545057 Control of Kinetic and Diffusive Length-Scales during Absorptive Hydrogen Removal for Enhanced Aromatics Yield in Methane Dehydroaromatization on Mo/H-ZSM-5 Catalysts

Monday, June 3, 2019: 4:00 PM
Texas Ballroom D (Grand Hyatt San Antonio)
Neil Razdan, Anurag Kumar and Aditya Bhan, Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN

Control of kinetic and diffusive length-scales during absorptive hydrogen removal for enhanced aromatics yield in methane dehydroaromatization on Mo/H-ZSM-5 catalysts

Neil Razdan1, Anurag Kumar1, and Aditya Bhan1,*

1University of Minnesota, Minneapolis MN 55455 (USA)

*abhan@umn.edu

Introduction

Direct, non-oxidative conversion of methane to benzene requires high reaction temperatures (950 – 1023 K) and is limited by thermodynamic constraints (6CH4 → C6H6 + 9H2) to ~10% equilibrium conversion. Mo/H-ZSM-5 catalysts have been shown to convert methane to benzene at greater than 70% selectivity with forward rates unaffected by hydrogen pressure [1]. We demonstrate interpellet physical mixtures and staged packed-bed configurations of pre-carburized MoCx/H-ZSM-5 catalyst and Zr, a known H2 absorbent, lift equilibrium limitations, thereby increasing single-pass conversion to near the kinetic limit (~14% conversion to benzene), as dictated by forward synthesis rates. A reaction-transport model rigorously accounts for catalyst-absorbent proximity effects through consideration of kinetic, diffusive, and convective length scales. Simulation results demonstrate introduction of zirconium metal enhances net methane pyrolysis rates throughout the catalyst bed and accurately predict benzene yield and effluent H2 partial pressure for various reactor configurations.

Materials and Methods

 NH4-ZSM-5 (Si/Al = 11.5, CBV 2314) was converted to H-ZSM-5 by heating in dry air (0.67 cm3 s-1) from room temperature to 773 K at 0.0165 K s-1 and holding for ~36 h to decompose NH4+ to H+ and NH3(g). Mo/H-ZSM-5 formulations were prepared from intimate physical mixtures of MoO3 and H-ZSM-5 powders (Mo:Alf ~0.25) heated from room temperature to 623 K at 0.0167 K s-1, held at this temperature for 15 h in dry air (0.67 cm3 s-1), and finally heated to 973 K at 0.0167 K s-1 and held for 10 h to promote migration of MoOx into the zeolite pores.

MoCx/H-ZSM-5 catalysts were prepared by treating pelletized and sieved (mesh 40-80) Mo/H-ZSM-5 samples in 0.27 cm3 s-1 of a 90%/10% CH4/Ar mixture for ~15.5 ks at 973 K. Upon carburization, MoOx moieties convert to zeolite-encapsulated MoCx nanoparticles of 0.5-2 nm and ~10 Mo atoms with concomitant oxygen removal [2, 3].

Reactor configurations of MoCx/H-ZSM-5 with pelletized Zr particles of the same size (mesh 40-80) were prepared in a glove-bag under inert (helium) atmosphere to prevent oxidation of the carburized catalyst.

Results and Discussion

CH4 dehydroaromatization on MoCx/H-ZSM-5 with and without Zr

Methane dehydroaromatization rates were measured employing: (i) a MoCx/H-ZSM-5 catalyst bed, (ii) an interpellet mixture of MoCx/H-ZSM-5 and Zr, and (iii) MoCx/H-ZSM-5 with a succeeding Zr bed. Interpellet physical mixtures showed single-pass conversion as high as ~28%, as depicted in Fig. 1(a). The identity and distribution of products is unaltered upon addition of zirconium, suggesting that the bifunctional reaction pathways of methane pyrolysis were unperturbed by hydrogen removal. Interpellet formulations were partially regenerated by treatment in helium at 973 K, removing all absorbed H2 and yielding above-equilibrium methane conversion in successive regeneration cycles.

Fig. 1(b) shows benzene yield as a function of time-on-stream (TOS) for all three reactor configurations. Addition of Zr, in both case (ii) and (iii), enhances benzene yield ~2x. The increase in methane conversion and aromatic production arises as a consequence of continuous hydrogen scavenging circumventing equilibrium constraints, as is evident in comparison of benzene yield to thermodynamic and kinetic limits shown in Fig. 1(b). Benzene synthesis rates decrease with time-on-stream owing to carbonaceous deposits that limit accessibility of residual Brønsted acid sites. With addition of Zr, the rate of deactivation is accelerated by transient saturation of hydrogen-removal sites.

Kinetic-Transport Model

We present a kinetic-transport model inclusive to all reactor configurations. Differential mole balance in the catalyst bed yields a non-dimensional equation

   .

The last term in Eq. (1) is only included in case (ii). Case (iii) requires consideration of a second differential mole balance for the Zr bed placed downstream of Mo/H-ZSM-5

  .

Approach to equilibrium for benzene production, η, is calculated from gas-phase partial pressures and equilibrium constant, Keq

  .     

Non-dimensionalization of design equations naturally gives rise to dimensionless parameters

                    .

