Research Article Cite This: ACS Catal. 2017, 7, 8235-8246
pubs.acs.org/acscatalysis
A Straightforward Descriptor for the Deactivation of Zeolite Catalyst H‑ZSM‑5 Daniel Rojo-Gama,†,‡,∥ Malte Nielsen,†,‡,∥ David S. Wragg,† Michael Dyballa,† Julian Holzinger,§ Hanne Falsig,‡ Lars Fahl Lundegaard,‡ Pablo Beato,*,‡ Rasmus Yding Brogaard,†,‡ Karl Petter Lillerud,† Unni Olsbye,† and Stian Svelle*,† †
Center for Materials Science and Nanotechnology (SMN), Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway ‡ Haldor Topsøe A/S, Haldor Topsøes Allé 1, 2800 Kgs. Lyngby, Denmark § Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark S Supporting Information *
ABSTRACT: ZSM-5 is a widely used zeolite catalyst and is employed industrially for the methanol to gasoline (MTG) process. Even so, deactivation of ZSM-5 by coke formation constitutes a major technical and also fundamental challenge. We investigate the deactivation of a range of ZSM-5 catalysts through catalytic testing, physicochemical characterization, and powder X-ray diffraction (XRD). It is demonstrated that the unit cell changes upon deactivation. Periodic density functional theory is used to show that the change is induced by certain methyl substituted benzenes in the channel intersection in ZSM-5. This finding is corroborated by Rietveld refinement of XRD data obtained for deactivated catalysts. We are able to establish a direct correlation between the difference in the length of the a- and b-unit cell vectors and the total amount of coke, the remaining acidity, and the remaining surface area of the catalysts. This a- minus b-parameter is a straightforward descriptor that carries the essential information regarding the degree of deactivation of a ZSM-5 catalyst, and a routine measurement of a diffractogram of the catalyst can be used to quantitatively assess the degree of deactivation. KEYWORDS: ZSM-5, symmetry, deactivation, zeolite, MTH, DFT, XRD
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INTRODUCTION Zeolites are crystalline aluminosilicates with a well-defined structure of micropores and cavities. Zeolites are versatile materials that are employed in processes such as adsorption, ion exchange, gas separation, and catalysis.1,2 The conversion of methanol to hydrocarbons (MTH), which was first reported by Chang and Silvestri in 1977,3 is one of the processes that relies on the use of acidic zeolites for the transformation of methanol into liquid hydrocarbons. Even though more than 200 different zeolite structures are currently described,4 few of them are industrially applied. Among them, ZSM-5 is the preferred catalyst in the MTH process, owing to its high catalytic activity and slow deactivation.5 Despite the outstanding performance of ZSM-5, hydrocarbon conversion processes cause the formation of nonreactive species, i.e., coke, which induce a loss of activity and eventually the complete deactivation of the catalyst.6 This accumulation of hydrocarbon residues in the zeolite pores is reversible, and the activity can be restored by oxidative regeneration at temperatures between 300 and 800 °C. In © XXXX American Chemical Society
addition, the permanent loss of zeolite activity, induced by the removal of Al atoms from the zeolite framework, i.e., dealumination, may occur across multiple reaction/regeneration cycles.7,8 The deactivation of zeolites in the MTH process has been a topic of thorough study for the past decades.6,9−13 Haw and Marcus proposed in 2005 the “burning cigar” model for fixed bed reactors.14 This model was used to describe how the deactivation over SAPO-34 materials occurs inhomogeneously throughout the bed. Once the induction period (which can resemble the lighting of the cigar and is characterized by very low methanol conversion) is over and a sufficient concentration of reactive intermediates (hydrocarbon pool species) has been reached, the catalyst bed is active and the deactivation proceeds from the inlet to the outlet of the bed, leaving the deactivated catalyst in its wake.14 Some years later, a similar deactivation Received: July 5, 2017 Revised: September 30, 2017 Published: October 23, 2017 8235
DOI: 10.1021/acscatal.7b02193 ACS Catal. 2017, 7, 8235−8246
Research Article
ACS Catalysis
vectors and identifying the exact compounds involved. Finally, we are able to correlate the observed change in the diffractograms to a number of catalyst properties related to deactivation. Essentially, recording a standard XRD pattern and refining the unit cell dimensions provides a quantitative measure of the total coke content, the remaining Brønsted acidity, and the remaining surface area. Clearly, this provides detailed information with very little effort compared to that required to actually measure these key properties directly. Overall, the difference between these unit cell vectors (a minus b) is a potent descriptor for the deactivation of ZSM-5 catalysts. We foresee that this parmeter will be a very efficient handle when it comes to catalyst optimization and the monitoring of plant operation. It may also be noted that the a- minus bparameter obviously would lend itself well for operando studies of deactivation. We have employed 6 different ZSM-5 catalysts, 4 commercially available. Four catalysts were tested at industrial conditions, whereas the remaining 2 samples were tested at typical laboratory conditions. Overall, the findings are based on the evaluation of a large data set comprising 30 fresh and increasingly deactivated ZSM-5 catalyst samples.
