Topics in Catalysis

, Volume 52, Issue 9, pp 1175–1181

Shape Selective Chemistries with Modified Mordenite Zeolites

Authors

    • The Dow Chemical Company
    • JMG LLC
  • Michael M. Olken
    • Dow Chemical, Chemistry & Catalysis R&D
  • G. John Lee
    • The Dow Chemical Company
  • Garry R. Meima
    • Dow Benelux N.V., Chemistry & Catalysis R&D
  • Pierre A. Jacobs
    • Laboratorium voor OppervlaktechemieKatholieke Universitat Leuven
  • Johan A. Martens
    • Laboratorium voor OppervlaktechemieKatholieke Universitat Leuven
Original Paper

DOI: 10.1007/s11244-009-9269-2

Cite this article as:
Garcés, J.M., Olken, M.M., John Lee, G. et al. Top Catal (2009) 52: 1175. doi:10.1007/s11244-009-9269-2
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Abstract

This is a review of work carried at Dow Chemical on the design and characterization of novel mordenite catalysts with a unique micro and meso-porous structure characterized by a variety of methods including, TEM, BET surface area and pore size distribution, and n-decane hydro-conversion. The catalytic performance of these catalysts in shape selective reactions is illustrated with results from n-decane hydro-conversion and applications to alkylation and trans-alkylation reactions of mono and polynuclear aromatic hydrocarbons and other chemistries.

Keywords

Mordenite3 DDM catalystMesoporosityBifunctional hydro-conversion of n-decaneAlkylationTrans-alkylation monoPolynuclear aromatics

1 Introduction

In the mid 1980s Dow R&D management challenged its catalyst community to develop a low-cost route to linear polycyclic aromatic monomers, needed for the commercial development of thermo-tropic liquid crystal polymers. These exhibit unique thermal and mechanical properties, are moldable and strong, and possess good flame resistance and low solubility in most solvents. Shape selective catalysis with acidic zeolite catalysts was proposed by the Dow’s Catalysis Lab as a potential new route to produce p,p′ dialkylated polynuclear aromatics.

Weisz and Frilette were the first to report shape selective catalysis in zeolites [1]. Csicsery [2] reviewed shape selective catalysis in zeolites, and Derouane [3] discussed shape selective catalysis with ZSM-5 zeolites. Shape-selective alkylation of naphthalene to methylnaphthalene on ZSM-5 zeolite catalysts was reported by Fraenkel et al. in 1986 [4]. A key challenge for us was how to create zeolite catalysts for a reaction that was already known to lead to coke formation and rapid catalyst deactivation [5]. Coke deposits in the pores of acidic zeolites increased with the Al framework density. Hence, the development of zeolite catalysts with low Al framework density became part of our strategy to make p,p′ dialkylated poly-nuclear aromatics. We tried to limit deactivation by reducing the concentration of the aluminum (Al) sites, and to improve diffusion of reactants and products by creation of secondary pores via chemical and thermal treatments, following, in part, the work of Beyer et al. on aluminum deficient mordenites [6]. Zeolites with 10- and 12-ring pores were selected as potential catalysts precursors. The acidic forms of these zeolites, including mordenite, were evaluated as potential catalysts in the shape selective alkylation of biphenyl C2-C4 olefins. This work at Dow Chemical pioneered the use of mordenite in alkylation of poly-nuclear aromatics [7].

The evolution of the pore structure of the mordenite catalysts was followed by BET surface area measurements, pore size distributions, and high resolution microscopy, complemented by the catalytic test reaction Bifunctional Hydro conversion of n-Decane (BHD). This test reaction can be used to explore the pore structure of zeolites by studying the cracking pattern of n-decane over Pt-loaded hydrogen-zeolites [810]. A joint effort between Dow and Prof. P.A. Jacobs and J.A. Martens, Leuven U. was set up to study the pore structure of the Dow catalysts using the BHD methods. Some of the results of that work have been previously reported [10].

