Zinc Hinders Deactivation of Copper-Mordenite: Dimethyl Ether

Letter pubs.acs.org/acscatalysis

Zinc Hinders Deactivation of Copper-Mordenite: Dimethyl Ether Carbonylation Allen A.C. Reule and Natalia Semagina* Department of Chemical and Material Engineering, University of Alberta, 9211, 116 Street Edmonton, Alberta T6G 1H9, Canada S Supporting Information *

ABSTRACT: A plethora of previously unimaginable low-temperature C1 and C2 valorization reactions have become possible after the discovery of metal-exchanged solid-acid catalysts, with the most promising candidate being copper-mordenite. We show the dramatic effect of zinc addition to copper-exchanged mordenite in maintaining high Cu dispersion and reducing the catalyst poisoning as it relates to carbonylation of dimethyl ether to methyl acetate. Zinc maintains 90%+ selectivity even during deactivation versus 60% for the Cu-mordenite, which leads to 6-fold higher product yield. The concept of Zn addition is recommended for further exploration in the conversion of methane to methanol or acetic acid. KEYWORDS: zeolites, DME, methyl acetate, stability, copper−zinc

I

halide promoters. The process has received increased attention in the past decade because DME can be produced directly from biomass-derived syngas with an exotic CO−H2 ratio of 1:118−20 or via dehydration of methanol.21 An acidic mordenite was discovered to be selective for DME carbonylation to MeOAc,9,11,22 with specific Brønsted acid sites in 8-membered rings being selective, while the sites in the 12-membered rings lead to other reactions and result in the formation of byproducts, which may poison the catalyst.11 The rapid deactivation set up the race to find a simple means to stabilize H-MOR and make its industrial use a reality. Literature on lifetime extension reports Cu- and Ag- ion exchange,3 selective dealumination of MOR,23,24 selective poisoning with pyridine,25 and nanomordenites,26 but the challenge still remains and impedes the process of industrialization. In this work, it was hypothesized and verified that Zn could fulfill its already known role of increasing and maintaining Cu dispersion and potentially block unselective acid sites on mordenite, thus leading to increased stability. Herein, the preparation of the bimetallic catalysts included competitive ion-exchange of Cu and Zn ions with a total metal loading of ca. 3 wt % based on Si/Al = 6.5 mordenite. No residual Na was detected in the final materials. Reactions were conducted at 483 K, 1 MPa CO, 48 kPa DME (2 MPa total pressure, 50.0% CO/2.4% DME/2.9% H2/44.7% He). The concentrations were analyzed online with a calibrated mass

on-exchanged copper-mordenite materials (Cu/H-MOR) have recently attracted significant attention in natural and syngas valorization reactions, specifically for the conversion of methane to methanol or acetic acid and for the carbonylation of methanol or dimethyl ether.1−8 For the former application, specific Cu locations in the zeolite framework were postulated to be of the utmost importance, with single-site oxo−Cu complexes being the active sites.5−7 Another study suggested small copper clusters as active sites.8 Although it is known that acidic mordenite with no ion-exchanged Cu can successfully catalyze the carbonylation of dimethyl ether (DME) via the formation of methoxy groups,9,10 the rate-determining step has been theorized to be the insertion of CO into the methoxy group to create an acetoxy group.11 To assist in this critical step in carbonylation, it has been believed that copper could be added to mordenite, increasing the rate of carbonylation through the formation of Cu−CO complexes, which may also interact with DME.12 Irrespective of the Cu/H-MOR application, it appears that a high dispersion of copper, either as single-site oxygen complexes or as nanoparticles, is paramount for the observed enhanced performance. As is well-known from the methanol synthesis on Cu/ZnO/ Al2O3,13−16 the addition of Zn helps to maintain high Cu dispersion, and recent studies recommend Zn-rich (4:1 Zn-toCu ratio) catalysts for the methanol synthesis.17 Surprisingly, Cu−Zn bimetallic materials have not yet been explored in solid-acid-catalyzed C1 or C2 upgrading reactions. This study is the first attempt to evaluate the effect of the addition of Zn to Cu/H-MOR as it relates to DME carbonylation. Solid-acid-catalyzed DME carbonylation to methyl acetate (MeOAc) is an environmentally friendly alternative to industrial acetic acid synthesis, which employs © XXXX American Chemical Society

Received: May 24, 2016 Revised: June 24, 2016

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DOI: 10.1021/acscatal.6b01464 ACS Catal. 2016, 6, 4972−4975

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ACS Catalysis

catalysts. The improvement depended strongly on the Zn-toCu ratio and was enhanced with the higher ratio. Zn/H-MOR was not as active as the bimetallic catalysts, but its presence was beneficial for maintaining high postpeak selectivity and hindering catalyst deactivation. The peak activity for H-MOR and Cu/H-MOR were also lower as compared to the bimetallic catalysts, which is very likely due to the rapid deactivation preventing the catalyst from achieving its highest possible peak activity. The temperature-programmed oxidation (TPO) of the spent catalysts (Figure 2a) confirms the beneficial effect of Zn in

spectrometer. Details can be found in the Supporting Information. Figure 1 and Table 1 illustrate the dramatic improvement of Cu/H-MOR stability in DME carbonylation upon Zn addition.

