Stian Svelle

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.

Post-synthetic modification of the metal–organic framework compound UiO-66

Post-synthetic modification is a viable route for the introduction of surface sites with new chemical properties in metal–organic framework compounds. Herein we demonstrate that it is possible to perform covalent post-synthetic modifications of the UiO-66–NH2 MOF with four different acid anhydrides. FT-IR is employed to monitor the reactions and the extent of reaction depends on the bulkiness of the anhydrides. For the smallest one, acetic anhydride, 100% conversion to UiO-66–NHCOCH3 was observed.

Shape‐Selective Conversion of Methanol to Hydrocarbons Over 10‐Ring Unidirectional‐Channel Acidic H‐ZSM‐22

With the forecasted depletion in global oil reserves, new routes to petrochemical products from natural gas, coal, or biomass are becoming increasingly important. The methanolto-hydrocarbons (MTH) reaction constitutes the final step in one such route. The MTH reaction proceeds over Brønstedacidic zeolite or zeotype catalysts, and near-commercial processes exist for the methanol-to-gasoline (MTG) reaction over ZSM-5, as well as the methanol-to-olefin (MTO) reaction over SAPO-34. A breakthrough in the mechanistic understanding of the MTH reaction was the formulation of the “hydrocarbon pool mechanism” by Dahl and Kolboe, 3] which postulates that methanol is continuously added to aromatic reaction centers, from which light alkenes are split off in later reaction steps. Recently, the importance of methylation and cracking of alkenes over ZSM-5 was highlighted. 6] ZSM-22 (TON) is less well studied as a MTH catalyst. It incorporates one-dimensional non-interacting 10-ring channels with diameters of 0.46 0.57 nm. Song and co-workers reported the failure of ZSM-22 to convert methanol into olefins. Their studies of ZSM-22 showed a low production of olefins during the first pulses of methanol, however the amount of olefin quickly decreased to essentially zero. This failure as an MTH catalyst was ascribed to the narrow pores, which were assumed to be too small to accommodate the complete catalytic cycle of the hydrocarbon pool mechanism. Flow experiments (at 250– 400 8C) afforded relatively constant yields of trace amounts of ethene and propene. The low reactivity was believed to be the result of traces of ZSM-11, impurities in the methanol (acetone) and/or external acid sites. Herein, we report studies of the MTH reaction over ZSM-22 at a wider range of reaction conditions and demonstrate that the previous conclusions are not universally valid. Under suitable conditions, ZSM-22 has a conversion capacity comparable to that of SAPO-34, reaction intermediates reside within the pores, and the product spectrum is intermediate to those found for reactions for the MTO and MTG processes. Several batches of ZSM-22 with different Si/Al ratios were synthesized and all were found to be active catalysts for the MTH reaction. The crystallinity and purity of the product were confirmed by X-ray diffraction. Scanning electron microscopy (SEM) revealed needle shaped crystals of 2–3 mm length. Al NMR spectroscopy indicated that, for the samples discussed here, Al was located exclusively in the framework, both for as-made and calcined/ion-exchanged samples. BET surface areas were in the range 160–207 m g . Two different ZSM-22 catalysts (Si/Al = 30 by inductively coupled plasma atomic emission spectroscopy) with BET surface areas of 173 and 207 m g , denoted ZSM-22 ACHTUNGTRENNUNG(173) and ZSM-22 ACHTUNGTRENNUNG(207) respectively, will be discussed. At temperatures above 350 8C, the initial conversion of methanol over ZSM-22ACHTUNGTRENNUNG(173) (weight hourly space velocity (WHSV) = 2.05 h ) was 100 % and appreciable conversion took place for several hours (Figure 1 a) . However, at 350 8C, deactivation was very rapid. The feed rate in this case was lower than that used in previous studies: WHSV = 48 h 1 was used by Song and co-workers and WHSV = 10 h 1 was used by Li et al.

In Situ Infrared Spectroscopic and Gravimetric Characterisation of the Solvent Removal and Dehydroxylation of the Metal Organic Frameworks UiO-66 and UiO-67

Herein, the desolvation, dehydroxylation and rehydroxylation of the metal organic frameworks UiO-66 and -67 are followed by in situ DRIFTS and TG–DSC. The spectra recorded on UiO-66 feature multiple bands corresponding to chemically inequivalent isolated hydroxyl groups, whereas UiO-67 has the expected single μ3-OH band from the Zr6O4(OH)4 cornerstone. Complete rehydration is demonstrated on both materials. Based on further experimental insights, hypotheses are given to explain the observed differences between UiO-66 and -67. Quantum chemical calculations are employed in order to deduce the feasibility of one possible explanation for the observed behaviour on UiO-66.

Synthesis and Characterization of Amine-Functionalized Mixed- Ligand Metal−Organic Frameworks of UiO-66 Topology

A series of amine-functionalized mixed-linker metal-organic frameworks (MOFs) of idealized structural formula Zr6O4(OH)4(BDC)(6-6X)(ABDC)6X (where BDC = benzene-1,4-dicarboxylic acid, ABDC = 2-aminobenzene-1,4-dicarboxylic acid) has been prepared by solvothermal synthesis. The materials have been characterized by thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and Fourier transform infrared (FTIR) spectroscopy with the aim of elucidating the effect that varying the degrees of amine functionalization has on the stability (thermal and chemical) and porosity of the framework. This work includes the first application of ultraviolet-visible light (UV-vis) spectroscopy in the quantification of ABDC in mixed-linker MOFs.

