extraction were carried out at the more modest temperature of 150 °C. Initially, the copper-exchanged ZSM-5 materials were activated using nitric oxide whilst extraction was carried out using steam. Although a lower methanol production (0.69 μmol gcat−1) was obtained by Sheppard et al. [14], it was reported that the methanol production was significantly greater than that obtained by the same experiment carried 2

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Halenda (BJH) model. The mesopore volume (Vmeso) was estimated using Eq. (1):

cycling. Finally in situ UV–vis spectroscopy will be carried out to observe the evolution of copper active sites during activation and reaction respectively whilst a series of characterisation techniques will be employed to provide a detailed analysis of the physicochemical properties of copper-exchanged ZSM-5 zeolite materials.

Vmeso = Vp/po - 0.99 - Vmicro

(1)

2.2.4. Thermal gravimetric analysis Thermogravimetric analysis (TGA) was carried out using a Netzsch STA 449 F1 Jupiter Co and Aëolos QMS 403 C mass spectrometer to determine the mass change of the samples as a function of temperature. This analysis was carried exclusively for the pelletised cooper-exchanged ZSM-5 zeolite materials. A mass of 40 mg was loaded into the equipment and ramped at a rate of 5°C/min using argon flowing at 40 mL/min until the material reached a temperature of 500°C.

2. Experimental 2.1. Preparation of the copper-exchanged ZSM-5 zeolite materials Copper containing ZSM-5 zeolites were prepared by the ion-exchange method [20]. For this commercial NH4-ZSM-5 zeolites (Alfa Aesar) were firstly calcined at 500 °C (1 °C/min) for 8 h in a static furnace (SNOL 3/1100 LHM01). Then, they were transferred to a beaker containing 200 mL aliquot of 0.01 M copper salt solution (initial pH = 4), Copper (II) nitrate hemi(pentahydrate) (Alfa Aesar, 98%), and stirred using a magnetic stirrer for a period of 24 h. Afterwards, the solution (final pH = 3) was centrifuged (Centurion K242 Large Centrifuge) to separate the solution from the solid. In order to remove physisorbed precursor species the solid was washed with distilled water and centrifuged serval times. Then, the solid was dried in an oven (SciQuip Oven-55S) at 100°C for a period of 18 h. From drying, the different copper-exchanged ZSM-5 zeolite materials were calcined in a furnace (SNOL 3/1100 LHM01) at 500°C in air for a time of 2 h and a ramp rate of 1 °C/min. The samples were pelletised by applying a load of 10 t in a 13 mm diameter cylinder for 90 s using a hydraulic press (Atlas™ Manual 15 T Hydraulic Press). These pellets were then crushed and sieved to sizes ranging from 125 μm to 250 μm using stainless-steel sieves (Fieldmaster 78–800). The different copper-exchanged ZSM-5 zeolite materials were labelled and assigned by their parent commercial zeolite as follows; CuZSM-5 (20:1), Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1), Cu-ZSM-5 (80:1), and Cu-ZSM-5 (200-400:1), where (20:1), (30:1), (50:1), (80:1), and (200400:1) refers to the Si:Al ratio. Characterization and catalytic activity tests were carried out using the pelletised samples.

2.2.5. Scanning Electron microscopy and energy-dispersive X-ray spectroscopy Scanning electron microscopy (SEM) on the different materials was carried out with a JEOL JSM 5600 SEM with an Oxford INCA Energy 200 EDX analyser to determine copper, silicon, and aluminium content. SEM images for each of the five materials can be found within the supplementary information. Elemental composition analysis for the different copper-exchanged zeolite materials here studied is summarised in Table 2. 2.2.6. In-situ UV–vis spectroscopy in situ UV–vis characterisation under similar DMTM reaction conditions was carried out using a Shimadzu UV-3600 Plus UV-VIS-NIR spectrophotometer. Baselining and zeroing of the spectrophotometer was carried out using samples of the commercial zeolite: NH4-ZSM-5 (Alfa Aesar). The scanning range on the spectrophotometer computer interface was set to 850 nm – 300 nm (12,000 cm−1 – 33,000 cm−1). During measurement the material was ramped at a rate of 10 °C/min to 500°C under a steady stream of oxygen (BOC, 99.5%). Once the material reached 500°C, it was held there for a total activation period of 3 h under oxygen, then the sample was cooled to 200°C under an oxygen atmosphere. From there, the sample was flushed with argon at 50 mL/min for 1 min to clear the system of oxygen. It was then reacted with methane (BOC, 10.18% in argon) at a flowrate of 30 mL/min for 30 min. Spectra were taken every 60 s throughout the activation and reaction period.

2.2. Characterisation of the copper-exchanged ZSM-5 zeolite materials 2.2.1. Transmission Electron microscopy The surface topography of the materials was studied using a Transmission Electron Microscope (TEM) (JEOL JEM-2011) operated at 200 kV. The samples were dispersed in ethanol and treated in an ultrasound bath for 1 min. Then two drops of the dispersion were deposited onto carbon coated copper grids and allowed to dry in open air.