pattern was observed for ZSM-5 catalysts: Schulz performed the MTH reaction in a plug flow reactor at 475 °C using ZSM5 catalysts and reported that the extent of deactivation was dependent on the exact axial position in the catalyst bed. The deactivation was most pronounced in the inlet part of the reactor, which was observed to be black due to coking. A middle zone, where the actual methanol conversion was believed to take place, was observed to be gray. Finally, a less deactivated zone having a light blue color was observed toward the end of the bed.15 Bleken et al. evaluated spatially the deactivation on ZSM-5 catalysts, and similar conclusions were drawn. On a purely microporous sample, the area of the bed closer to the inlet was deactivated to a larger extent than the bottom part of the reactor.16 More recently, we have evaluated in time (time on stream) and space (axial reactor coordinate) the deactivation over a large number of different zeolite topologies. We have shown that the deactivation pattern along the catalyst bed depends on the pore size of the zeolite used. When using medium pore size zeolites, the deactivation is gradual along the bed and follows the “burning cigar” model. On the other hand, a more uniform degree of deactivation was observed across the different areas of the bed when large pore size zeolites such as Beta and Mordenite were employed as acid catalysts.17 Previous studies have shown that the buildup of hydrocarbons in the zeolite structure may induce changes in the structure of microporous catalysts. Wragg and co-workers used time- and space-resolved X-ray diffraction (XRD) to study the accumulation of hydrocarbon species over SAPO-34,18 the preferred catalyst for the methanol to olefins process (MTO).13 The results revealed that c-axis expansion started 2−3 mm into the bed and then spread gradually toward both the inlet and outlet until the expansion had occurred in the entire catalyst bed. Using a similar approach, del Campo et al.19 studied the deactivation with time-resolved operando X-ray diffraction over the unidimensional ZSM-22 catalyst, observing a similar unit cell expansion with deactivation but with a less clear spatial evolution. In a key study of relevance to the present work, Alvarez et al.20 reported that coking of ZSM-5 resulted in distinct changes in the XRD patterns. This was interpreted as a symmetry change of the material. As is well-known, fresh ZSM-5 catalysts are orthorhombic, with the a- and b-unit cell vectors differing by approximately 0.3 Å (space group Pnma, no. 62). Alvarez et al.20 reported that the deactivated catalysts displayed a tetragonal symmetry, (i.e., identical a- and b-unit cell vectors, space group P42212, no. 94). This apparent change in symmetry was linked to the buildup of coke in the structure. The work of Alvarez et al. has received remarkably little attention and is cited by only a handful of studies that report similar changes in the diffractograms upon deactivation.21−24 In this work, we elaborate on the studies described above. We demonstrate that the changes observed in the diffractograms upon deactivation of ZSM-5 are not an actual symmetry change but rather a gradual transition following the degree of deactivation. The difference in the length of the a- and b-unit cell vector does become smaller for deactivated catalysts, but this difference becomes close to zero only for some catalyst samples, and the apparent change to tetragonal symmetry is coincidental. Further, we analyze the coke species in deactivated catalysts and combine this with DFT calculations to model the forces that these species exert on the framework, thereby finding a driving force for the change in unit cell