2 Shape Selective Mordenite Catalysts

Weisz et al. in early work on zeolite shape-selectivity, “engineered” a Pt-loaded mordenite to hydrogenate ethylene but not propylene [11]. It is remarkable that the mordenite could be “reengineered” at Dow, to produce a catalyst for selective alkylation of biphenyl with propylene [7]. The “reengineering” of the conventional acidic mordenite catalysts into the Dow shape selective catalysts for biphenyl alkylation was a tremendous challenge.

W. H. Meier at Caltech working under Prof. L. Pauling first determined the structure of sodium mordenite, Na8Al8Si40O96.24H2O. Single crystals were provided by Prof. C. Frondel (Harvard) and Dr.Donald H. Breck (Linde Res. Lab). The remarkable stability of mordenite, the most siliceous of the natural zeolites, was associated with the abundance of 5- and 6-rings in the structure. Stacking faults are responsible for the unexpected adsorption properties of mordenite. The structure has Cmcm symmetry and exhibits 12-ring and 8-ring channels running parallel to [001] and [010] [12].

Studies by Sherman and Bennet and others [13] on the framework structures related to mordenite, gave “tentative evidence for the existence of more than one mordenite” This was later confirmed in our work when it became clear that the nature of the structural differences, probably associated with c-axis faulting, was important to select the ideal precursors to make the best mordenite catalysts [14]. A Symmetry Index, SI, defined as the ratio SI = {[111] + [241]}/[350], where the brackets are the peak heights of the corresponding reflections, was adopted as a practical number to estimate the degree of faulting in synthetic samples and also to follow the changes produced by chemical and thermal treatments on catalyst samples [14].

Dealumination was achieved by a concerted combination of chemical and thermal treatments, including exchange with 1 M HCl solutions, thermal activation between 673 and 973 K, and extraction with 6 M HNO3 solutions. These treatments were repeated as needed. Catalysts with different SAR (SiO2/Al2O3) were produced by this method. The SI increases with dealumination, and the changes in SAR and SI depend on the source of mordenite [7].

3 Characterization of Mordenite Catalysts by BHD Method

Dealumination data for commercial synthetic mordenite zeolites TSZ-620 (Tosoh) and Norton-type (NOR) are given in Table 1. Repeated dealumination results in higher SAR, BET surface area, and total pore volume. However, at higher levels of dealumination the changes tend to level out. The NOR and TSZ-620 samples exhibit differences in the evolution of BET surface area and pore volume details, in good agreement with the differences in morphology, composition and source of the crystals [7].
Table 1

Composition and porosity of mordenite and dealuminated mordenites

#

Description

SAR

BET

Micropore

Total PV

PV ratio

  

SiO2/Al2O3

m2/g

ml/g

ml/g

 

1

TSZ-620

14.8

315

0.15

0.17

0.12

2

TSZ620 3DDM1

102

432

0.18

0.26

0.31

3

TSZ620 3DDM2

181

339

0.13

0.32

0.59

4

TSZ-620 3DDM3

216

408

0.18

0.32

0.44

5

NOR

10.6

238

0.11

0.19

0.42

6

NOR 3DDM1

144

321

0.15

0.22

0.31

7

NOR 3DDM2

256

446

0.17

0.36

0.53

8

NOR 3DDM3

368

431

0.17

0.31

0.45

Details of the pore structure of a typical TSZ-620-3DDM catalyst can be seen in Fig. 1. The image shows crystalline domains of about 5 to 50 nm of highly crystalline MOR permeated by meso- and macro-pores [15].
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig1_HTML.jpg
Fig. 1

HRTEM image of TSZ-620-3DDM-3 catalyst

Analyses of the BHD results summarized in Table 2 show that the refined Constraint Index (CI°) values for both sets of mordenite decrease, in general, with dealumination, similarly to ZSM-5 and ZSM-11 zeolites [9]. However, the presence of ethyloctanes, EC8, in the range of 5–12% show that the 3-dimensional dealuminated (3DDM) samples cracking pattern is more typical of the large pore 12-ring zeolites (FAU, BEA, LTL) [8]. The 3EC8/4EC8 ratio at 5% isomerization exhibits values in the range of 0.5–0.9, which is in line with mordenite or more open zeolites such as FAU, BEA and LTL [8]. Hence the 3DDM mordenites have a very open structure with limited spatial constraints, as supported by the presence of mesopores and macropores within the crystals.
Table 2