2.3. Performance of the copper-exchanged ZSM-5 zeolite materials during the DMTM reaction 2.3.1. Experimental apparatus The experimental apparatus used to carry out the DMTM reaction was equipped with: i) a packed bed reactor (PBR) unit, ii) a reactor furnace (Elite Thermal Systems Limited. Model No: TSV12/32/150), iii) a network of electrical line heaters, iv) a water delivery system (IGI Systems), v) a gas delivery system and vi) a mass spectrometer (EcoSysP™ Mass Spectrometer). A novel feature of this experimental setup was the use computer programme written using LabVIEW systems engineering software which allowed the control of i) opening and closing of the twelve inlet solenoid valves of the gas delivery system, ii) parameters of the water delivery system (WDS), iii) regulation of the electrical line heaters. The computer system also allowed real time readings from the mass spectrometer and the water composition of the WDS to be monitored and adjusted accordingly. Condensation within the lines was prevented by a network of electrical line heating tape and supplemented by thermal insulating fabric, which kept the line temperatures above 100 °C. The temperature of each of these lines was monitored using several k-type thermocouples, distributed throughout the equipment which displayed to the computer interface.

2.2.2. X-ray diffraction An X-ray diffraction (XRD) analysis was carried out to determine the phase and crystallinity of each of the materials here studied. A Panalytical X’Pert PRO Multipurposed Diffractometer was used to record powder x-ray diffraction (XRD) patterns using Co K(α) irradiation (λ = 1.790307 Å). Diffraction patterns were collected at a step size of 0.02° between the range of 10°–90° (2θ). 2.2.3. Nitrogen adsorption/desorption isotherms The surface area and pore volume of the materials was determined at three different stages of preparation (non-calcined commercial NH4ZSM-5 zeolites, powder copper-exchanged ZSM-5 zeolite materials, and pellets copper-exchanged ZSM-5 zeolite materials) using nitrogen adsorption/desorption isotherms. The materials were degassed under helium at 200°C for 2 h before nitrogen adsorption/desorption measurements were taken at -196°C (Micrometrics Gemini VI Surface Area and Pore Size Analyzer) [21]. The BET method was used to calculate the specific surface area of the samples [22]. The micropore volume (Vmicro) and the mesopore surface area (Smeso) were found using the tplot method whilst the mesopore size and volume distributions were calculated from the nitrogen desorption data using the Barrett-Joyner3

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materials the BET surface area increased when compared to their parent NH4-ZSM-5 commercial zeolite. Across all Si:Al ratios the relative difference between the BET surface area of powdered and pelletised materials remains constant whilst commercial and pelletised materials have approximately equal surface areas for Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1). The surface areas calculated for NH4-ZSM5 (20:1) and NH4-ZSM- 5 (200–400) are significantly lower than those of their pelletised counterparts. All the materials showed a characteristic type I BDDT nitrogen adsorption/desorption isotherm which suggests the presence of micropores within their structures [25]. The gradual rise in the quantity of nitrogen adsorbed can be attributed to the slow filling of micropores whilst the sharp uptake at higher relative pressures suggests a high contribution of mesoporosity within all the materials. The high contribution of mesoporosity can be explained by the filling of the interparticle voids, created by agglomeration of nano-sized ZSM-5 crystals [25]. Hence, the higher maximum uptake of the pelletised copper-exchanged ZSM-5 materials at higher relative pressures, compared to NH4-ZSM-5 commercial zeolites and powder copper-exchanged ZSM-5 materials, suggests a particularly high number of mesopores. This is further verified by the measured volume of the mesopores which shows greater mesopore volumes (Vmeso) for the pelletised copper-exchanged ZSM-5 materials when compared to NH4-ZSM-5 commercial zeolites and powder copper-exchanged ZSM-5 materials.

2.3.2. Procedure during the DMTM reaction experiments In a typical DMTM reaction experiment the material was exposed to the following gaseous environments carried out in series: a) 60 min activation in air (BOC, 99.9%), b) 5 min argon purge (BOC, 99.9%), c) 60 min reaction in 10 vol.% methane balanced in argon (BOC, 10.18%), d) 5 min argon purge (BOC, 99.9%), e) 60 min reaction in 5 vol.% steam balanced in argon. All the experiments were carried out under mild conditions (T = 200°C; P = 1 atm) and the feed gas flow rate was kept at a constant 150 mL/min (STP). The effluent gas was monitored by inline mass spectrometry. 2.3.3. Procedure during the Isothermal DMTM Cycling experiments In order to improve the methanol production the DMTM reaction can be repeated in a concept known as cycling. A typical isothermal DMTM Cycling experiment involved 5 cycles. In each cycle the copperexchanged ZSM-5 zeolite material was exposed to the gaseous environments already mentioned in section 2.3.2. All the experiments were carried out under mild conditions (T = 200°C; P = 1 atm). 3. Results 3.1. Characterisation of the copper-exchanged ZSM-5 zeolite materials 3.1.1. Transmission Electron microscopy TEM images for each of the five copper-exchanged ZSM-5 materials, taken after calcination in air at 500°C for a period of 2 h using a ramp rate of 1°C/min, are shown in Fig. 2. Copper nanoparticles of 5 nm are observed in Cu-ZSM-5 (20:1), which exhibiting a high degree of uniformity across the surface of the sample. Notice that these particles cannot be seen within any of the remaining materials. Fig. 2b shows a high resolution TEM (HRTEM) of one of the nanoparticles showing dspacing of 0.2068 nm featuring the distance between the (111) planes of metallic copper crystals.