Figure 1. (a) DME conversion versus time on stream, (b) selectivity to methyl acetate versus postpeak DME conversion. Conditions: 483 K, 1 MPa CO, 48 kPa DME. Complete selectivity versus time on stream plots can be found in the Supporting Information.

Table 1. Catalyst Performance in DME Carbonylation

Figure 2. Mass spectrum signal for (a) CO2 evolution during the TPO of the spent catalysts normalized for the lifetime, (b) intact DME desorbed during the TPD of preadsorbed DME on fresh catalysts.

a

catalyst

MeOAc yield (kgMeOAc kgcat−1)

peak activity (gMeOAc kgcat−1 h−1)

lifetimeb (h)

H-MOR Cu/H-MOR 2Cu-1Zn/H-MOR 1Cu-1Zn/H-MOR 1Cu-4Zn/H-MOR Zn/H-MOR

3.57 2.51 5.53 7.12 14.24 6.00

213 206 238 246 240 217

25 20 37 41 86 45

preventing the poisoning of the catalyst. Copper-only addition to the mordenite results in a similar amount of coke formation, as evidenced by the similar areas of the CO2 peak. This agrees with the similar lifetimes of H-MOR and Cu/H-MOR (Table 1). The presence of Cu changes the chemisorption strength and/or mode of carbonaceous species, which can be removed at lower temperatures as compared to the metal-free mordenite. It was suggested that a similar decrease of the high-temperature peak for the Cu/H-MOR, compared to that of the H-MOR, occurred due to the formation of new Lewis acid sites, which bind CO and suppress hydrocarbon formation.27 When Zn is added, the overall coke formation is significantly depressed, which is concomitant with the increased lifetime. Fresh catalysts chemisorbed similar DME quantities (0.78, 0.72, 0.68, and 0.74 mol CH3/mol Al for H-MOR, Cu/H-MOR, 1Cu-4Zn/H-MOR, and Zn/H-MOR, respectively), but the adsorption strength and mode changed for bimetallic catalysts,

a

483 K, 1 MPa CO, 48 kPa DME. bTime to 10% postpeak DME conversion.

The 4:1 Zn-to-Cu catalyst’s lifetime increased 4-fold and, overall, allowed for a 6-fold higher MeOAc yield as compared to the Cu/H-MOR. The peak activity for all catalysts was similar, but Zn strongly affected MeOAc selectivity upon deactivation; that is, the 1Cu-4Zn catalyst maintained 90+% selectivity to MeOAc even at 15% postpeak DME conversion, with the value being ca. 60% for H-MOR and Cu/H-MOR 4973

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after completion of the catalytic reaction in the reductive CO atmosphere, suggesting their negligible catalytic function. Ionexchanged zinc most likely blocks unselective sites, which are responsible for selectivity decline upon deactivation, while copper facilitates the carbonylation (Figure 1) via CO activation, followed by DME adsorption, as suggested earlier.12 The presence of zinc also seems beneficial for the copper functionality because it prevents Cu’s in situ agglomeration. The nature of the copper and zinc active species, their loci, and their catalytic functions require further investigation. An example of bimetallic catalyst regeneration in a hydrogen stream at 673 K, as suggested by Becker et al. for the mordenite-based DME carbonylation catalysts,30 is shown in Figure 4. Elevated H2 pressure and temperatures as low as 573

