7 Mobil’s Olefin-to-Gasoline and Distillate Process (MOGD)

22.8 Summary and Outlook



obtainable. The problem in calculating equilibrium yields is that an extremely large

number of compounds is involved and very few of the required free energy data

are available. The computations can be greatly simplified by the fact that when a

group of isomers are in equilibrium with each other they can be treated as a single

compound while calculating their equilibrium with other compounds [66].

22.7.3

Technical Process



The transformation of these olefin reactions into a large-scale process requires that

a number of technical aspects be taken into account. The two main concerns for the

MOGD process are the heat control of the reaction and maximization of the yield

of either the gasoline- or the distillate/diesel-range products. The solution chosen

consists of four fixed-bed reactors – three on-line and one in the regeneration

mode – during the course of technical operation. The three on-line reactors are

operated in series with interstage cooling and condensed liquid recycle to control

the heat of reaction. The olefins feed is mixed with a gasoline recycle stream and

passed, after heating, through the three reactors. The conceptual design allows that

both the maximum gasoline as well as the maximum diesel modes be envisaged by

shifting of the reactor temperature and recycle composition. In order to generate

a gasoline-rich stream for recycle to the reactors, a fractionation is applied. The

recycle also improves the distillate/diesel selectivity [60, 66].



22.8

Summary and Outlook



Light olefins like ethene and propene are the key building blocks in the petrochemistry, with annual productions (2006 figures) of about 110 × 106 and

65 × 106 tons per annum, respectively. Almost all ethene production comes from

steam cracking of naphtha and natural gas liquids, whereas about two-third of

the propene production is obtained as a coproduct of steam cracking. The second largest source of propene, again as a coproduct, is from the FCC units of

the crude oil refineries (Figure 22.8). The future need of ethene and propene is

expected to increase, especially with respect to propene. A significant challenge

is the availability of feedstocks and whether these feedstocks can meet the future

demand, again especially regarding propene. Traditional naphtha steam crackers

and refinery FCC units cannot meet the future propene demand. This gap has to

be bridged via propane dehydrogenation and the MTO technology. In addition,

the costs of the conventional olefin feedstocks are strongly connected to the crude

oil price [44]. Finally, both lignocellulosic biomass and coal emerge as potential

feedstocks for light olefins through MTO because both can be used as raw materials

in the production of methanol [44].

Currently, several commercial MTP and MTO projects are at different levels of

development. These technologies are usually connected to the utilization of remote



707



708



22 Methanol to Olefins (MTO) and Methanol to Gasoline (MTG)



natural gas often located far from the market. Therefore, the gas must be converted

into an easily transportable product in order to be cost effective. Consequently,

the logistic aspects are important with respect to the operation of MTO and MTP

units, in particular, for the UOP/Norsk Hydro MTO process and the Lurgi MTP

technology. The overall driving forces for the realization of these new technologies

are the search for alternative, lower-cost feedstocks for the olefin production,

monetization of remote natural gas, and developments in methanol technology.

Both technologies are based on innovative catalyst systems [44]. In conclusion, a

broad variety of well-proven technologies for the production of hydrocarbons from

methanol is established; however, their future perspectives depend strongly on the

gasoline, methanol, and natural gas prices besides the logistic aspects [60].

Regarding the catalysts investigated, there is no doubt that the ZSM-5 and

SAPO-34 systems have shown to be the best catalysts so far for the MTG and

MTO processes, respectively. Although SAPO-34 deactivates faster than ZSM-5, it

is, however, regenerable and more resistant to deactivation than the isomorphous

zeolites due to the lower acid strength and acid site density [44].

Furthermore, a number of other catalysts have been tested for the MTO reaction,

including Beta-zeolite, MeAPSOs, MeAPOs, and others; however, none of them

have resulted in any commercial importance [44].

Anyway, the monetization of natural gas, crude oil, tar sand, shale bitumen,

coal, and renewable energy sources with respect to the methanol conversion will be

driven by the technology available, the fuel and light olefins demand worldwide, and

secure energy supply (among others), as outlined by G.A. Olah et al. in the recently

published monograph entitled ‘‘Beyond Oil and Gas: The Methanol Economy’’ [67].



22.9

Outlook



Even though the described catalytic systems for the MTG and MTO processes

are already well developed, there is still room for improvement, like the enhanced hydrothermal stability of SAPO-34 by ammonia treatment [68]. Completely

new microporous materials are under development, like the recently introduced

inorganic–organic hybrid materials (MOFs, COFs, ZIFs, etc.), and tailoring these

new type of porous materials toward application in catalysis (by introducing proper

acidity, thermal stability, etc.) might be one of the future challenges, not necessarily

with respect to the MTG and MTO reactions.