BHD data for mordenites (1,5) and dealuminated mordenites (2-4, 6-8)

#

Description

CI°

EC8 (%)

3EC8/4EC8

PC7 (%)

2,7dMC8 (%)

iC5 (%)

C3-C7

C4-C6

DI

1

TSZ-620*

1.63

7.76

0.68

0.62

6.29*

39.88

14.39

21.08

35.47

2

TSZ620 3DDM1*

1.34

8.02

0.93

0.69

6.65

29.42

0.58

1.61

2.19

3

TSZ620 3DDM2*

1.27

7.94

0.91

0.69

5.82

21.43

2.24

1.98

4.23

4

TSZ-620 3DDM3*

2.05

12.06

0.46

1.09

6.96

58.91

3.69

4.28

7.97

5

NOR*

1.82

9.05

0.63

0.99

10.91

47.15

13.49

12.73

26.23

6

NOR 3DDM1*

1.59

5.53

0.76

0.18

7.57

59.66

5.78

14.53

20.31

7

NOR 3DDM2*

1.23

7.47

0.84

0.63

5.87

41.33

1.56

2.8

4.36

8

NOR 3DDM3*

1.24

6.94

0.88

0.60

6.13

38.83

3.21

3.72

6.93

* Activation conditions: 0.5wt% Pt impregnation; oxidation and reduction at 400 °C for 1 h

CIo Constraint index base on 2-/5-Methylnonane at 5% isomerization; EC8 Percent ethyloctanes in monobranched isodecanes at 5% isomerization; 3EC8/4EC8 3-/4- Ethyloctane ratio at 5% isomerization; 2,7 dMC8 2,7 di-Methyloctanes at 5% isomerization; PC7 Propylheptane in monobranched isomers at 75% total conversion; iC5 Moles of isopentane per 100 moles cracked at 35% cracking; Dimensionality index (ca. 15-50% cracking) DI = l(C3-C7)l-l(C4-C6)l

The yield of PC7 mono-branched isomers at 75% n-decane conversion increases with SAR for the TSZ-620 3-DDM mordenites, suggesting that more open structures are produced on dealumination. In contrast, the NOR 3-DDM samples show changes in PC7 that do not follow the SAR trend. This is a significant result because the PC7 criterion is a very sensitive tool to measure changes in effective pore size in 12-ring zeolites [9].

Values for the 2,7 dimethyloctanes at 5% isomerization given in Table 2 are very low and more typical of 12-ring 2-3-D zeolites [9]. They are also consistent with the CIo values observed for the 3-DDM samples in Table 2.

The values obtained from BHD analyses for CI, %EC8, and %PC7 and DI suggest that the parent mordenite samples are 12-ring with a 1-D channel system. After dealumination the 3DDM catalysts appear to behave more like higher dimensional (2-D/3-D) zeolites [10]. These observations provided the impetus to create the name 3DDM to describe these three-dimensional (3D) dealuminated mordenites (DM).