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EXPERIMENTAL SECTION Materials. Six different ZSM-5 samples were used. ZSM-5MFI-27 and ZSM-5-Pentasil were received from Sud Chemie; ZSM-5-PZ-2-100H was from Zeochem, and ZSM-5-CBV-8014 was from Zeolyst. In addition, the CBV-8014 was mildly steamed for 5 h and 450 °C obtaining ZSM-5-S-CBV-8014. Finally, ZSM-5-hm (homemade) was synthesized as reported elsewhere.25 Generation of Deactivated Catalyst Samples. An extensive set of partially and fully deactivated ZSM-5 samples were obtained by converting methanol to hydrocarbons in two different fixed bed reactors, operated at different conditions. For ZSM-5-MFI-27 and ZSM-5-hm, the reaction was carried out using 200 mg of catalyst at atmospheric pressure (130 mbar methanol in inert) and 400 °C using a weight hourly space velocity (WHSV) of 2 gMeOH/(gcat h) with a 5 mm inner diameter reactor, whereas on the remaining 4 ZSM-5 catalysts, the reaction was performed at 20 barg (6 mol % methanol in inert), 370 °C, and WHSV of 8 gMeOH/(gcat h) using a reactor with an internal diameter of 2 mm and 150 mg of sample. For the experiments at atmospheric pressure, the catalyst bed was ca. 25 mm tall, and the linear gas velocity was ca. 200 cm min−1 (based on a flow expressed in NmL). For the high pressure tests, the catalyst bed was ca. 100 mm tall, and the linear gas velocity was ca. 3800 cm min−1 (again based on a flow expressed in NmL). As outlined in the Introduction, there will be significant axial gradients in the degree of deactivation. To reduce the impact of this behavior and to be able to generate many partially deactivated samples efficiently, we decided to separate the catalysts beds into sections or layers. These layers are labeled top, middle, and bottom. We operate in a down flow mode, so “top” corresponds to the reactor inlet. First, the catalysts were (nearly) fully deactivated to evaluate their catalyst lifetime and to generate a completely deactivated final sample. In a second, identical MTH experiment, the reaction was quenched at ca. 80% of the reaction time required until breakthrough of methanol, and the catalyst bed was axially divided into 3 layers to evaluate spatially the degree of deactivation along the bed. We expect no axial gradients along the catalyst bed for severely 8236
DOI: 10.1021/acscatal.7b02193 ACS Catal. 2017, 7, 8235−8246
Research Article
ACS Catalysis
(λ = 1.5406 Å). For these diffractometers, samples were prepared on glass plates. The pattern of some ZSM-5 samples was measured on the multipurpose diffractometer at beamline BM01 of the European Synchrotron Radiation Facility (ESRF) using a wavelength of 0.7743 Å and a Pilatus 2 M area detector. Samples were prepared in 0.5 mm internal diameter capillaries. Data were azimuthally integrated using the Pylatus suite.28 To verify the calculated models for the position and amount of the different internal coke molecules (see below for details on calculations), Rietveld refinements were carried out against high resolution powder X-ray diffraction data. A high resolution powder pattern on a completely deactivated ZSM-5-MFI-27 was collected at room temperature on a Bruker D8 Advance diffractometer in capillary mode with monochromatic Cu Kα1 radiation and a LynxEye XE detector. Rietveld29 refinement was carried out using TOPAS 5. The ADDSYM30 program in Platon was used to determine the correct space group (Pnma, no. 62) from the P1 calculated structure. The instrumental peak shape was modeled using the fundamental parameters method with both Gaussian and Lorentzian crystallite size broadening parameters, and the background was fitted with a 9-term Chebyshev function and a broad peak used to fit the amorphous background from the capillary. Orthorhombic lattice parameters and all symmetry independent framework atom positions were refined for both models. Two shared thermal parameters (Biso) were used for all silicon and all oxygen atoms in the zeolite framework. To