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Catalysis, Vol. 137, 2nd edn (eds H. van

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p. 747.

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porous Mater., 82, 257.

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Yurchak), Elsevier, Amsterdam, p. 307.



66. Tabak, S.A., Krambeck, F.J., and



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67. Olah, G.A., Goeppert, A., and

Surya Prakash, G.K. (2006) Beyond

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Wiley-VCH Verlag GmbH & Co KGaA,

Weinheim.

68. Mees, F.D.P., Martens, L.R.M., Janssen,

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44.



711



713



23

Metals in Zeolites for Oxidation Catalysis

Takashi Tatsumi



23.1

Introduction



Zeolites have long been used as solid acid catalysts. The fluid catalytic cracking

of heavy fractions of petroleum by the use of Y-type zeolites included in catalyst

matrices is the world’s largest catalytic process. Acidic zeolites have also widely

replaced mineral and Lewis acids in large-scale chemical manufacturing such

as alkylation of aromatics and the Beckmann rearrangement. Zeolites are also

endowed with the redox property by the incorporation of a variety of metals, and

this chapter deals with incorporation of metals and the resultant oxidation catalysis.

The ways to introduce heteroatoms into zeolites are classified into two categories:

heteroatoms can be introduced into the framework as well as into voids as

extraframework species. Most zeolites have an intrinsic ability to exchange cations

[1]. This ability to exchange is a result of isomorphous substitution of a cation of

trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a

consequence of this substitution, a net negative charge develops on the framework

of the zeolite, which has to be neutralized by cations present within the channels or

cages that constitute the microporous part of the crystalline zeolite. These cations

may be any of the metals, metal complexes, or alkylammonium cations. If these

cations are transition metals with redox properties, they can act as active sites for

oxidation reactions. As a pioneering work, Wacker-type reactions were catalyzed by

Y zeolite into which Pd2+ and Cu2+ were incorporated by ion exchange [2].

Research on coordination chemistry in zeolites started in 1970s and early work

was summarized by Lunsford [3]. A metal complex of the appropriate dimensions

can be encapsulated in a zeolite, being viewed as a bridge between homogeneous

and heterogeneous systems. Complexes that are smaller than the free diameters of

the channels and windows have access to the cavities. On the other hand, complexes

that are larger than the diameters of the windows must be synthesized in situ,

namely, by the adsorption of the ligands into the zeolites containing transition

metal ions or by the synthesis of the ligands in those zeolites [4–6]. Herron et al. first

referred to such zeolite guest molecules as ship-in-a-bottle complexes [7]. Cationic

complexes can be tethered to zeolites through the electrostatic interaction. However,

Zeolites and Catalysis, Synthesis, Reactions and Applications. Vol. 2.

Edited by Jiˇ´ Cejka, Avelino Corma, and Stacey Zones

rı ˇ

Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-32514-6



714



23 Metals in Zeolites for Oxidation Catalysis



it is noteworthy that ship-in-a-bottle complexes, even if they are neutral, can be

stabilized in zeolite pores [8]. This type of complex encapsulation does not require

interaction between the host zeolite and the guest complex, allowing the catalytic

properties of homogeneous complexes to be mimicked closely. Since the first report

on the synthesis of a metal phthalocyanine inside zeolite Na-Y in 1977, numerous

examples of encapsulation of metal phthalocyanine complexes have been provided.

Related heme-, polyaza-type, and N,N -bis(salicylidene)ethylenediimine (SALEN)

complexes have also been trapped in a zeolite cavity that has restricted apertures.

These are typical examples of ship-in-a-bottle complexes, being biomimetic models

for dioxygen binding, oxygenase, and photosystems [9].

The other way to introduce heterometals is their isomorphous substitution for Si

in the framework, in a similar manner to the isomorphous substitution of Al. The

heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type

and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom

precursors in the synthesis mixture. Breck has reviewed the early literature where

Ga, P, and Ge ions were potentially incorporated into a few zeolite structures via

a primary synthesis route [10]. However, until the late 1970s, exchangeable cations

and other extraframework species had been the primary focus of the researchers.