3.1.4. Thermal gravimetric analysis The change in mass of each of the pelletised copper-exchanged ZSM5 materials as it was ramped at 5°C /min from room temperature to 500°C under vacuum are presented in Fig. 5. Each of the five materials showed a mass loss upon thermal treatment as water is being driven from their internal structures. Among all the materials studied, Cu-ZSM-5 (20:1) shows the greatest drop in mass with a final mass loss of 8%. On the other hand, Cu-ZSM-5 (200-400:1) shows the smallest drop with approximately 1% of its mass being lost after treatment at 500°C. With decreasing the Si:Al ratio each of the materials show increasing mass loss with Cu-ZSM-5 (80:1) and Cu-ZSM-5 (50:1) losing approximately 3% of their masses whilst Cu-ZSM-5 (30:1) lost approximately 5%. The first derivative of the TGA profiles is depicted on Fig. 5b; CuZSM-5 (20:1), Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1), Cu-ZSM-5 (80:1) and Cu-ZSM-5 (20-400:1) have inflection points of 125°C, 112°C, 107°C, 102°C, and 50°C, respectively. TGA first derivative shows the temperature were the rate of dehydration is maximized. According to these profiles, the lower the Si content, the higher the water content and the higher the temperature necessary to dehydrate the sample.

3.1.2. X-ray diffraction XRD diffractograms for the different copper-exchanged ZSM-5 zeolites after calcination for 2 h in air at a temperature of 500 °C using a ramp rate of 1°C/min, are shown in Fig. 3. All materials show that the different Al:Si ratio ZSM-5 zeolites employed in this work have a MFI crystal structure with characteristics peaks at 2θ = 10.5°,15.8°,16.5°, 17.7°, 18.6°, 27.4°, 28.4°, 30.2° and 35.7°, which correspond to (111), (102), (112), (131), (022), (051), (313), (323) and (062) planes, respectively [23]. The XRD diffractograms pattern for the different copper-exchanged ZSM-5 zeolites were compared to the JCPDS 00-0450937. Magnification of the 2θ region between 40-60° shown in Fig. 3b highlights that all reflections featuring the support ZSM-5 but reflections of any copper species are not detected. For instance main reflection of tenorite at 42.5° and 46.6° corresponding to (11-1) and (111) planes, respectively, cannot be seen [24]. This suggests copper particles with a very small particle size, as the cooper particles observed in sample Cu-ZSM-5 (20:1) by TEM, or/and amorphous phases of copper. Similarly, the inability of XRD characterisation techniques to pick out small crystalline compounds at relatively low concentrations is a wellknown limitation of XRD analysis [22].

3.2. Performance of copper-exchanged zeolite materials during the DMTM reaction A typical DMTM reaction experiment involved 3 steps; i) 60 min activation in air (BOC, 99.9%), ii) 60 min reaction in 10 vol.% methane balanced in argon (BOC, 10.18%), iii) 60 min reaction in 5 vol.% steam balanced in argon. All three steps were was carried out under mild conditions (T = 200°C; P = 1 atm). The instantaneous methanol concentrations taken during the extraction process using 5% Vol steam in argon at 200°C are shown in Fig. 6a. Samples Cu-ZSM-5 (20:1), Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1) follow a similar extraction profile, with each material exhibiting a rapidly increases of the methanol concentration reaching their maxima within the first ten minutes of the extraction process. The maximum methanol extraction rate obtained from each material were as follows; Cu-ZSM-5 (30:1) maximum of 1.07 μmol min−1 after 5 min, Cu-ZSM-5 (50:1) maximum of 0.54 μmol min−1 after 4 min, Cu-ZSM-5 (80:1) maximum of 0.36 μmol min−1 after 4 min, and Cu-ZSM-5 (200-400:1) maximum of 0.18 μmol min-1 after 3 min. From there the methanol concentration

3.1.3. Nitrogen adsorption/desorption isotherms BET surface areas plotted against Si:Al ratios for commercial zeolites (NH4-ZSM- 5), powdered and pelletised copper-exchanged ZSM-5 materials are shown in Fig. 4. Both powdered and pelletised copper-exchanged ZSM-5 materials were calcined at 500 °C for a period of 2 h using a ramp rate of 1°C/min. All the samples were degassed at 200°C for 2 h before nitrogen adsorption/desorption measurements were taken at -196°C. Fig. 4 shows that in all cases the BET surface increased with the Si continent. For both powdered and pelletised copper-exchanged ZSM-5 4

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Fig. 2. TEM images A) Cu-ZSM-5 (20:1), B) highlighted phenomena on the surface of Cu-ZSM-5 (20:1), C) Cu-ZSM-5 (30:1), D) Cu-ZSM-5 (50:1), E) Cu-ZSM-5 (80:1), and F) Cu-ZSM-5 (200-400:1).

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Fig. 3. X-ray diffraction patterns after calcination for 2 h in air at 500 °C A) Cu-ZSM-5 (20:1), B) Cu-ZSM-5 (30:1), C) Cu-ZSM-5 (50:1), D) Cu-ZSM-5 (80:1), E) CuZSM-5 (200-400:1).

gradually decreases levelling off to 0 μmol min−1 after 1 h of extraction. However Cu-ZSM-5 (20:1) did not follow the same rate profile as the other materials. The methanol extraction rate for Cu-ZSM-5 (20:1) slowly increases to a maximum concentration of approximately 0.11 μmol min−1 after 12 min of extraction before beginning a gradual drop to 0 μmol min−1 as the extraction continues. A summary of the methanol yield, calculated from determining the area under each of the five methanol concentration curves in Fig. 6a, is presented within Table 1. Among all the materials Cu-ZSM-5 (30:1) had the highest methanol yield with 26.6 ± 0.1 μmol·gcat−1 of methanol being produced. Cu-ZSM-5 (50:1) shows the second highest yield with a calculated yield of 12.5 ± 0.1 μmol·gcat-1 whilst Cu-ZSM-5 (80:1) presented a similar yield of 10.8 ± 0.1 μmol·gcat-1. As expected from the particularly low content of aluminium within its structure, Cu-ZSM-5 (200-400:1) had a particularly low methanol yield with only 3.2 ± 0.1 μmol·gcat−1 being produced. Although containing the highest aluminium content Cu-ZSM-5 (20:1) produced the lowest yield

Fig. 4. BET surface area for the commercial zeolites, powder copper-exchanged ZSM-5 zeolite materials, and pelletised copper-exchanged ZSM-5 zeolite materials.

