Desilication Seminar Report

8/3/2019 Desilication Seminar Report

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Desilication: Novel Technique to Enhance Catalytic Activity


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Abstract

Catalytic activities as well as hierarchal properties of various species of zeolites used

commercially were investigated after alkali treating at various treatment conditions and

compared with parent. In some cases vide array of parameters like alkali concentrations,

temperature of bath, time of bath as well as Si/Al ratios were used to study dominating

parameter. These results were linked with original species to find out better. Various

cases were studied and finally it was inferred that Alkali treatment resulted to selective

removal of silica from catalyst framework which generated mesopores, conserving the

acidity of catalysts. The generation of mesopores created scope in reaction kinetics,

iomerizations, cracking, diffusivity and in overcoming restricted access of discriminated

species.


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1.Introduction to Zeolites and their Properties

Zeolites can be referred chemically as alumino-silicates with well organized spatial

arrangement of ions. Cronstedt was the person who discovered in 1756 the first naturallyoccurring mineral stilbite that lost a substantial amount of steam upon heating. Accordingly,

he named the materialzeolite after the Greek zeo (to boil) and lithos (stone). In 1948 the first

synthetic zeolite was successfully prepared by Richard Barrer [1]. Approximately there exist

over 40 natural and 100 synthetic zeolites, whereas many of the synthetic zeolites do not have

natural counterparts.

Compositionally, zeolites are similar to clay minerals. More specifically, both are

alumino-silicates. They differ, however, in their crystalline structure. Many clays have a

layered crystalline structure (similar to a deck of cards) and are subject to shrinking and

swelling as water is absorbed and removed between the layers. In contrast, zeolites have

a rigid, 3-dimensional crystalline structure (similar to a honeycomb) consisting of a

network of interconnected tunnels and cages. Water moves freely in and out of these

pores but the zeolite framework remains rigid. Another special aspect of this structure is

that the pore and channel sizes are nearly uniform, allowing the crystal to act as a

molecular sieve [2].

Some properties of zeolites can be studied as follows.

1.1. Ion Exchange

Ingredients of zeolite include cations (e.g., Na+, K+, or NH4+) after the synthesis. These

cations are required to balance the negative net-

charge caused by trivalent aluminum cations which

are coordinated tetrahedrally by oxygen anions. By

exposing a sodium containing zeolite to a solution

containing other cations, the sodium ions can be

exchanged by these other cations provided they are

AlSi_

Na + K+ NH4+

Fig 1. A typical Zeolitic Structure, with Si-O-Si bond.


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not excluded from the pores due to their size (including the water molecules coordinating

the respective cations).

1.2. Molecular Sieve Effect

The pore sizes of zeolites are determined by their structures and may be varied slightly by

ion-exchanging the zeolite. By this process smaller cations can be positioned in windows

making them wider. Specific cations may also be positioned on other sites than in the

windows, thus leading to even larger open windows. The window sizes determine the

accessibility of the zeolite pore system for other (e.g. organic) molecules.

1.3. Acidity

As already mentioned, protonated zeolites have acidic properties. The protons which

balance the negative charge of a zeolite framework are not strongly bound to the

framework and are able to move within the pores and react with molecules which

penetrate into the zeolite pore system. A protonated zeolite thus can act as a Bronsted

acid. Furthermore, Lewis acidity can be caused by cations within the pores.

Bronsted Acidity

Window Opening

Species larger than window

opening dimensions are

discriminated using sievingeffect.

Fig 2. Zeolite Window sizeFigure Courtesy: http://www.co2crc.com.au/images/imagelibrary/cap_diag/zeolites._adsorption_media.jpg
http://www.co2crc.com.au/images/imagelibrary/cap_diag/zeolites._adsorption_media.jpghttp://www.co2crc.com.au/images/imagelibrary/cap_diag/zeolites._adsorption_media.jpg


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Al OH Si Terminal Silanol group

Lewis Acidity

AlO+ , Al(OH)3 x H2O Metal Cations [3]

Significance of zeolites is held in their appreciable morphological characteristics. Some

of the promising properties of zeolites includes :

High surface area

Uniform micropore size

High Hydro thermal stability

Intrinsic acidity

Ability to accommodate active metal species

Introducing constraints to undesired species by molecular sieving effect ( Shape

Selectivity) [4]

Environmentally harmless

Non-corrosive

Show ease of separation from reaction mixture compared with homogeneous catalysts [5]

Due to such properties zeolites have increased attention in commercial application for

several Petrochemical and Industrial processes. But theses benefits associated with

zeolites are accompanied with some drawbacks like intraparticular diffusional

constraints, geometrical constraints, coking or carbon deposits etc. To overcome these

drawbacks or even to alter post synthesis properties some modifications are done, of

which includes desilication, dealumination and carbon templating. Theses three strategies

are widely used to improve transport properties of zeolites to overcome mentioned

constraints. A comprehensive description of former method will be followed after

briefing of all three methods.