The isomorphous substitution of Ti for Si was claimed by Taramasso et al. in

1983 [11]. The resulting material has the structure of silicalite-1 (pure silica MFI)

with Ti in the framework positions and named titanium silicalite-1. or (TS-1) The

new findings including the claim that other metals can be inserted into the zeolite

lattice met with skepticism. Ione et al. predicted the probability of isomorphous

substitution of metal ion (Mn+ ) and the stability of the Mn+ position in the

tetrahedrally surrounding oxygen atoms by using the Pauling criterion [12]. On the

basis of the ratio of ionic radii ρ of the cation and anion, the value for Ti and O (ρ =

0.515) falls out of the range (ρ = 0.225–0.414) for which tetrahedral coordination is

expected [13]. The allowed cation would include only Al3 + , Mn4 + , Ge4 + , V5 + , Cr6 + ,

Si4 + , P5 + , Se6 + , and Be2 + . Presumably this type of estimate is surely effective,

which can explain the preference of B3 + for trigonal coordination and the resultant

instability of B3+ in the zeolite matrix. However, it is a very rough approximation

since the completely ionic character of T–O bond is not the case and the model

assumes that the atoms have a perfect round shape.

In this chapter, the oxidation catalysis exhibited by isolated metals incorporated

into the framework of zeolites will mainly be dealt with. TS-1 proved to be a very

good catalyst for liquid-phase oxidation of various organic compounds using H2 O2

as oxidant, and several industrial processes utilizing TS-1 are being operated [14].

Most of this chapter is devoted to titanium-containing zeolites, although zeolites

incorporating vanadium, chromium, cobalt, and tin are briefly mentioned. The

success of TS-1 has encouraged the researchers to synthesize other titanosilicates

with different zeolite structures, especially those with larger pores, since TS-1

encountered with a limitation of inapplicability to bulky molecules owing to the

medium pores of 10-ring. Table 23.1 lists the representative titanosilicates prepared

by various techniques.



23.2 Titanium-Containing Zeolites

Table 23.1



Representative titanosilicates.



Material



Structure code



Channel system



Preparation methodsa



References



TS-1

TS-2

Ti-ZSM-48

Ti-beta

TAPSO-5

Ti-ZSM-12

Ti-MOR

Ti-ITQ-7

Ti-MWW

T-YNU-2



MFI

MEL

N.A.b

*BEA

AFI

MTW

MOR

ISV

MWW

MSE



10-10

10-10

10

12-12-12

12

12

12-8-8

12-12-12

10, 10

12-10-10



HTS

HTS

HTS

HTS, F – , DGC

HTS

HTS

PS

HTS

HTS, PS

PS



[10]

[13]

[14]

[15–17]

[18]

[19]

[20]

[21, 22]

[23, 24]

[25]



a HTS, hydrothermal synthesis in alkali media; dry gel conversion (DGC); PS, postsynthesis; F− ,

fluoride media method.

b Not assigned.



23.2

Titanium-Containing Zeolites

23.2.1

TS-1



A very comprehensive review was made on TS-1 and other titanium-containing

molecular sieves [26]. TS-1 was synthesized by the hydrothermal crystallization

of a gel obtained from Si(OC2 H5 )4 and Ti(OC2 H5 )4 [11, 27] (Enichem method,

hereafter named method A). The incorporation of Ti into the framework of MFI

structure was demonstrated by the increase in unit-cell size in XRD pattern, as

shown in Figure 23.1 [28], and the appearance of tetrahedral Ti species in the

UV–vis spectra. The maximum amount of Ti that can be accommodated in the

framework positions is claimed to be limited to x = Ti/(Ti+Si) of 0.025.

TS-1 is capable of serving as a highly efficient catalyst for the oxidation of various

organic substrates, for example, alkanes, alkenes, alcohols, and aromatics, with

H2 O2 as an oxidant under mild conditions [15, 29–32]. The epoxidation reaction

catalyzed by TS-1 may be performed under mild conditions in dilute aqueous

or methanolic solution. The active oxygen content of H2 O2 , 47 wt% (16/34), is

much higher than that of organic peracids and hydroperoxides; water is the only

co-product. Besides epoxidation, TS-1 catalyzes a broad range of oxidation reactions

with hydrogen peroxide as the oxidant, as shown in Table 23.2.

It is to be noted that, in the presence of alkalis, extraframework Ti species are

formed, giving rise to inferior catalytic properties [15]. It has been widely believed

that the presence of an alkali metal, even in very small amounts, cramps the activity

of TS-1 for the oxidations with H2 O2 by preventing the insertion of titanium into

the silicalite framework. However, Khouw and Davis reported that the presence



715



23 Metals in Zeolites for Oxidation Catalysis

20.00



20.14



19.95



Axis b (Å)



Axis a (Å)



20.16



20.12

20.10

20.08

0.000



0.010

0.005



0.015



Unit-cell volume V (Å3)



13.40

13.38

13.36



(c)



0.010

0.005



0.020



0.005

0.015

0.025

Atomic ratio Ti/(Ti + Si)

(d)



0.020

0.015



0.025



Atomic ratio Ti/(Ti + Si)



(b)



13.42



0.010



19.95



0.025



13.44



0.000



19.90



19.80

0.000



0.020



Atomic ratio Ti/(Ti + Si)



(a)



Axis c (Å)



716



5400

5380

5360

5340

5320

0.000



0.010

0.020

0.005

0.015

0.025

Atomic ratio Ti/(Ti +Si)



Figure 23.1 The increase in unit-cell size against the incorporation of Ti into the framework of MFI structure [26].