arrive at a suitable descriptor for the degree of deactivation of the partially deactivated samples of catalysts, Le Bail refinement was carried out on each (partially) deactivated catalyst sample, the unit cell parameters were determined, and the difference between the lengths of the aand b-unit cell vectors was calculated. This quantity, referred to as the a- minus b-parameter, or (a−b), was used as a descriptor throughout the work to describe the degree of deactivation. Computational Details. Periodic density functional theory calculations were performed using the Quantum Espresso code31 interfaced with the Atomic Simulation Environment.32 The BEEF-vdw exchange correlation functional33 was applied using a plane wave basis with kinetic energy cutoff at 700 eV and charge density cutoff at 7000 eV, using only the Γ-point to sample Brillouin zone. These parameters were based on a convergence analysis of geometry optimization combined with unit cell relaxation showing convergence to 0.01 Å in any direction. The calculations modeled the ZSM-5 zeolite by constructing four acid sites in the MFI unit cell, corresponding to a Si/Al ratio of 23. Three different types of acid site position were used: one in the straight channel (site 11), one in the zigzag channel (site 4), and one in the intersection (site 12), where the numbering of the T-sites follows the numbering in ref 34. When a molecule is placed in the intersection, straight channel, or sinusoidal channel, the acid site of the zeolite is placed accordingly. When evaluating the effect on the unit cell of a given molecule, the calculations were performed in the following manner: First, the aromatic molecule was inserted into the zeolite structure. Unless otherwise stated, the loading was four molecules per unit cell. The atomic positions were then relaxed to a maximum force of 0.01 eV/Å, using the BFGS optimization algorithm. After this, the unit cell and atoms were simultaneously relaxed by the “VC-relax” algorithm in
deactivated catalysts, i.e., when ending the experiment at negligible conversion. Basic Catalyst Characterization. The chemical composition of the fresh catalysts was measured using a microwave plasma atomic emission spectrometer (MP-AES 4100) from Agilent Technologies. The particle size was investigated by scanning electron microscopy (SEM) on a Hitachi SU 8230 FE-SERM instrument. Uncoated samples in powder form were glued on the holder with carbon tape. The amount of coke species in deactivated catalysts was quantified by thermogravimetric analysis (TGA) on a Rheometric Scientific STA. Approximately 15 mg of catalyst was heated under 25 mL/min of synthetic air to 650 °C at a rate of 5 °C/min. Then, the final hold time at this temperature was 40 min. The total amount of coke is given in wt % relative to the regenerated catalyst (final mass in the TGA profiles). NH3 TPD was performed on a Mettler-Toledo TGA-DSC 1 instrument. Samples were heated to 500 °C with a flow of He at a rate of 20 °C min−1, and this temperature was held for 110 min. The adsorption of NH3 was carried out at 150 °C using a stream of 2% NH3 in He for 30 min. Subsequently, the temperature was kept at 150 °C for 4 h while a flow of He was fed to desorb the excess ammonia. The temperature was then increased to 600 °C at 10 °C min−1 and kept at this temperature for 15 min. The desorption rate is calculated on the basis of the changes of weight with time. The acidity of fresh as well as on partially deactivated samples of ZSM-5-MFI-27 and ZSM-5-hm was quantified with pyridine as a probe molecule on a Bruker Vertex 80 instrument with a mercury cadmium telluride (MCT) detector, using a resolution of 2 cm−1. An in-house designed transmission cell with KBr windows was used. Catalysts were prepared as thin wafers supported in a gold envelope and pretreated in vacuum (