Fig. 5. TGA profiles (A) and their first derivative curves (B) for the different copper-exchanged ZSM-5 zeolite materials here studied: A) Cu-ZSM-5 (20:1), B) Cu-ZSM5 (30:1), C) Cu-ZSM-5 (50:1), D) Cu-ZSM-5 (80:1), E) Cu-ZSM-5 (200-400:1). 6

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Fig. 6. A) in situ mass spectrometer-detected signal of m/z = 31 attributed to methanol during the exposure of each of the materials to 5 vol.% steam balanced in argon at 200 °C. The materials have previously undergone activation in air at 200 °C before being reacted with CH4 (BOC, 10.18% in Ar) at 200 °C. B) Methanol production during the DMTM reaction as a function of the Si:Al ratio.

of methanol of 2.8 ± 0.1 μmol·gcat-1. The effect of the hydrophilicity of the zeolite in the methanol production was studied by comparing methanol yield and Si:Al ratio. Fig. 6b shows that the methanol yield decreased exponentially as the Si:Al ratio increased for all materials with the exception of Cu-ZSM-5 (20:1) which showed the lowest methanol production (2.8 ± 0.1 μmol·gcat−1). Total yield of methanol and Methanol/Copper ratio as a function of the copper concentration, is plotted in Fig. 7A and B, respectively. The volcano shape curve observed in both plots suggest that an optimal Cu:Si:Al ratio exits, with methanol yield decreasing on both sides of this value.

powdered copper-exchanged ZSM-5 materials d-d and LMCT transitions result in a sweeping absorption bands around 15,000 cm−1 and 31,000 cm−1, respectively. The LMCT transition absorption bands are originate from the transfer of electrons from oxygen to copper within copper-oxo structures (O2−Cu2+ → O−Cu+). The areas of each spectrum associated with d-d and LMCT transitions are highlighted in sections I) and II), respectively, in each of the plots shown in Fig. 8. As each of the materials is treated with methane a gradual drop in absorption can be seen at both of these absorption bands [26]. Fig. 8a shows that the interaction of Cu-ZSM-5 (20:1) with methane resulted in a prominent reduction of both d-d and LMCT transition absorption bands. Likewise, Cu-ZSM-5 (30:1) showed a large decrease in its absorption spectrum after the treatment in methane, see Fig. 8b. It is worth to point out that Cu-ZSM-5 (30:1) presents a noticeable kink in its LMCT transition adsorption band around 31,500 cm−1. Instead, a very little effect was observed when Cu-ZSM-5 (50:1) interacted with methane. Indeed, Fig. 8c shows that changes in both d-d and LMCT transition adsorption bands after methane treatment for Cu-ZSM-5 (50:1) were significantly smaller when compared to those presented in Cu-ZSM-5 (20:1) and Cu-ZSM-5 (30:1). Likewise, no change in d-d adsorption transitions was observed when Cu-ZSM-5 (80:1) was exposed to methane, see Fig. 8d. However, some changes were observed in the LMCT transition adsorption band of this material after the methane treatment. Notice that Cu-ZSM-5 (80:1) showed a kink in the LMCT transition absorption band around 31,000 cm−1, similar to that found in Cu-ZSM-5 (20:1). Finally, Fig. 8e shows that the LMCT transition adsorption band for Cu-ZSM-5 (200-400:1) remained relatively unaffected by the presence of methane.

3.3. In situ UV-vis spectroscopy characterization in situ UV-vis spectroscopy characterisation for all the powder copper-exchanged ZSM-5 materials under similar DMTM reaction conditions are presented in Fig. 8. The top spectrum in each plot represents the oxygen activated sample just before interaction with methane takes place, whilst the bottom spectrum represents the absorbance measured after 30 min of reaction with methane. Across each of the five plots presented within Fig. 8 absorption bands attributed to d-d and Ligand to Metal Charge Transfer (LMCT) transitions can be seen with varying magnitude. Commonly explained from crystal field theory d-d transitions are caused by a splitting of the five-fold degenerated 2D term within the octahedral ligand field of the cupric complex, resulting in a distortion of these complexes. This distortion causes the splitting of the ground and excited states allowing electrons to be transferred between them. Within the UV–vis spectra of

Table 1 Comparison of the performance of the copper-exchanged zeolite materials here studied with those in the literature during the DMTM reaction. Materials

Cu-ZSM-5 (20:1) Cu-ZSM-5 (30:1) Cu-ZSM-5 (50:1) Cu-ZSM-5 (80:1) Cu-ZSM-5 (200-400:1) Fe-ZSM-5 (50:1) Cu-ZSM-5 Cu-ZSM-5 Cu-MOR Cu-SSZ-13 (Cu-1.21-Na) Cu-SSZ-13 (Cu-1.21-Na)

Conditions Activation

Reaction

Extraction

Air / T = 200 °C Air / T = 200 °C Air / T = 200 °C Air / T = 200 °C Air / T = 200 °C N2O / T = 240 °C O2 / T = 450 °C N2O / T = 150 °C O2 / T = 450 °C O2 / 450 °C O2 / 550 °C