Carbon templating: A hydrothermal zeolite synthesis is carried out in the presence of an

additional carbon source, e.g. carbon black or carbon nanofibers [6]. Extra voids are

formed due to combustion, after it is calcined in presence of this extra source of carbon


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Dealumination: It is selective removal ofaluminium from the zeolite framework, which

is an established post-synthesis treatment to stabilize zeolites [7]. Dealumination can be

done by acid treating or steam treating the zeolite. Dealumination is mainly employed to

improve mesoporosity in matrix, to improve acidity and to stabilize the catalyst.

Desilication: It is selective removal of framework silica from zeolite matrix. Usually

most of the zeolite frameworks contain a higher concentration of silicon than aluminium

and accordingly formation of extra pores can be conveniently achieved. It is known that

treatment in alkaline and even acidic medium can be used to dissolve amorphous silica

[8]. Many references proved with compared to dealumination, silica removal is

affordable if acidic properties are not to be changed. With the help of these references we

are able to conclude that desilication is opted when mesoporosity is to be altered but not

at the cost of significant acidity changes, while dealumination is opted when acidity is to

be altered [9].


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2. Desilication Methodology

In precise, desilication is achieved by treating the parent zeolite in bath of basic

chemicals (mainly alkali) with different concentrations. A generalized method can bedescribed as follows. Alkali solution is prepared in water either at room temperatures or

at elevated ones, well stirring ensures uniform distribution of alkali over solution. Then,

catalyst is introduced in this solution and stirred for several hours. This catalyst-alkali

solution is cooled, filtered, washed neutralized and dried over long duration (generally

overnight) at ambient or elevated temperatures. Then finally daughter catalyst is

exchanged with ammonium nitrate if desired [4] [9] [10] [11]. So we can thus conclude

that governing parameters which decides amount of silica to be extracted are alkali

concentration, temperature of bath and time in bath. Severity in conditions will lead to

more difference in properties.


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3. Characterization

Various characterizations includes following parameters

crystallinity, morphology, Si/Al or SAR, porosity, diffusion, acidity, microporous and

mesoporous surface area per weight of catalyst, microporous and mesoporous volume

respectively. Following table will help us to select method for particular characterization.

Characterizations can be summarized.

3.1 Hierarchal Properties

Hierarchal properties mainly include morphology of the zeolitic framework and itscrystallinity. Study of these properties of zeolites reveals crystal structure of geometry of

zeolite and crystallinity gives us information on stability of framework. Crystallinity can

be studied on X-Ray Diffraction Patterns by comparing Intensity of peaks w.r.t. 2 .

Morphological studies are done with the help of Scanning Electron Microscopic (SEM)

or Transmission Electron Microscope (TEM).

3.2 Si/Al or SAR

Silicon to Aluminum Ratio is a very important parameter to be characterized in

desilication. This ratio gives amount of Silicon per Aluminum. This ratio helps us to

decide extent of desilication. Various methods are employed for its characterization such

as X-Ray Flourescence (XRF), Inductively Coupled Plasma individually and combined

with Optical Emission Spectroscopy and Atomic Emission Spectroscopy (ICP, ICP-OES,

ICP-AES), as well as Nuclear Magnetic Resonance (NMR) as well as some other

techniques are also employed.


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3.3 Porosimetry

N2 Adsorption and Desorption isotherms are mainly used to study pore size as well as

pore size distribution. Amount of N2 desorbed is related with saturation pressure. Some

conclusions can be made from the relations such as, a vertical hystersis loop indicates

cylindrical mesopores, while horizontal hysteresis loop indicates ink bottle type

mesopoers.

3.4 Acidity

Acidity of the zeolite samples can be measured by adsorbing a basic compound such as

NH3 or Pyridine adsorption. Acidity characterizes amount of catalyst activity by giving

us information on acidic sites.

3.5 Diffusivity

Diffusivity of various probe molecules can be studied with the help of Thermo

Gravimetric Analysis, Intelligent Gravimetric Analysis, DRIFTS Diffuse Refluctance

Infrared Fourier Transform Spectroscopy.

3.6 Aluminum Characterization

Aluminum Characterization is performed with the help of Fourier Transform Infrared

Spectroscopy (FT-IR), Argon Adsorption Desorption.

3.7 Coke Characterizations

Coke characterizations are also performed to compare and measure amount of coke

formed for different species of catalysts.


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Characterizations can be summarized.

