Abstract
The present study investigates the possibility of improving the selective oxidation of methane to formaldehyde over V-SBA-15 catalysts in two different ways. In a classical approach of catalyst optimization, the in situ synthesis of V-SBA-15 catalysts was optimized with regard to the applied pH value. Among the set of catalysts synthesized, a higher amount of incorporated vanadium, a higher content of polymeric VOx species as well as a less ordered structure of the support material were observed by increasing the pH values from 2.0 to 3.0. An optimum in performance during the selective oxidation of methane to formaldehyde with respect to activity and selectivity was found over V-SBA-15 prepared at a pH value of 2.5. With this knowledge, we have now evaluated the possibilities of reaction control using this catalyst. Specifically, artificial neural network modelling was applied after the collection of 232 training samples for obtaining insight into the influence of different reaction parameters (temperature; gas hourly space velocity (GHSV); and concentration of O2, N2 and H2O) onto methane conversion and selectivity towards formaldehyde. This optimization of reaction conditions resulted in an outstanding high space-time yield of 13.6 kgCH2O∙kgcat∙h−1.
Highlights
Methane, the main component of natural gas, is the simplest and most abundant hydrocarbon with estimated global deposits of 199·1018 m3 [1]
We investigated the influence of the pH value during synthesis on the structure of V-SBA-15 catalysts and the consequences on their performance in the selective oxidation of methane to formaldehyde
The metal loading and the textural properties of the synthesized V-SBA-15 catalysts are given in 771 to 574 m2 ·g−1, and the pore volume varies between 0.92 and 1.00 cm3 ·g−1 for the samples synthesized at pH values below 3.0 and decreases to 0.82 cm3 ·g−1 at the pH value of 3.0
Summary
The main component of natural gas, is the simplest and most abundant hydrocarbon with estimated global deposits of 199·1018 m3 [1]. Due to its molecular symmetry and the high strength of the C–H bonds, high temperatures are normally required for methane activation [2] in chemical transformations. As a result of harsh reaction conditions, only few processes which transform methane directly into value-added products, such as HCN, CH3 Cl, CS2 or C2 H2 [3], were transferred from laboratory to industrial scale. Processes directly yielding oxygenates like methanol or formaldehyde often suffer from overoxidation towards CO and CO2 , especially at high methane conversion [4]. Among the variety of evaluated catalytic systems for the selective oxidation of methane towards formaldehyde, silica-supported vanadium catalysts are one of the most selective and most productive candidates to date [4,5,6,7]. The most prominent ones are wet and incipient wetness impregnation [8,9] and ion exchange [10,11] of vanadium salts, grafting of molecular
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