T = 200 °C T = 200 °C T = 200 °C T = 200 °C T = 200 °C T = 25 °C T = 200 °C T = 150 °C T = 200 °C T = 200 °C T = 200 °C

Steam / T = 200 °C Steam / T = 200 °C Steam / T = 200 °C Steam / T = 200 °C Steam / T = 200 °C Liquid Water Steam / Liquid Water Steam / T = 150 °C Steam / T = 200 °C Steam / T = 20 °C Steam / T = 20 °C

7

Methanol Yield [μmol·gcat−1]

Ref

2.8 ± 0.1 26.6 ± 0.1 12.5 ± 0.1 10.8 ± 0.1 3.2 ± 0.1 5.0 9.0 0.7 13.0 8.2 9.0

– – – – – [9] [11] [12] [16] [40] [40]

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Fig. 7. Total yield of methanol and Methanol/Copper ratio as a function of the copper concentration, A and B, respectively.

role during the copper exchange process. Likewise, the higher the copper content the higher likelihood to sinter and form big surface particles after calcination.

3.4. Isothermal DMTM cycling experiments In order to improve the methanol production the DMTM reaction can be repeated in a concept known as cycling. Fig. 9 shows the evolution of the methanol production during five cycles of DMTM for both Cu-ZSM-5 (30:1) and Cu-ZSM-5 (200-400:1) materials. It can be seen that Cu-ZSM-5 (200-400:1) provided a much more consistent methanol production when compared with Cu-ZSM-5 (30:1). Hence, after the first cycle Cu-ZSM-5 (30:1) produced 26.6 ± 0.1 μmol gcat−1 of methanol but no methanol was produced in the subsequent cycles. However, even though Cu-ZSM-5 (200-400:1) produced 3.2 ± 0.1 μmol gcat−1 of methanol after the first cycle, its production remained relatively stable at 1.6 ± 0.1 μmol gcat−1 in the subsequent cycles. Similar results were obtained by Bozbag et al. that reported a total of eight cycles of DMTM using Cu-MOR (8:1) [13]. However, methanol quantification was carried out using ex situ methods, either by condensation or using liquid water, which required the sample be removed from the reactor after each cycle.

4.1.2. Nitrogen adsorption/desorption isotherms As expected the process of pelletising did not have a significant effect upon the materials surface area. All copper-exchanged ZSM-5 materials, except Cu-ZSM-5 (20:1), showed an acceptable surface area between 300-400 m2/g comparable with those found within literature [28]. The lowest BET surface area observed for Cu-ZSM-5 (20:1) (i.e. 296 m2/g) suggests that copper nanoparticles observed in TEM images could be blocking the pores in the ZSM-5 structure.

4.1.3. Thermal gravimetric analysis It is well known that the lower the Si:Al ratio is, the more hydrophilic the zeolites are and therefore more likely to uptake water from their environment [22]. As expected, Fig. 5a shows that materials with a lower Si:Al ratio lost more water than materials with a larger Si:Al ratio (i.e. mass loss of 8 wt% and 1 wt% for Cu-ZSM-5 (20:1) and CuZSM-5 (200-400:1), respectively), which can be attributed due to the higher hydrophilicity of large aluminium content zeolites. The inflection point of each TGA profile provides a numerical value for the temperatures at which each material has lost approximate half of its water. With the inflection point of Cu-ZSM-5 (20:1) being two and a half times larger than that of Cu-ZSM-5 (200-400:1), the relationship between Si:Al ratio and the temperature at which the material can be considered to be dry can be formed. Hence, materials with a lower Si:Al ratio lose the majority of their water content at significantly higher temperatures when compared to those with a higher Si:Al ratio. Therefore, TGA analysis will also provide a qualitative understanding of the behaviour of these materials during the extraction process with 5 vol.% steam balanced in argon (T = 200 °C; P = 1 atm). Similar, TGA results were reported by Ganemi et al. who observed inflection points at 75 °C and 350 °C for mass losses associated with the removal of water for Cu-ZSM-5 (26:1) and Cu-ZSM-5 (200:80) materials, respectively. In addition, Ganemi et al. reported a second mass lost at higher temperature (i.e. T = 500) associated with the dehydroxilation of the zeolite, removal of structural OH [29].

4. Discussion 4.1. Characterisation of the copper-exchanged ZSM-5 zeolite materials 4.1.1. Transmission Electron microscopy Among all the materials here studied Cu-ZSM-5 (20:1) is the only one that presents copper nanoparticles on its surface, see Fig. 2a and b. Similar copper nanoparticles have been previously reported in the literature for copper-exchanged zeolite materials with low Si:Al ratios. For instance, Yamaguchi et al. observed larger copper nanoparticles between 10–20 nm in copper-exchanged ZSM-5 materials prepared by ion exchange using copper nitrate solution [27]. Likewise, copper nanoparticles of a similar size to those reported here have been observed by Beznis et al. and Sheppard et al. [12,28]. Beznis et al. determined that prolonged exposure to the electron beam during TEM image analysis could led to the sintering of copper as particles with a diameter between 2–5 nm [28]. However, if that was the case for the materials here studied then copper nanoparticles would be present across all samples. The fact that the copper content in Cu-ZSM-5 (20:1) is significant larger than in any of the other materials (i.e. copper content 1.5 wt%, see Table 2) suggests that inherent materials’ properties such as number of ion exchanges sites and hydrophobicity play an important 8

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Fig. 8. In situ UV–vis spectrums A) Cu-ZSM-5 (20:1), B) Cu-ZSM-5 (30:1), C) Cu-ZSM-5 (50:1), D) Cu-ZSM-5 (80:1), and E) Cu-ZSM-5 (200-400:1). The top spectrum in each plot represents the oxygen activated sample, just before interaction with methane takes place, whilst the bottom spectrum represents the absorbance measured after 30 min of reaction with methane. Highlighted in each spectra are I) d-d transitions and II) LMCT transitions.