4 Hydrocracking Fragments in Different Mordenite Samples

In the absence of spatial constraints, ideal hydrocracking yields a symmetrical carbon fragment distribution [810]. BHD analysis of dealuminated mordenites in this work shows that TSZ-620, Table 2, produces a non-symmetrical distribution of carbon fragments at 54% cracking yield (Fig. 2). The pores have spatial constraints, typical of 1-D pore systems or high dimensional systems whose channels are encumbered with occluded material. In contrast, the carbon fragment distribution of TSZ-620-3DDM3 at 46% cracking yield (Fig. 3) is highly symmetrical and more typical of a zeolite with a 2D or 3D channel system. This change in the cracking product distribution by dealumination is assigned to the creation of large pores in the samples that relieves diffusion constrictions. This phenomenon seems to be the basis for the improved performance of the catalysts in biphenyl alkylation with propylene [7]. Notice that the PV ratio, Table 2, goes from 0.12 to 0.44 from TSZ-620 to TSZ-620 3DDM3 resulting from a more open structure, with a higher mesopore and macropore contribution to the pore volume, and with less spatial constrains as shown by the more symmetrical distribution at high cracking yields (Fig. 2, 3). NOR 3DDM3 mordenite also shows a more symmetrical carbon fragment distribution at 55% cracking yields than the NOR before dealumination at 62% carbon fragment yield (Fig. 4, 5). Moreover, the TSZ-620 3DDM3 at 46% cracking yield is more symmetrical that the NOR 3DDM3 at 62% carbon yield (Fig. 3, 5). Thus, TSZ-620 3DDM3 has a carbon fragment distribution that is more typical of a three-dimensional zeolite Y (FAU) catalyst (Fig. 6).
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig2_HTML.gif
Fig. 2

TSZ-620. Carbon fragment versus mol/100 mol cracked at 54% yield

https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig3_HTML.gif
Fig. 3

TSZ-620-DDM3. Carbon fragment versus mol/100 mol cracked at 46% yield

https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig4_HTML.gif
Fig. 4

NOR. Carbon fragment versus mol/100 mol cracked at 62% yield

https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig5_HTML.gif
Fig. 5

NOR 3DDM3. Carbon fragment versus mol/100 mol cracked at 55% yield

https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig6_HTML.gif
Fig. 6

FAU. Carbon fragment versus mol/100 mol cracked at 44% yield

5 Biphenyl Alkylation with Propylene on 3DDM Mordenite Catalyst

The alkylation of biphenyl (BP) with propylene on 3DDM mordenite catalysts [7] is in very good agreement with the BHD results discussed above. The TSZ-620 (DHM, Ref. 7) catalysts are more active and selective for biphenyl alkylation than the NOR (Zeolon 100, Ref. 7). The n-decane cracking pattern of TSZ-620 3DDM3 samples (Fig. 3) is closer to the FAU cracking pattern (Fig. 6) than the pattern for NOR 3DDM3 (Fig. 5). Also, the DHM samples do not deactivate as readily as the NOR (Zeolon 100 Ref. 7). This is well supported by the PC7 results discussed above. The high p,p′ selectivity in BP alkylation of the TSZ-620 3DDM catalyst appears to be controlled by reactions within the 12-ring micropores. Hence, mesopores are produced at the expense of defects in the parent materials resulting in localized regions containing a highly crystalline MOR lattice surrounded by mesopores, after the sequential dealumination and thermal treatments. The presence of color centers in the NOR samples and the strong interaction with BP are in contrast with the results for the TSZ-620 3DDM materials that do not show color centers [7]. This suggests that the nature of the defects in the TSZ-620 samples leads to a more open structure with more perfect nano-crystalline mordenite domains separated by meso- and macro-pores. The formation of almost exclusively meta and para alkyalted BP and the high selectivity to the p,p′ DIPB provide additional support for this argument [7]. The active sites for alkylation are located almost exclusively within the crystalline domains and not in the mesopores. Otherwise we could not account for these unique selectivities. Finally, the size of the perfect crystalline domains appears to be sufficiently large (5–50 nm) to support highly selective alkylation of polynuclear aromatics larger than BP, as is the case of diphenylether, and biphenylphenylether [16]. In addition, these catalysts show high yields of para-substitution of alkylbenzenes in alkylation with long chain alpha olefins [17]. The presence of mesopores next to the nano-crystalline domains allows for facile diffusion of reactants and products, and the low concentration of acid sites in the mesopores limits the formation of undesired products. Finally, the low concentration of active sites in the mesopores, and the small size of the nano-crystalline domains, accounts for the long life on stream and the absence of coke formation in a zeolite that at one time was the model for deactivation in alkylation reactions. The recent review by K. P. de Jong et al. [18] illustrates the key role played in modern catalysis, and to be played in the future, by the proper design of catalysts that comprise unique combinations of crystalline nano-domains and mesopores.

