Microwaves effectively examine the extent and type of coking over acid zeolite catalysts

Coking leads to the deactivation of solid acid catalyst. This phenomenon is a ubiquitous problem in the modern petrochemical and energy transformation industries. Here, we show a method based on microwave cavity perturbation analysis for an effective examination of both the amount and the chemical composition of cokes formed over acid zeolite catalysts. The employed microwave cavity can rapidly and non-intrusively measure the catalytically coked zeolites with sample full body penetration. The overall coke amount is reflected by the obtained dielectric loss (ε″) value, where different coke compositions lead to dramatically different absorption efficiencies (ε″/cokes’ wt%). The deeper-dehydrogenated coke compounds (e.g., polyaromatics) lead to an apparently higher ε″/wt% value thus can be effectively separated from lightly coked compounds. The measurement is based on the nature of coke formation during catalytic reactions, from saturated status (e.g., aliphatic) to graphitized status (e.g., polyaromatics), with more delocalized electrons obtained for enhanced Maxwell–Wagner polarization.

Page 6: change "dispersions in zeolite lead to dramatic increase" to "dispersions in zeolite lead to large increase" Page 6: split sentence "This indicates that real coke deposits formed and dispersed naturally in the zeolite structure during reactions possess much higher microwave absorption efficiency than those mechanical mixtures (even mixing the same coked sample with pure zeolite cannot achieve the same effect), which are most possibly attributed to a more uniform, contiguous, thinner-layer dispersion of carbon in the zeolite system29, and can only be achieved by the conditions of a catalytic reaction, or the interactions between the coke species and zeolite frameworks (of course, this has sparked our interest for a further study in the future)." into two sentences.

Reviewer #1:
This is a sound paper dealing with the analysis of coke formation occurring in zeolites.
The dielectric response is clearly a measure for the degree of coke in the zeolite. The paper is well written and it should be published.
The idea to determine the coke in zeolites by microwaves is unique, however, when searching "( coke AND formation AND catalyst AND microwave )" in Scopus, I found N. Müller et al., "Initial tests to detect quantitatively the coke loading of reforming catalysts by a contactless microwave method", Chemical Engineering and Processing: Process Intensification, 50(8), pp. 729-731. It appears that this study and later papers of this (and probably other groups) use a similar method to measure the extent of coke formation; not on zeolites but on fixed-bed catalysts and automotive filters.
Nevertheless, the paper provides enough novelty to justify publication.
English is used without obvious errors, and the article contains a common thread.
The results seem to be sound and reliable.

Reply to Reviewer #1:
We sincerely appreciate the Reviewer 1 for these very encouraging comments. His/her understanding on our work is admirable and very professional, also the reviewer has highlighted that dielectric response is clearly a measure for the degree of cokes in zeolite structure which is the critical, central element of the study.
As the Reviewer 1 has noted, our novelty is based on our studies on zeolites, which is the most important and widespread catalysts in the petrochemical industry with intensive coke formation in any operating plant. Furthermore, we have developed the method to enable it to separate different coke compositions with our designed calibration profiles.

