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Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

Characterization of Cu-exchanged SSZ-13: a comparative FTIR, UV-Vis, and EPR study with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios.

Cu-SSZ-13 has been characterized by different spectroscopic techniques and compared with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios and prepared by the same ion exchange procedure. On vacuum activated samples, low temperature FTIR spectroscopy allowed us to appreciate a high concentration of reduced copper centres, i.e. isolated Cu(+) ions located in different environments, able to form Cu(+)(N2), Cu(+)(CO)n (n = 1, 2, 3), and Cu(+)(NO)n (n = 1, 2) upon interaction with N2, CO and NO probe molecules, respectively. Low temperature FTIR, DRUV-Vis and EPR analysis on O2 activated samples revealed the presence of different Cu(2+) species. New data and discussion are devoted to (i) [Cu-OH](+) species likely balanced by one framework Al atom; (ii) mono(μ-oxo)dicopper [Cu2(μ-O)](2+) dimers observed in Cu-ZSM-5 and Cu-β, but not in Cu-SSZ-13. UV-Vis-NIR spectra of O2 activated samples reveal an intense and finely structured d-d quadruplet, unique to Cu-SSZ-13, which is persistent under SCR conditions. This differs from the 22,700 cm(-1) band of the mono(μ-oxo)dicopper species of the O2 activated Cu-ZSM-5, which disappears under SCR conditions. The EPR signal intensity sets Cu-β apart from the others.

Interaction of NH3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study

In the typical NH3-SCR temperature range (100-500 °C), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. Therefore, the study of adsorbed ammonia at high temperature is a key step for the understanding of its role in the NH3-SCR catalytic cycle. We employed different spectroscopic techniques to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, that is, (i) NH3 bonded to copper centers, (ii) NH3 bonded to Brønsted sites, and (iii) NH4(+)·nNH3 associations. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu(+) species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.

The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH3-Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES

The small-pore Cu-CHA zeolite is today the object of intensive research efforts to rationalize its outstanding performance in the NH3-assisted selective catalytic reduction (SCR) of harmful nitrogen oxides and to unveil the SCR mechanism. Herein we exploit operando X-ray spectroscopies to monitor the Cu-CHA catalyst in action during NH3-SCR in the 150-400 °C range, targeting Cu oxidation state, mobility, and preferential N or O ligation as a function of reaction temperature. By combining operando XANES, EXAFS, and vtc-XES, we unambiguously identify two distinct regimes for the atomic-scale behavior of Cu active-sites. Low-temperature SCR, up to ∼200 °C, is characterized by balanced populations of Cu(I)/Cu(II) sites and dominated by mobile NH3-solvated Cu-species. From 250 °C upward, in correspondence to the steep increase in catalytic activity, the largely dominant Cu-species are framework-coordinated Cu(II) sites, likely representing the active sites for high-temperature SCR.

Methane to Methanol: Structure–Activity Relationships for Cu-CHA

Cu-exchanged zeolites possess active sites that are able to cleave the C-H bond of methane at temperatures ≤200 °C, enabling its selective partial oxidation to methanol. Herein we explore this process over Cu-SSZ-13 materials. We combine activity tests and X-ray absorption spectroscopy (XAS) to thoroughly investigate the influence of reaction parameters and material elemental composition on the productivity and Cu speciation during the key process steps. We find that the CuII moieties responsible for the conversion are formed in the presence of O2 and that high temperature together with prolonged activation time increases the population of such active sites. We evidence a linear correlation between the reducibility of the materials and their methanol productivity. By optimizing the process conditions and material composition, we are able to reach a methanol productivity as high as 0.2 mol CH3OH/mol Cu (125 μmol/g), the highest value reported to date for Cu-SSZ-13. Our results clearly demonstrate that high populations of 2Al Z2CuII sites in 6r, favored at low values of both Si:Al and Cu:Al ratios, inhibit the material performance by being inactive for the conversion. Z[CuIIOH] complexes, although shown to be inactive, are identified as the precursors to the methane-converting active sites. By critical examination of the reported catalytic and spectroscopic evidence, we propose different possible routes for active-site formation.

Probing the surface of nanosheet H-ZSM-5 with FTIR spectroscopy

Herein we report FTIR in situ adsorption of molecular hydrogen, carbon monoxide, water, methanol, pyridine and 2,4,6-trimethylpyridine (collidine) on nanosheet H-ZSM-5 which was recently studied in the methanol to hydrocarbons (MTH) reaction. The nature of the hydroxyl groups and surface species are described in detail. The IR spectrum of nanosheet H-ZSM-5 is dominated by silanols, which saturate the external surfaces. The acidity of Si(OH)Al is comparable to that observed in the case of standard microcrystalline H-ZSM-5. The study of the external surface allows the recognition of Si(OH)Al species located at the channel entrance, which are mostly all accessible to hindered molecules such as collidine.