6 Shape Selective Alkylation of Mononuclear Aromatics

The above described successful work on the shape selective alkylation of polynuclear aromatics sparked interest in a different R&D division at Dow. At the Terneuzen site Dow operated a cumene process that was using Solid Phosphoric Acid (SPA) as catalyst. This was one of the only two catalysts used commercially for cumene production at that time (the eighties); the other being anhydrous AlCl3 used homogeneously. By far the SPA process was the most used [19].

Besides environmental issues associated with the use of these catalysts a main disadvantage of the SPA-catalyzed process was the formation of polyalkylates or so-called ‘heavies’. This resulted in a yield loss of about 5%. Typically, these heavies were used as fuel having a much lower value than cumene. Transalkylation of these polyalkylates into cumene was part of a research project at Dow Terneuzen in the late nineteen eighties to mitigate this loss. The 3-DDM zeolites proved to have superior stability over a large number of (solid) acid catalysts tested. All of these acid catalysts deactivated strongly when used with the heavies stream. After some optimization, 3-DDM was the first zeolite to be commercialized in a cumene process in an add-on transalkylation process (in 1992). The unique performance and stability was demonstrated by the fact that the undistilled heavies stream was used. Due to the high degree of dealumination in 3-DDM, external crystallite activity was largely diminished and size exclusion effects limited the further reaction of heavy components. The catalyst operated successfully for several years until Dow’s Terneuzen plant was retrofitted to an all-liquid phase zeolite process. These liquid phase processes were developed soon after the application of 3-DDM for the transalkylation step and since then the SPA- and AlCl3-based processes are becoming obsolete [20].

An interesting feature of the 3-DDM catalysts is their shape selectivity. During alkylation only para and meta diisopropylbenzene (DIPB) is formed and no ortho. At lower temperatures (<150 °C) there is a clear tendency to form only cumene and p-DIPB during alkylation of benzene with propylene. Also, when transalkylating a mixture of DIPB’s at low temperature typically no ortho conversion was observed whereas only para conversion took place. In some cases slight isomerization to meta took place leading to an increase of this component in the product.

Higher conversions of the meta and ortho isomers at lower temperatures can be obtained by adding a second zeolite that more easily converts these isomers [21] These effects are due to the varying size of the specific di-isomers and are illustrated in Fig. 7. Representative data are supplied in Table 3 [22]. This shape selective effect of 3-DDM catalysts can be utilized for the production of important diphenols such as resorcinol and hydroquinone. These chemicals are now produced from DIPB mixtures in which especially the ortho and meta isomer are difficult to separate by distillation (boiling points 203.8 and 203.2 °C, respectively).
https://static-content.springer.com/image/art%3A10.1007%2Fs11244-009-9269-2/MediaObjects/11244_2009_9269_Fig7_HTML.gif
Fig. 7

Varying size of the specific di-isomers with respect to the mordenite 12-ring channel

Table 3

Feed composition and results obtained at various temperatures using a 3-DDM catalyst with a SiO2/Al2O3 molar ratio of 156

Feed component wt%

Benzene

91.4

no change in feed composition

91.0

87.4

93.4

Propylene

8.5

8.9

12.4

6.5

Propane

0.1

0.1

0.2

0.1

B/P (molar)

5.8

5.5

3.8

7.7

Temp. (oC)

130

145

155

165

175

185

175

175

Benz. sel. (%)

71.1

74.0

78.8

86.4

91.6

94.6

91.1

94.7

C3 = sel. (%)

54.6

59.2

65.6

73.6

83.4

88.0

83.0

87.0

DIPB sel. (%)

28.5

25.0

20.1

13.7

7.9

5.1

8.6

5.1

Meta (%)

4.9

9.9

12.2

9.3

5.3

3.3

5.7

3.4

Ortho (%)

Para (%)

23.6

15.1

7.9

4.4

2.6

1.7

2.9

1.7

Full propylene conversion. WHSV = 1 h−1. (Selectivity defined on a molar basis. DIPB sel. = ∆ moles DIPB*100/∆ moles benzene). Taken from [22]

7 Detergent Alkylate

Alkylation with long chain olefins, C18–24, on TSZ-620 3DDM catalysts lead to high selectivity to monoalkylated benzene or substituted benzenes (toluene, cumene, phenol, etc.) and to high selectivity (>95%) to the two position of the olefins, with little or no isomerization. Interestingly, alkylation with small olefins, C6–10, lead to dialkylation with high selectivity to the two position of the olefins [17]. Again, little or no isomerization is observed in this case. This suggests several things:
  • Long chain alkylation takes place only in the restricted 12-rings of 3DDM catalysts and not in the mesopores.