Table 23.2



Catalytic chemistry with TS-1.



Substrate



Product



Olefins

Olefins and methanol

Diolefins

Phenol

Benzene

Paraffins

Primary alcohols

Secondary alcohols

Ammoximation of cyclohexanone

N,N-Dialkylamines

Thioethers



Epoxides

Glycol monomethyl ethers

Monoepoxides

Hydroquinone and catechol

Phenol

Alcohols and ketones

Aldehydes

Ketones

Oximes

N,N-Dialkylhydroxylamines

Sulfoxides



of alkali-metal ion in the preformed TS-1 does not have any significant effect on

the activity [33]; although neither sodium-exchanged TS-1 nor TS-1 synthesized in

the presence of high alkali-metal concentrations (Si/Na < 20) is active for alkane

oxidation, the catalytic activity can be restored by washing the solid with acid

solution. The restoration of the activity may be ascribed to the conversion of the

Na-exchanged TS-1 into its original form as shown in Scheme 23.1 No satisfactory

explanation has yet been offered for the lack of activity of sodium-exchanged

Ti species. If this acid treatment is generally applicable, it will be useful for



23.2 Titanium-Containing Zeolites

Si



Si



O− O

Na+



H2SO4



O



Ti

O



O



Si



Scheme 23.1



H



Si

NaOH



O



O



Si



Si



Si

O

Ti

O

Si



Possible interconversion between TS-1 and Na-exchanged TS-1.



synthesizing a variety of titanium silicate structures that require the presence of

alkali-metal ions for their crystallization.

The catalytic properties of TS-1 depend on the lattice Ti content, which is usually

less than 2 wt% [34, 35]. The effective way to increase the Ti content in the

framework of TS-1 is still a big challenge. Thangaraj and Sivasanker reported

that eight Ti ions could be incorporated in the lattice sites per unit cell (Si/Ti =

about 10) by an improved method (method B) in which titanium tetra-n-butoxide

was first dissolved in isopropyl alcohol before addition to the aqueous solution of

hydrolyzed tetraethyl orthosilicate for the purpose of avoiding the formation of TiO2

precipitate by reducing the hydrolysis rate of the alkoxide [36], but Schuchardt and

his coworkers could not reproduce it, and found that there was no difference in the

framework Ti content between the samples synthesized by the methods A and B [37].

To synthesize Ti-rich TS-1, it is necessary and helpful to make its crystallization

mechanism clear. However, very few reports have devoted to the study of this

subject [38]. The crystallization process of titanosilicates is much more complex

than that of aluminosilicates, because Ti4+ has a weak structure-directing role

and is much more difficult to be incorporated into the framework than Al3+ .

Isomorphous substitution of metal atoms for Si in zeolites is not only related

to zeolite structures/framework composition flexibility and the chemical nature

of metals but also strongly influenced by the crystallization mechanism. The

framework composition flexibility of zeolites is chemically important. Ti K-edge

EXAFS studies have shown that the Ti–O bond length of tetrahedral Ti(OSi)4

˚

˚

species is about 1.80 A in contrast to 1.61 A for Si–O [39, 40]. The Ti–O bond is

much longer than the Si–O bond, probably making the local structure around Ti

seriously distorted. This results in the slow inclusion of Ti into the framework,

compared to Si ions. If crystallization proceeded too fast, Ti ions would not have

enough time to be incorporated into the lattice. However, crystallization that is too

slow would possibly lead to the formation of transition metal oxides, preventing

metal cations from being incorporated into the framework. In addition, the difficult

crystallization may also result from the strong competition between the interaction

of soluble silicate ions and mother liquor and the condensation of silicate ions.

Thirdly, a mismatch among hydrolysis of Ti and Si alkoxides, polymerization

of Ti4+ and/or Si4+ ions, nucleation, and crystal growth would lead to much

difficulty in the inclusion of Ti in the framework. Since the chemical nature of Ti

and the rigidity of the framework of TS-1 cannot be altered, finding an effective

crystallization-mediating agent would be the sole way to increase the lattice Ti

content in TS-1 by harmonizing the hydrolysis rate of Ti alkoxide with that of

silicate species as well as the nucleation and crystal growth rates.



717