Computational exploration of newly synthesized zirconium metal–organic frameworks UiO-66, -67, -68 and analogues

One of the major weaknesses of metal–organic framework (MOF) materials is their rather low thermal, hydrothermal, and chemical stabilities. Identification of stable and solvent resistant MOF materials will be key to their real world utilization. Recently, Lillerud and coworkers reported the synthesis of a new class of Zr MOF materials. These materials have very high surface area and exceptional thermal stability, are resistant to water and some solvents, acids, bases, and remain crystalline at high pressure. The newly synthesized Zr metal–organic frameworks (UiO-66, -67, and -68) as well as analogues substituting Ti and Hf for Zr, are explored using density functional theory calculations. The crystal structure, phase stability, bulk modulus, electronic structure, formation enthalpies, powder X-ray diffraction, chemical bonding, and optical properties are studied. We find bulk moduli of 36.6, 22.1, and 14.8 GPa for UiO-66, -67, and -68 respectively. As the linkers are extended, the bulk modulus drops. The highest occupied crystal orbital to lowest unoccupied crystal orbital gaps range from 2.9 to 4.1 eV. The compounds have similar electronic structure properties. Experimental powder X-ray diffraction patterns compare well with simulation. The large formation enthalpies (−40 to −90 kJ mol−1) for the series indicate high stability. This is consistent with the fact that these materials have very high decomposition temperatures. A detailed analysis of chemical bonding is carried out. Potential applications for these new materials include organic semiconducting devices such as field-effect transistors, solar cells, and organic light-emitting devices. We hope that the present study will stimulate research on UiO-based photocatalysis and will open new perspectives for the development of photocatalysts for water splitting and CO2 reduction. The large surface areas also make these materials good candidates for gas adsorption, storage, and separation.

Does an Ethene/Benzenium Ion Complex Exist? A Discrepancy between B3LYP and MP2 Predictions

To investigate the possible existence of a complex between the benzenium ion and ethene, computations employing B3LYP and MP2 were carried out. The two methodologies gave conflicting answers; B3LYP confirmed the existence, but according to MP2, the structure found by B3LYP transforms into an ethylbenzenium ion. Computations utilizing the CCSD and QCISD methods showed the B3LYP result to be correct; 21 kJ/mol is needed to separate the two moieties.

Synthesis of Titanium Chabazite: A New Shape Selective Oxidation Catalyst with Small Pore Openings and Application in the Production of Methyl Formate from Methanol

Isomorphous substitution of a small fraction of titanium(IV) in the tetrahedral sites of zeolitic frameworks provides materials with excellent catalytic properties for partial oxidation reactions with H2O2. [1] The original TS-1 material performs with an enzyme like activity and selectivity and is of great industrial importance in partial oxidation reactions of small organic substrates. Titanium incorporation in silicates has been extensively studied both on amorphous phases (dispersed in microporous silica and grafted to mesoporous molecular sieves), in layered delaminated zeolite precursors and in zeolitic frameworks; such as Ti-Beta, TS-2, Ti-ZSM-48, Ti-MWW, and Ti-MTF. Here we report the synthesis of Ti-CHA zeolites with and without aluminum and demonstrate their activity in partial oxidation reactions with H2O2. The pores in most titanium silicates are medium to large; however in this material they are 8-membered rings. This opens up new possibilities within shape-selective oxidation catalysis. CHA is a zeolite topology with a 3dimensional pore structure comprising an elongated cage with 8-ring windows. Different CHA materials (SAPO-34 and SSZ-13) have been studied as catalysts in the methanol-to-olefin process due to their shape selective properties. The CHA topology is rare in that it contains only one crystallographic independent T site and is, therefore, an interesting model material to study Ti insertion in the framework, as testified by previous theoretical periodic studies. It was recently reported that CHA zeolites can be prepared over a wide range of silicon/aluminum ratios, including the purely siliceous. This creates new doping options, for example in Ti-CHA and Ti-Al-CHA, in which both redox and acidic properties can work together. For example, a possible application of the Ti-Al-CHA material is in the production of methyl formate from methanol. Methyl formate is of great importance as an intermediate in the production of chemicals such as formamide, dimethyl formamide, and formic acid. Formic acid alone is produced in several hundred thousands of tons per year. In contrast to the conventional synthetic route, this method operates at very mild conditions, making it economically and environmentally advantageous. Detailed results are reported for two Ti-containing samples (Ti-CHA with Si/Ti = 246 and Ti-Al-CHA with Si/Ti = 95 and Si/ Al = 17) with comparison to their respective blank matrices (pure siliceousand Al-CHA with Si/Al = 20). The elemental analysis reported in Table 1 shows that the Ti loading is low, as already observed in other Ti-containing zeolites, and that Ti in-

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.