  • Isomerization does not occur in the 12-rings because of restrictions in the transition states of the intermediates and/or absence of active sites in the mesopores.

  • The length of the crystalline domains in the 3DDM catalysts is large enough to house a benzene molecule with two short chains or a benzene molecule with a small alkyl group and a long chain. This suggests that the crystalline domains with lengths of about 5–50 nm are effective for this chemistry. This is in good agreement with TEM and adsorption data, and also in excellent agreement with the lifetime of the cumene heavies transalkylation catalyst mentioned above. Clearly, the crystalline domains are not large enough to allow formation of coke deposits.

The products of this type of alkylation can be converted into highly selective ore flotation surfactants via sulfonation of the aromatic ring [17].

8 Near or Near Linear Alkylbiphenyls

The 3DDM catalysts were also used to produce Dowtherm MX, a Dow high-temperature heat-transfer fluid, enriched in linear or near linear di(isopropyl)biphenyl containing mainly the p,p- and p,m-isomers [23]. In related chemistry, o-methylbiphenyl (o-phenyltoluene) was isomerized to p, m-methylbiphenyl used for Dow thermal fluids. The ortho isomer is very difficult to remove by other means, such as distillation.

9 Other Mordenite Catalyst Processes

Dow also developed a process to produce diphenyloxide DPO by dehydration of phenol on 3DDM catalyst [24]. Other important contributions from Dow include the use of a guard bed to adsorb oxygen impurities [25]. In addition, Dow patented the conversion of ethylbenzene plants using the AlCl3 catalyst to liquid phase alkylation plants using zeolite catalysts [26]. Dow has used the conversion of CCl4 into CO2 and HCl with high selectivity on acidic mordenite catalysts at its Pittsburgh CA plant [27].

10 Summary

The development of Dow’s 3DDM catalysts is a major contribution to a variety of important hydrocarbon chemistry and process breakthroughs and zeolite technology advancements. Among the most important topics and issues discussed in this paper are:
  • The first practical demonstration of shape selective alkylation of polynuclear aromatics with olefins in zeolite catalysts.

  • The first zeolite based liquid phase alkylation process to make cumene.

  • The first industrial zeolite process for transalkylation of cumene heavies to cumene.

  • A process for obtaining higher conversions of diisopropylbenzenes at lower temperatures during transalkylation by using a mixture of zeolites.

  • The technology for liquid phase alkylation of aromatics resulting in a change in the industry with more and more processes using liquid phase alkylation as the best route to achieve long life on stream and better selectivity.

  • Process for production of biphenyloxide via dehydration of phenol.

  • Guard bed for alkylation of aromatics with zeolites.

  • Use to the BHD method to follow structural and catalytic changes in dealuminated Mordenite zeolites.

  • Conversion of 1-D Mordenite into a 2D/3D Mordenite.

  • Use of 3DDM catalyst to produce thermal fluids.

  • Process to remove oxygenated impurities in alkylation reactors.

  • Retrofit AlCl3 EB liquid phase process into liquid phase zeolite process

Acknowledgements

The Dow authors of this paper celebrate the contributions of Prof. P.A. Jacobs to Catalysis and Zeolite Science, and in particular to various Dow supported, internal and external projects, where his input and wisdom made a difference. They also thank the Dow Chemical Company for permission to publish this manuscript. Finally we thank our colleagues at Dow and Leuven U and other institutions for their contributions to this work.

Copyright information

© Springer Science+Business Media, LLC 2009