9

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lower oxygen chemical potential of dinitrogen monoxide compare with air, could lead into the formation of copper complexes with lower average coordination number which seems to be less active towards the DMTM reaction [33–37]. This agrees with Groothaert et al. [11] results that reported a higher methanol production to ours (i.e. 8.9 μmol gcat−1) for Cu-ZSM-5 (Si:Al ratio = 12 and 2.4 w%Cu) using pure oxygen during the activation step and a reaction temperature of 200 °C. Nevertheless, Groothaert et al. [11] used higher temperature than us during the activation of the materials (i.e. 450 °C vs 200 °C) and higher copper loading (2.4 w%Cu vs 1.5 w%Cu), which could explain their higher methanol production. Although, Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1) materials have similar copper content (i.e. 0.6 w%, 0.5 w% and 0.7 w%, respectively), the methanol production for Cu-ZSM-5 (30:1) was larger than those obtained for Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1), see Fig. 7a. Indeed, Fig. 7b shows that the methanol/copper production for Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1) materials was 0.11 and 0.09, respectively, whereas it was 0.27 for Cu-ZSM-5 (30:1). This behaviour could be explained due to a different arrangement of copper atoms within the Cu-ZSM-5 (30:1) material. As shown in Fig. 1, the copper could have been deposited in a variety of locations and as part of several clusters within ZSM-5. According to Rice et al. [38] and Sushkevich et al. [11] in materials with high Si/Al ratios, the probability of forming mono-copper species is significantly higher than in materials with low Si/Al ratios. This agree with the methanol/copper production ratio observed for Cu-ZSM-5 (30:1), which suggests a bigger population of multi-copper active sites in this material than in Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1) materials. Based on its performance, it seems that for Cu-ZSM-5 (30:1) copper could have formed the more favourable complexes within the zeolite for the DMTM reaction. This suggests that under this reaction conditions mono-copper species are lest active than multi-copper species. Fig. 7b compares the performance of our catalysts with those published by Markovits et al. [39], a series of copper-exchanged ZSM-5 zeolites with high and low Al content. Markovits et al. [39] also concluded that the differences observed in the catalytic activity during the DMTM reaction could be associated with a particular arrangement of copper species occur during ion exchange and subsequent activation treatment. Another aspect to take into account is the hydrophobicity of the materials; the larger the Si:Al ratio, the less efficient the extraction process is since water cannot go into the zeolite structure. Hence methanol produced in large Si:Al ratio materials such as Cu-ZSM-5 (80:1) and Cu-ZSM-5 (50:1) may not be fully removed during the extraction process with steam. In this respect, the low performance of Cu-ZSM-5 (200-400:1) during the DMTM reaction (i.e. methanol production = 3.2 ± 0.1 μmol·gcat−1) can be explained due to a combination of its high hydrophobicity and low copper content. The low aluminium content of the ZSM-5 (200-400:1) provides few opportunities for copper ion exchange resulting in a material containing less than < 0.1 w% copper content and low probability of founding bimetallic and there metallic copper clusters. Likewise, methanol produced may not be fully removed during the extraction process due to the high hydrophobicity of the Cu-ZSM-5 (200-400:1). As it can be seen in Fig. 5. Cu-ZSM-5 (50:1) and Cu-ZSM-5 (80:1)

Fig. 9. Comparison of the methanol production after five consecutive cycles for Cu-ZSM-5 (30:1) (dotted line) and Cu-ZSM-5 (200-400:1) (solid line).

4.2. Performance of copper-exchanged zeolite materials during the DMTM reaction The instantaneous methanol concentrations profiles taken during the extraction process using 5% Vol steam in argon at 200°C are shown in Fig. 6a. It can be observed that the rate of methanol desorption increased with the Si/Al ratio. According to Sushevih et al. [30] the slow methanol desorption observed in samples with low Si/Al ratio can be related with their higher amount of aluminium and residual Brønsted sites, which can interact with methanol molecules owing their high affinity to polar molecules. The performance the different copper-exchanged ZSM-5 materials studied in this work during the DMTM reaction is summarised in Table 1. The lowest methanol production of all five materials (i.e. 2.8 ± 0.1 μmol·gcat−1) corresponds to Cu-ZSM-5 (20:1), which is also an exception to the exponentially decreasing relationship observed in Fig. 6b. An explanation of this behaviour can be found within the TEM images shown in Fig. 2. Among all the materials, Cu-ZSM-5 (20:1) is the only one that shows copper nanoparticles with diameters of approximately 5 nm on its surface. Lai et al. [31] have reported that precipitation of copper nanoparticles on the surface can limit the access to the adsorbate molecules and therefore the reactants into the zeolite. Moreover, it is well known that copper nanoparticles are not active in the DMTM reaction at the reaction conditions study here [32]. Likewise, their presence suggests that very little of the copper has gone on to form the desired copper complex structures within the ZSM-5 (20:1) framework. Sheppard et al. [12] using the same ion exchange method for the preparation of their materials, reported a methanol production of 0.69 μmol gcat−1 for Cu-ZSM-5 (Si:Al ratio = 12 and 1.88 wt% Cu). Their lower methanol production compared with our similar material Cu-ZSM-5 (20.1) (i.e. 2.84 μmol gcat−1 and 1.5 wt% Cu) could be explained due to the different buffer gas employed during the activation step (i.e. dinitrogen monoxide rather than air) and the lower reaction and extraction temperature (i.e. T = 150 °C). It is well known that the