as evidenced by the temperature-programmed desorption (TPD) tracing the mass-spectrum signal of intact DME (Figure 2b). The metal addition allows for the DME desorption in the high-temperature region without its conversion to coke intermediates, and this effect is more profound with bimetallic compositions versus Cu only. It was suggested that the reaction of surface methoxy groups with DME yielding trimethyl oxonium cations (TMO+) lead to hydrocarbon formation and catalyst deactivation.28 Although the deactivation mechanism was out of the scope of the current work and requires further investigation, our TPO and DME TPD results are in line with the proposed hypothesis on the carbonaceous-deposit formation from TMO+ because higher hard-coke formation in the case of H-MOR is concomitant with the absence of intact desorbed DME molecules in the high-temperature region. The presence of the metal(s) stabilizes DME (and/or methoxy groups) against oligomerization even at temperatures above 673 K. Zinc is known to maintain high copper dispersion in bimetallic catalysts for methanol synthesis,13−16 which was also the case in the current study. Cu dispersion in the fresh catalysts as determined via CO adsorption isotherms was 42% for the Cu/H-MOR catalyst and 62% for the 1Cu-4Zn/HMOR. More importantly, the presence of zinc prevented the copper from sintering during the catalytic reaction and ensured its high dispersion as seen from the transmission electron microscopy (TEM) of the fresh calcined and spent catalysts (Figure 3). Similar copper/copper oxide nanoparticles were reported for ion-exchanged mordenites,8,27,29 and their presence was suggested to be concomitant with the activity loss in DME carbonylation because of the active species migration from ion-exchanged positions to the external surface and agglomeration.29 No detectable nanoparticles were observed with the most efficient 1Cu-4Zn/H-MOR catalyst

Figure 4. Performance of the 1Cu-1Zn/H-MOR catalyst before and after regeneration with pure H2 at 673 K. Reaction at 483 K, 1 MPa CO, 48 kPa DME (details in the Supporting Information).

K are also known to effectively clean the surfaces of zeolite alkylation catalysts from the majority of carbonaceous deposits.31 Although the regeneration procedure warrants further optimization, along with the characterization of the regenerated catalyst, it demonstrates the possibility for integrated reaction−regeneration cycles as a means of dealing with the moderate Cu−Zn catalyst decay. The data reported herein thus shows the dramatic improvement of Cu/H-MOR catalysts in DME carbonylation with the addition of Zn. Zinc maintains Cu stability and dispersion while it inhibits the formation of unselective species and hard coke. The mechanism of the zinc effect warrants further study. With the discovery of this catalyst, the industrialization of DME-toMeOAc conversion is a far more attractive venture. The process conditions, such as temperature, pressure, weight hourly space velocity, and feed composition, should be varied to assess Zn’s promoting effect and to optimize the product yield. The concept of the zinc addition may also be attractive to the Cu/ H-MOR-catalyzed conversions of methane to methanol and methane to acetic acid,1,5,7 helping to improve Cu dispersion and/or contribute to the stabilization of its specific loci.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01464. Selectivity versus time on stream, catalyst synthesis procedure and catalyst compositions, procedure of DME carbonylation tests, regeneration study procedure, procedures of catalyst characterizations (EDX, TEM,

Figure 3. TEM images of the fresh calcined and spent catalysts (after the reactions in Table 1). The total metal loading is ca. 3 wt %. 4974

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(24) Armitage, G. G.; Sunley, J. G. U.S. Patent (BP Chemicals Limited) 8,956,588, February 17, 2015. (25) Liu, J.; Xue, H.; Huang, X.; Wu, P.; Huang, S.; Liu, S.; Shen, W. Chin. J. Catal. 2010, 31 (7), 729−738. (26) Xue, H.; Huang, X.; Ditzel, E.; Zhan, E.; Ma, M.; Shen, W. Ind. Eng. Chem. Res. 2013, 52, 11510−11515. (27) Wang, S.; Guo, W.; Zhu, L.; Wang, H.; Qiu, K.; Cen, K. J. Phys. Chem. C 2015, 119, 524−533. (28) Boronat, M.; Martinez, C.; Corma, A. Phys. Chem. Chem. Phys. 2011, 13, 2603−2612. (29) Zhan, H.; Huang, S.; Li, Y.; Lv, J.; Wang, S.; Ma, X. Catal. Sci. Technol. 2015, 5, 4378−4389. (30) Becker, E. J.; Ditzel, E. J.; Kaiser, H.; Morris, G. E.; Roberts, M. S.; Schunk, S. A.; Smit, M.; Sunley, J. G. U.S. Patent (BP Chemicals Limited) 8,329,606, December 11, 2012. (31) Josl, R.; Klingmann, R.; Traa, Y.; Glaser, R.; Weitkamp, J. Catal. Commun. 2004, 5, 239−241.

CO isotherms, TPO, DME titrations and DME TPD) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jing Shen and Dr. James Sawada for fruitful discussions and technical support with TEM and CO isotherm adsorption measurements. Financial support from the Canada Foundation for Innovation (CFI, Leaders Opportunity Fund), NSERC (CRD grant), and Enerkem is gratefully acknowledged.

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DOI: 10.1021/acscatal.6b01464 ACS Catal. 2016, 6, 4972−4975