Performance of density functional theory for describing hetero‐metallic active‐site motifs for methane‐to‐methanol conversion in metal‐exchanged zeolites

Abstract

Methane‐to‐methanol conversion (MMC) can be facilitated with high methanol selectivities by copper‐exchanged zeolites. There are however two open questions regarding the use of these zeolites to facilitate the MMC process. The first concerns the possibility of operating the three cycles in the stepwise MMC process by these zeolites in an isothermal fashion. The second concerns the possibility of improving the methanol yields by systematic substitution of some copper centers in these active sites with other earth‐abundant transition metals. Quantum‐mechanical computations can be used to compare methane activation by copper oxide species and analogous mixed‐metal systems. To carry out such screening, it is important that we use theoretical methods that are accurate and computationally affordable for describing the properties of the hetero‐metallic catalytic species. We have examined the performance of 47 exchange‐correlation density functionals for predicting the relative spin‐state energies and chemical reactivities of six hetero‐metallic [M‐O‐Cu]2+ and [M‐O2‐Cu]2+, (where MCo, Fe, and Ni), species by comparison with coupled cluster theory including iterative single, double excitations as well as perturbative treatment of triple excitations, CCSD(T). We also performed multireference calculations on some of these systems. We considered two types of reactions (hydrogen addition and oxygen addition) that are relevant to MMC. We recommend the use of τ‐HCTH and OLYP to determine the spin‐state energy splittings in the hetero‐metallic motifs. ωB97, ωB97X, ωB97X‐D3, and MN15 performed best for predicting the energies of the hydrogen and oxygen addition reactions. In contrast, local, and semilocal functionals do poorly for chemical reactivity. Using [Fe‐O‐Cu]2+ as a test, we see that the nonlocal functionals perform well for the methane CH activation barrier. In contrast, the semilocal functionals perform rather poorly. © 2018 Wiley Periodicals, Inc.

On the Accuracy of Density Functional Theory in Zeolite Catalysis

“…In order to model steric effects imposed by the zeolite framework accurately, periodic DFT calculations representing the zeolite framework in its entirety are commonly the method of choice and imply calculations of on the order of 100 to 300 atoms depending on the zeolite investigated, e. g. the unit cell of H‐ZSM‐5 has 288 atoms. Similarly, the accuracy of DFT calculations for these materials is also highly interesting . While these sizes limit the computational method to DFT, Sauer and co‐workers have introduced a hierarchical cluster approach, that represents an elegant method allowing to employ higher level calculations as well .…”

“…Similarly, the accuracy of DFT calculations for these materials is also highly interesting. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] While these sizes limit the computational method to DFT, Sauer and co-workers have introduced a hierarchical cluster approach, that represents an elegant method allowing to employ higher level calculations as well. [27][28][29][30][31] In this approach, nonperiodic cluster models of the active site are cut out of the zeolite structure and used to supplement DFT with higher level methods such as MP2 or CCSD(T) calculations as shown exemplary in Figure 1.…”

Approaches for Selective Oxidation of Methane to Methanol

“…At this temperature, the methanol selectivity ranges from 70% to 98% for Cu-based ZSM-5 and mordenite zeolites, respectively. Nevertheless, subsequent methanol desorption from the zeolites is quite challenging [71][72][73]. This is because the desorption activation energy for zeolites, such as [Cu 2 (µ-O)] 2+ -AEI, -CHA, -AFX, and -MFI, is quite high, amounting to 13, 23, 22, and 29 kJ/mol, respectively [66].…”

“…Methanol desorption from ZSM-5 occurs at 400-550 K, which often results in a fraction of methanol being dehydrated, yielding dimethyl ether [71] and formaldehyde, in addition to other minor products that are difficult to separate from each other [13,55]. Further increasing temperature results in poor methanol selectivity, with CO and CO 2 as major products [73]. In comparison to zeolites, methanol desorption from ceria ranges between 150 to 250 K, suggesting that methanol desorption is not hindered in ceria.…”

Analisis Molekular dan Karakteristik Hidrogen Sianida (HCN)

“…143,1 kj/mol Tabel 3. Sifat Termodinamika Hidrogen Sianida pembuatan asam sianida (HCN) yang paling sering dilakukan menggunakan metode, yang diciptakan oleh Leonid Andrussow, dimana metana (155)(156)(157)(158)(159)(160)(161)(162)(163)(164)(165)(166)(167)(168)(169)(170)(171)(172)(173)(174) , dan amonia bereaksi pada suhu [206] sekitar 1200 o C menggunakan katalis platina (175)(176)(177)(178)(179)(180)(181)(182)(183)(184)(185)(186)(187)(188)(189)(190)(191)(192)(193)(194) , dengan reaksi sebagai berikut :…”