Table 2 Elemental composition analysis for the different copper-exchanged zeolite materials here studied. Atomic percent (at.%) O Cu-ZSM-5 Cu-ZSM-5 Cu-ZSM-5 Cu-ZSM-5 Cu-ZSM-5

(20:1) (30:1) (50:1) (80:1) (200-400:1)

70.8 71.4 70.5 69.8 70.8

Weight percent (wt%) Si

± ± ± ± ±

0.1 0.1 0.1 0.1 0.1

26.9 27.1 28.7 29.4 29.0

Al ± ± ± ± ±

0.1 0.1 0.1 0.1 0.1

Cu

1.8 1.3 0.7 0.6 <

± 0.1 ± 0.1 ± 0.1 ± 0.1 0.2

10

0.4 0.2 0.1 0.2 <

Cu/Al ratio

Cu ± 0.1 ± 0.1 ± 0.1 ± 0.1 0.1

1.5 0.6 0.5 0.7 <

± 0.1 ± 0.1 ± 0.1 ± 0.1 0.2

0.2 0.1 0.1 0.3 0.5

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show very similar TGA profiles, which suggests that both materials have comparable hydrophobicity. These similarities in hydrophobicity and copper contend could explain the alike performance observed in CuZSM-5 (50:1) and Cu-ZSM-5 (80:1) materials, methanol/copper ratio 0.16 and 0.10, respectively. Although, Brønsted sites, copper content, hydrophobicity, and sample preparation method provide useful information to understand the performance of copper-exchanged ZSM-5 materials during the DMTM reaction, information about copper active sites is key to have a more complete view of the reaction process. In order to have a deeper understanding of copper active sites involve in the DMTM reaction in situ UV-vis characterisation of copper-exchanged ZSM-5 materials was carried out under the reaction conditions.

Table 3 Electronic absorption features of some copper complex, ṽmax refers to the maximum of the absorption. ṽmax (cm−1)

Ref.

bis(μ-oxo) dicopper(III)

32,700 – 30,800

[41]

μ-(η2:η2) peroxo dicopper(II)

Planar: 25,000 – 22,300 29,600 – 27,300 Bent: 19,700 17,000 ˜ 27,800 23,800 – 20,400 ˜ 18,200 22,700 32,000 30,000

[41]

Complex Name

4.3. In-situ UV–vis spectroscopy experiments

Structure

[Cu2(μ-O)]2+ Mono(μ-oxo) dicopper

In-situ UV-vis spectroscopy characterisation for all the powder copper-exchanged ZSM-5 materials under DMTM mild conditions (T = 200°C; P = 1 atm) and oxygen activation (T = 500°C; P = 1 atm) are presented in Fig. 8. The top spectrum in each plot represents the oxygen activated sample just before interaction with methane takes place, whilst the bottom spectrum represents the absorbance measured after 30 min of reaction with methane. Before discussing the outcomes of in-situ UV–vis spectroscopy experiments it is important to highlight that, although during DMTM performance experiments samples are activated at 200°C in air, all of them have been previously calcined at 500°C in air. Therefore, the conditions employed during the in-situ UV–vis experiments (500°C in oxygen) are close to those employed during the DMTM performance experiments. Nevertheless, the ex-situ calcination at 500 °C before DMTM performance experiments does not ensure that the state of the zeolites is the same as when doing the in-situ UV–vis experiments. Copper-exchanged zeolite materials are air and moisture sensitive, hence subsequent hydration after the ex-situ calcination at 500 °C can result to different speciation in the copper-exchanged zeolite materials during DMTM performance experiments compared to in-situ UV–vis experiments. Despite these differences we consider that in-situ UV–vis experiments can provide some insight and understanding of copper active sites involve in the DMTM performance experiments. All five materials here studied show, to varying degrees, a sweeping absorption band at 31,000 cm−1 and 15,000 cm-1 associated with LMCT and d-d electron transitions, respectively. A summary of different copper complexes spectroscopically described in the literature is presented in Table 3. The band reported by Groothaert et al. at 22,700 cm−1 associated with the copper active complex for the DMTM reaction was not observed for any of the five materials here studied (11). Besides, having similar copper content the differences observed in UV–vis spectrums for Cu-ZSM-5 (30:1), Cu-ZSM-5 (50:1), and Cu-ZSM-5 (80:1) suggest that copper atoms in these materials are forming different complexes which could explain their different performance during the DMTM reaction. Hence, the high activity of Cu-ZSM-5 (30:1) during the DMTM reaction could be related with the absorption band upon interaction with methane observed at 28,000 cm−1. Likewise, CuZSM-5 (80:1) seems to have a similar interaction with methane also presenting an absorption band at 28,000 cm−1, which suggests that both materials may have similar copper complexes. However the methanol production during the DMTM reaction for Cu-ZSM-5 (80:1) was about twice smaller than that observed for Cu-ZSM-5 (30:1) (i.e. 10.8 ± 0.1 μmol·gcat−1 vs 26.68 ± 0.1 μmol gcat−1). This behaviour indicates that the higher hydrophilicity of Cu-ZSM-5 (30:1) compared with Cu-ZSM-5 (80:1) enhances methanol removal during the extraction process. Fig. 8 shows that the absorption band at 28,000 cm−1 cannot be seem for Cu-ZSM-5 (20:1), Cu-ZSM-5 (50:1), and Cu-ZSM-5 (200400:1), which suggests that these materials may have formed different