Da and Pe-1 reflect ratios of the characteristic convective length-scale to kinetic and diffusive length-scales. Pe and PeZr are Péclet numbers in the catalyst and Zr bed, both taken to be unity per numerical fit. Da = 0.16 is the Damköhler number for the pseudo 1st-order forward benzene synthesis rate, calculated using measured values taken from previous investigations [1]. H2 scavenging by Zr is rapid and precludes kinetic measurement in current reactor configurations. DaZr > 104 give essentially invariant axial methane conversion and H2 pressure profiles and are in quantitative agreement with net synthesis rates and outlet η, as shown in Fig. 2.

In case (iii), Danckwerts boundary conditions are applied at both bounds of the catalyst and absorbent beds.

                

                     

Eqs. (4) and (7), “closed” Danckwerts boundary conditions, account for discontinuous change in dispersion length-scales between entrance and reaction sections and absorptive and exit sections, respectively. Eqs. (5) and (6), “open” Danckwerts boundary conditions, demand continuity of mass and diffusive flux between catalyst and absorbent beds. Case (i) and (ii) simply require “closed” Danckwerts boundary conditions at each end of the single reactor bed.

Proximity and length-scale effects during polyfunctional catalysis

The impact of catalyst-absorbent proximity is evident in comparison of benzene yield in cases (ii) and (iii), as shown in Fig. 1(b). Physical interpellet mixtures of MoCx/H-ZSM-5 and Zr show the largest maximum benzene yield, but deactivate most rapidly. Intimacy of product formation and scavenging functionalities in interpellet mixtures disproportionately promotes formation of higher aromatics which congest zeolite pores and sterically hinder dehydrocyclization steps. Separation of catalytic sites from H2 absorptive function mitigates deactivation and increases cumulative benzene yield from 3.03 to 3.78 mol C6H6/mol Mo at 8.7 ks TOS from case (ii) to case (iii).

Profiles for benzene yield and approach to equilibrium along the catalyst bed predicted by the kinetic-transport model are shown in Fig. 2. Non-zero benzene yield and η values at the reactor inlet arise from “closed” Danckwerts boundary conditions which demand discontinuous change in species concentration from x = 0- to x = 0+ as dispersion considerations become relevant in the finite reaction section. Predicted values of η → 0 at reactor outlet in case (iii) result from rapid H­2 removal by formation of ZrHx such that PH2 → 0 at x = 1. Balance of convective and diffusive length scales, reflected in Pe ~ 1, permit response of H2 pressure profiles throughout the catalyst bed. In contrast, in ideal PFR reactors, dominance of convective mass transport (i.e. Pe → ∞) precludes bed-scale changes in axial profiles by successive product removal.

Significance

Introduction of a continuous H2 scavenging functionality in interpellet and staged-bed configurations of MoCx/H-ZSM-5 and Zr lifts thermodynamic equilibrium constraints for non-oxidative methane conversion, enhancing aromatic synthesis rates and cumulative product yields without changes to product distribution. Kinetic-transport models and dimensionless parameters presented herein quantitatively capture effects of hydrogen absorption and provide a framework to investigate performance of various reactor configurations through the formalism of Da and Pe which capture interplay between kinetic, diffusive, and convective length-scales.

Notation

 

yj

dimensionless partial pressure of component j, normalized by 1 bar

x

dimensionless length along reactor bed

us

superficial linear velocity [m/s]

L

length catalyst bed [m]

LZr

length of absorbent bed [m]

Deff

effective diffusivity [m2 s-1]

k

pseudo 1st-order rate constant for benzene production reaction [s-1]

kZr

pseudo 1st-order rate constant for H2 absorption [s-1]

 

Greek letters

 

νj

stoichiometric coefficient of component j in benzene production reaction

dH2,j

Kronecker delta, permitting absorption of only H2 by Zr

η

approach to equilibrium

References

[1] Bedard, J., Hong, D-Y., Bhan, A., J. Catal., 306, 58-65 (2013).

[2] Kumar, A., Song, K., Liu, L., Han, Y., Bhan, A. Angew. Chem. Int. Ed., (2018). In Review

[3] Ding, W., Li, S., Meitzner, G.D., Iglesia, E., J. Phys. Chem. B, 105, 506-513 (2001).

Figure 1: (a) CH4 conversion vs TOS for case (i) and case (ii) before (fresh) and after regeneration in He flow. Regenerations 1, 2, and 3 of interpellet mixtures by flushing in He flow  at 973 K for 61.2 ks, 84.6 ks, and 34.2 ks respectively. Dashed red line indicates ~10% equilibrium conversion. (b) Measured benzene yield vs TOS for cases (i), (ii), and (iii) in black, white, and gray-filled squares, respectively. Large circles are predictions of the kinetic-transport model in the absence of deactivation.

 

Figure 2: Axial profiles of benzene yield and approach to equilibrium, as predicted by the kinetic-transport model, for cases (i), (ii), and (iii), shown in solid, dashed, and dotted lines, respectively. Measured benzene yield (approach to equilibrium) for cases (i), (ii), and (iii) shown in black, white, and gray-filled circles (squares), respectively. Simulations predict η → 0 throughout the bed for interpellet mixtures.

 


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