Reviewer #2:
PAGE 2 Question 1: The Reviewer 2 has added a sentence before introducing the microwave cavity perturbation technique in the Introduction of this paper. He/she also made some necessary changes on the words and grammars used in this paragraph. (Page 2, right, line 21) These are reproduced below: Reply: We gratefully accept the Reviewer 2's corrections and have made the necessary changes. The new contents have been shown below.
"To overcome the above shortcomings we resort to using the microwave cavity perturbation technique which also enables sample interrogation in an electromagnetic field, and can show the growth of carbonaceous species in numbers, as well as the dielectric property change of the whole catalyst body even in-situ 17,18 ." According to the Nature Communications template, we have to start the final paragraph in Introduction, by starting with "Here, we show…", so we have combined the Reviewer 2's suggestions with the Editor's: "Here, we show a microwave cavity perturbation based method to effectively measure the coke accumulation in the whole structure of an acid zeolite catalyst (volumetrically), and separate different coke compositions. We have shown that different coking levels (they have different coke accumulations) of acid zeolite catalysts can be readily distinguished by their dielectric loss properties, as reflected in their different ε" values probed by the microwave cavity perturbation technique. The contribution to integral dielectric loss value of a coked sample by unit weight of cokes, given by ε"/wt%, is entirely characteristic of the coke composition formed under different reaction conditions. Particularly, we find that at the working frequencies near 2.45GHz, polyaromatics dominate in the microwave response, with outstanding ε"/wt% values, as compared to olefin/paraffin cokes. The observed results correspond closely with data obtained from previous coke characterization methods, e.g. Raman, TGA and 13 C NMR. The present technique possesses distinct advantages in terms of volumetric measurement with sample full body penetration, and higher sensitivity for deeply dehydrogenated cokes. This advance could provide critical information for monitoring catalyst coking and deactivation in important industrial processes (e.g. an industrial FCC process for petroleum refinery). The microwave based approach interrogates the nature of catalytic coke formation which is an evolution from sp 3 carbons to sp 2 carbons that possess a further delocalized bond electron distribution, i.e., from saturated alkanes/olefins to the coke graphite structures with a conjugated π electron system. By far, the available spectrum for catalyst analysis ranges from X-ray, to Ultraviolet, Visible, and Infrared, and here our findings embody the potential to extend this to the microwaves."

PAGE 3
Question 2: The Reviewer 2 has suggested: Changing all ε symbols into non-italics.
Changing the Vs in equation ε"AVs = 0 to V sub s.
These are reproduced below: Reply: Many thanks! We have made the corresponding changes.
All ε symbols in the manuscript have been changed into non-italics.
The equations have been changed: The Editor also made some suggestions on the formatting of symbols in this manuscript. "Please check proper formatting of symbols -see my e-mail for details. Scalar variables (e.g. x, V, χ) and constants (e.g. π, ħ, e) should be typeset in italics, and vectors (such as r, the wavevector k, or the magnetic field vector B) should be typeset in bold without italics. In contrast, subscripts and superscripts should only be italicized if they too are variables or constants. Those that are labels (such as the 'c' in the critical temperature, T_c, the 'F' in the Fermi energy, E_F, or the 'crit' in the critical current, I_crit) should be typeset in roman".
We have also made necessary changes by converting the following scalar variables and constants into the italics. 1) Scalar variables: S 21 (transmitted microwave power), f (frequency), f 0 (initial frequency of a sample), BW (bandwidth), Q (unloaded quality factor) and Q L (loaded quality factor).
2) Scalar constants (they are not changed once the experimental configurations have been applied): V s (sample volume), a (cavity radius), d (diameter), and r (tube inner radii).
The final manuscript formatting is a combination of suggestions from both the Reviewer and Editor. These have been replaced below: Reply: Many thanks for the very careful corrections and a great patience! We have made the necessary changes and accepted the corrections.
"Here f 0 is the unperturbed resonant frequency. A is a constant determined by the size and geometry of the cavity. For the present system, A is approximately 7.34×10 -3 , as detected using a PTFE sample of known complex permittivity 23 . V s is the effective volume of sample in the cavity (i.e. ~0.126cm 3 ) 19 . This is a non-destructive, non-invasive and contact-less measurement, plus data acquisition takes only milliseconds and shows excellent repeatability among multiple tests. Besides, previous research has shown that these measurements can be taken at higher temperatures which would benefit future in-situ applications and better contribute to the real-time monitoring of catalyst deactivation 17,24 ." "The microwave cavity is designed in a cylindrical shape, as shown schematically in Fig. 1b. The sample was placed in a thin-walled high-purity quartz tube and introduced axially through a small insertion hole in the centre of the top and bottom plates of the cavity (Fig. 1c). The cavity is made from aluminium with an unloaded quality factor (Q factor) of ~8000 at room temperature."