[Cu2(μ-OH)2]2+ Bis(μ-hydroxo) dicopper

[11]

[26]

μ-1,1-hydroperoxo dicopper(II)

bis(μ-hydroxy) dicopper(III)

trans-μ-1,2-peroxo dicopper(II) η1 -superoxo copper(II) η2 -superoxo copper(II)

η1 -hydroperoxo copper(II)

copper complexes than Cu-ZSM-5 (30:1) and Cu-ZSM-5 (80:1). Nevertheless, Cu-ZSM-5 (50:1) showed a reasonable activity during the DMTM reaction with a similar methanol production than that observed for Cu-ZSM-5 (80:1) (i.e. 10.8 8 ± 0.1 μmol•gcat−1). The low methanol production observed for ZSM-5 (200–400;1) (i.e. 3.2 ± 0.1 μmol•gcat−1) can be connected to its lower copper content and high hydrophobicity compare with other materials here studied. Finally, as it has been already mentioned the lowest performance of CuZSM-5 (20:1) during the DMTM reaction can be explained due to precipitation of copper nanoparticles on its surface, which are not active under the reaction conditions.

4.4. Isothermal DMTM cycling experiments The present experiments explore the behaviour of copper-exchanged ZSM-5 materials during isothermal DMTM cycling at mild conditions (T = 200°C; P = 1 atm). The hypothesis examined was that the performance of the different materials during isothermal DMTM cycling experiments not only depends on the activity shown during the DMTM reaction but also of their physical property such as hydrophobicity. Hence, two very different samples were chosen to study their ability to isothermal DMTM cycling: Cu-ZSM-5 (30:1) and Cu-ZSM-5 (200-400:1). The low performance of Cu-ZSM-5 (30:1) after the first cycle suggests that active oxygen species, created at 500 °C during the calcination, cannot be regenerated during the activation treatment in air at 200 °C. Furthermore, the high hydrophilicity of Cu-ZSM-5 (30:1) avoid the remained water within this material after the first extraction step to be removed during the second activation step. As shown by the TGA 11

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References

analysis in Fig. 4. the mild condition employed during the extraction step (i.e. 5 vol.% steam balanced in argon; T = 200°C; P = 1 atm) would result in only part of the residual water content within the CuZSM-5 (30:1) being removed. This would explain the inability of CuZSM-5 (30:1) to carrying out more than one cycle. On the other hand the same treatment carried out in Cu-ZSM-5 (200-400:1) would result in practically all the water being removed, see Fig. 4. Moreover, it seems that active oxygen species of Cu-ZSM-5 (200-400:1) can be partially regenerated at 200 °C during the activation treatment in air. This would explain that methanol production for this material remained relatively stable at 1.6 ± 0.1 μmol•gcat−1 after the first cycle. Based on the cycling results presented here, with the more active material having poor cycling ability and the less active material being better suited towards cycling, it is clear that the ideal cycling material must be good across each step of the DMTM process (i.e. activation, reaction, and extraction). Above results also have pointed out that activation temperature and hydrophobicity of the materials seems to play an important role on the material ability to be cycled.

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5. Conclusions Among all the copper-exchanged ZSM-5 materials here studied during the DMTM reaction, Cu-ZSM-5 (30:1) has showed the highest methanol production (i.e. 26.6 ± 0.1 μmol·gcat−1). A combination of factors can be explain the high methanol production observed for CuZSM-5 (30:1): (i) Si/Al ratio of the zeolite material (which defines the number of ion exchange sites), (ii) copper complex formed during the sample preparation (due to the Al atom distribution in the zeolite material), (iii) copper complex coordination number achieved during the activation process, (iv) hydrophilicity of the zeolite material. Isothermal DMTM Cycling experiments clearly show that the ideal cycling material must be good across each step of the DMTM process (i.e. activation, reaction, and extraction). A total of five isothermal DMTM cycles were carried out using Cu-ZSM-5 (200-400:1) material. Throughout the five cycles, Cu-ZSM-5 (200-400:1) dropped its methanol production from 3.2 ± 0.1 μmol•gcat−1 in the first cycle to 1.6 ± 0.1 μmol•gcat−1 in the second and remaining cycles. Comparing Cu-ZSM-5 (30:1) and Cu-ZSM-5 (200-400:1) performance during isothermal DMTM cycling experiments, it can be concluded that hydrophobicity of the materials seems to play an important role on the material ability to be cycled. in situ UV-vis spectroscopy characterisation under DMTM mild conditions (T = 200°C; P = 1 atm) and oxygen activation (T = 500°C; P = 1 atm) showed that all copper-exchanged ZSM-5 materials here studied presented to varying degrees sweeping absorption bands at 31,000 cm−1 and 15,000 cm-1 associated with LMCT and d-d electron transitions, respectively. As the materials were interacted with methane their UV–vis spectra changed highlighting the fact that active complexes were being created and then destroyed during the DMTM reaction. Declaration of Competing Interest None. Acknowledgments The authors gratefully acknowledge the research funding provided by EPSRC in the United Kingdom (J27318 - EPSRC Institutional Sponsorship). F.R.G.G. would like to thank Drochaid Research Services for their help and support during this research work.

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