Spectroscopic and Computational Study of Cr Oxide Structures and Their Anchoring Sites on ZSM-5 Zeolites

Abstract

Cr/ZSM-5 catalysts with 0.5−2.6 wt % Cr loadings and zeolites with 15−140 Si/Al ratios were characterized with in situ UV−vis, IR, and Raman spectroscopies, including operando Raman measurements under reaction conditions in methane conversion at 773−1123 K with a simultaneous online analysis of reaction products. DFT calculations with cluster and periodic models were performed with DMol 3 , Gaussian 09, and VASP software packages. Isolated Cr(VI) dioxo and Cr(III) mono-oxo structures on framework Al anchoring sites were identified as the dominant species under most conditions. In the absence of gas-phase O 2 (under Ar flow) at 773 K, the Cr(VI) dioxo species on framework Al anchoring sites autoreduce, and the Cr(III) mono-oxo species remain the only observable surface Cr oxide structures. For ZSM-5 zeolites with a relatively low concentration of framework Al atoms (Si/Al ≥ 25), exposure to gas-phase O 2 at 773 K forces surface Cr oxide species to migrate from framework Al anchoring sites to Si sites on the external surface of the zeolite and form dioxo (Si− O−) 2 Cr(O) 2 and mono-oxo (Si−O−) 4 Cr(O) structures. The activity of Cr/ZSM-5 catalysts in methane conversion with the production of benzene and hydrogen as the main products is lower than that of Mo/ZSM-5 catalysts. The rate of benzene formation over Cr/ZSM-5 catalysts, however, is relatively stable with time on stream, in comparison to a rapidly declining rate over Mo/ZSM-5 catalysts. The zeolite-supported Cr species are highly mobile under the reaction conditions and can migrate between zeolite particles.

Tracking the reasons for the peculiarity of Cr/Al2O3 catalyst in ethylene polymerization

“…The spectroscopic properties of Cr 6+ on Al 2 O 3 are well known in the specialized literature. While for Cr 6+ on SiO 2 there is still a debate on the aggregation state of the Cr 6+ species (both mono-and dichromates have been proposed [5, 7-9, 20, 22-24, 26, 34, 35], or even mono-oxo CrO 5 [27][28][29][30][31][32][33]), Cr 6+ exist primarily as monochromate species on Al 2 O 3 [9,[19][20][21][22][23][24]36] (the corresponding DR UV-Vis-NIR spectrum is shown in Figure S1a). Temperature-programmed reduction measurements have demonstrated that Cr 6+ on Al 2 O 3 are more reducible than Cr 6+ on SiO 2 (i.e.…”

A Single-Event MicroKinetic model for the cobalt catalyzed Fischer-Tropsch Synthesis

“…In this work, the outlet molar flow rates of C1 to C6 n-alkanes and C2 to C5 1-alkenes were used as responses in eq [6]. The statistical weights, , are calculated from the inverse of the covariance of the experimental errors, eq [7].…”

“…Initial estimates for the are determined from the inverse of calculated by eq [1]. The final regression was performed using the weights of eq [7] as only at a few experimental conditions replicate experiments were available.…”

“…Next to converting the syngas directly on transition metals, it can also be first converted to methanol which can subsequently be converted into hydrocarbons on acid catalysts such as H-SAPO34 or H-ZSM5 [5]. Furthermore, also routes which directly activate methane are explored such as methane aromatization [6][7][8] or oxidative coupling of methane [9,10].…”

“…Additionally, Co catalysts also exhibit a higher activity and better stability than Fe catalysts [1]. 7 The microkinetic simulation of FTS is of key importance to acquire fundamental insight for process design, optimization and simulation [11] as well as for rational catalyst development [12][13][14][15]. Certainly when considering the scale of the industrial application, an improved design, even if it only leads to a minor enhancement of the desired product selectivity, can have a major impact on the process economics.…”