Unit cell thick nanosheets of zeolite H-ZSM-5: Structure and activity

Nanosheets of zeolite H-ZSM-5 were synthesized and characterized by X-ray diffraction, transmission electron microscopy (TEM), N2-physisorption, FT-IR spectroscopy, 27Al and 29Si MAS NMR spectroscopy in addition to catalytic testing in conversion of methanol to hydrocarbons (MTH). It was found that Rietveld analysis, involving anisotropic broadening parameters, gave average crystallite dimensions in good agreement with TEM images. The selectivities in MTH is intact in the mesoporous nanosheet H-ZSM-5 with the largest difference being a higher C3/C2 ratio compared to regular H-ZSM-5.

New insights into catalyst deactivation and product distribution of zeolites in the methanol-to-hydrocarbons (MTH) reaction with methanol and dimethyl ether feeds

Methanol (MeOH) and dimethyl ether (DME) have been compared as feedstock for the methanol-to-hydrocarbons (MTH) reaction over H-ZSM-5 (MFI), H-SSZ-24 (AFI) and H-SAPO-5 (AFI) catalysts at 350 and 450 °C. Several clear observations were made. First, the MeOH–DME equilibrium is not always established in the MTH reaction, because the rate of MeOH dehydration to DME is similar to the rates of the methylation reactions over strong Bronsted acid sites. In the presence of weak acid sites (i.e. the AlPO framework of SAPO-5), which are nearly inactive to hydrocarbons formation, the MeOH–DME equilibrium can be reached. Second, the MTH activity is ostensibly higher for DME compared to MeOH. Third, the carbon conversion capacity of the catalysts is generally higher (up to 16 times higher under the conditions used in this work) with a DME feed compared to a MeOH feed. Incorporation of AlPO-5 as dehydration catalyst before or mixed with a H-SSZ-24 catalyst for MTH, leads to lower MeOH concentrations in the reaction mixture, and a significant increase of the conversion capacity. Finally, a MeOH feed results in a higher selectivity for aromatic products and ethylene, pointing to a larger contribution of the arene cycle, compared to a DME feed. We hypothesize, that MeOH causes formation of formaldehyde, while DME does not. Formaldehyde is a known coke precursor, which provides an explanation for the faster deactivation of zeolites in a MeOH feed.

Role of internal coke for deactivation of ZSM-5 catalysts after low temperature removal of coke with NO2

By treating a deactivated ZSM-5 catalyst for the conversion of methanol to hydrocarbons with NO2, coke deposits can be removed at around 350 °C, which potentially enables catalyst regeneration at 350–400 °C, which is about 200 °C lower compared to a conventional regeneration in oxygen. To evaluate the regeneration with NO2 at 350 °C, the activity of a used ZSM-5 catalyst was measured after treatment with 1% NO2/He and 0.7% NO2/7% O2/He at 350 °C, and 2% O2/He at 550 °C. After the treatments with NO2 at 350 °C, some activity was restored, but the catalysts showed a fast deactivation. Temperature programmed desorption of ammonia and 27Al MAS NMR measurements indicate that the amount of framework aluminium in the regenerated catalysts is about 60% of that in the fresh catalysts, and some redistribution of the aluminium takes place. Gravimetric temperature programmed oxidation showed that the catalysts still contain 0.3–0.6 wt% coke. GC-MS analysis of the retained species and very high-speed 1H MAS NMR revealed that the remaining coke species are methyl benzenes, which are located inside the micropores of the ZSM-5 zeolite. It is concluded that the deactivation not only depends on the amount of coke, but also on the location of the coke in the catalyst.

Nitrate–nitrite equilibrium in the reaction of NO with a Cu-CHA catalyst for NH3-SCR

The equilibrium reaction between NO and Cu-nitrate, Cu(II)-NO3− + NO(g) ⇌ Cu(II)-NO2− + NO2(g), has been proposed to be a key step in the selective catalytic reduction of NO by ammonia (NH3-SCR) over Cu-CHA catalysts and points to the presence of Cu-nitrites. Whereas the formation of gaseous NO2 has been observed, a direct observation of Cu-nitrite groups under conditions relevant to NH3-SCR has been so far unsuccessful. In an effort to identify and characterize Cu-nitrites, the reaction between Cu-nitrates hosted in the CHA zeolite and NO is investigated by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis) and X-ray absorption spectroscopy (XAS). We find that NO reacts with Cu-nitrates and that about half of the Cu-nitrate species are converted. After the reaction, the Cu(II) state is different from the original oxidized state. Analysis of XAS data indicates that the final state of the Cu-CHA catalyst is consistent with the partial conversion of the Cu-nitrate species to a bidentate Cu-nitrite configuration.