Parameter Avaliable methods

Crystallinity XRD

Morphology SEM, TEM,

Si/Al or SAR XRF, ICP, ICP-OES, ICP-AES, NMR Porosity N2 Adsorption Desorption, Ar Adsorption Desorption

Acidity NH3-TPD, Pyridine IR

microporous and

mesoporous surface area

per weight of catalyst

BET method, t-plot

Diffusivity TGA, IGA, DRIFTS

Aluminum

Characterization

FT-IR, Ar Adsorption Desorption


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4. Application to Catalysis

A) Advantages

4.1. Hydroxylation of Benzene to Phenol

Cumene process is conventionally used for hydroxylation of benzene to phenol [12].

But researchers proved that Fe-HZSM-5 was one of its alternatives [12,13]. Also for the

catalyst to be effective Fe species are prerequisite [14-16]. On the contrary, some

drawbacks accompanied by these catalysts are rapid deactivation of catalyst,

consequently leading to lower yield [17].

Main reaction involving in process is

Benzene (C6H6) + N2O Phenol (C6H6O)

Main drawbacks involving hindered activity of catalyst is limited diffusion due to

geometrical constraints. This can be overcome by introducing mesoporosity in catalyst by

treating it with 0.2 M NaOH solution (full procedure mentioned in [11]). Comparing the

results obtained for hydroxylation with the help of graphs and charts.

Fig 3 conv. of benzene temp


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a) b)

From these statistics we can come to a conclusion that after 360 min TOS yield of

phenol was 1 % for parent and 22 % for desilicated species. While selectivity didnt

changed markably for daughter while for parent, it reduced to 10 % from 70 % .

Conversion can also be studied as a function of temperature, and it is clear that it is

directly related with temperature i.e. as temperature increases conversion also increases.

Desired results can justified by following facts i) changes in active sites ii) mesopore

formation [15-17].

4.2. Benzene Alktylation [5]

Fig. 4 a) Yield of phenol TOS, b) Selectivity of phenol TOS


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This text describes that how reaction performance is enhanced after, selective removal

of silica from catalyst. Due to low micropore size, mass transport of bulky hydrocarbon

experience restricted access in the

catalyst for reaction this directly

affects the reaction

kinetics [18].

Reaction undergoing in the process

in presence of mordenite is

Benzene + Ethane (Alkane)

Ehtylbenzene

As in above case this reaction is

also affected by diffusional limitations due to geometrical constraints. So catalyst is

treated with 0.2 M NaOH (full procedure mentioned in [5]). The improved kinetics can

be studied as follows. After treating the catalyst with alkali, benzene is more likely to

reach upto intrapore active sites. Also since residence time is reduced, so further

alkylation of benzene is suppressed hence increase in Ethylbenzene yield is obvious.

4.3 Butene aromatization [23]

Mobil Oil disclosed that ZSM-5 was apt catalyst for transformation of light

hydrocarbons to aromatics due to promising properties discussed above [19-22]. But

together with these benefits, the catalyst enhances coke production which is undesired for

reaction kinetics. For treating the catalyst, it was introduced in the bath of low solutions

of different alkali concentration 0.1M (AT1) and 0.1 M (AT2). To monitor coke

formation qualitatively and quantitatively FT-IR and TG profiles were used respectively.

Reaction undergoing in presence of ZSM-5 catalyst:

Butene (C4H8) Aromatics

4.3.1 Conversion

Fig. 5 comparsion of yield and selectivity parent and treated specieswith time.


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Conversion decreased to around 93.5 % while it as not issue in case of alkali treated

catalysts after 34hr TOS which is evident form fig 6.

4.3.2. Selectivity of aromatics

Selectivity of aromatics during the reaction

is important aspect. For parent catalyst

initial selectivity was noted as 61.9 %

which decreased to 19.1 % after 34 hr TOS.

While same thing for treated samples

remained nearly constant, this can be

studied from fig 7

4.3.3 Coking

Coking was found to be decreased in

treated samples. Characterization of coke

revealed that treated samples had 149 and

147 mg/g cat. for AT1 and AT2

respectively. While same for parent was

152 mg/g cat. These evidences are

insufficient to prove that enhanced catalyst

activity is due to less coking, so it is

Fig 6 Benzene Conv. TOS

Fig 7 aromatics selectivity TOS

Fig 8 TPO of coked samples


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important to know the location of the coke in the catalyst as this will provide that extra

coke deposited in the parent catalyst was

covering the active site due to which reaction kinetics were apparent. Fig 8 helps us to

find the location of coke with TPO.

4.3. Production of propylene from methanol [27]

Propylene has many industrial applications some of which includes as a raw material

for production of polypropylene, polyacrylonitrile, acrolein and acrylic acid. Propylene is

mainly obtained as a byproduct of petroleum refining and by naphtha cracking. But, due

to increased demand of this alkane a promising process emerged known as Methanol to

Propylene (MTP) [24-26]. For this task ZSM-5 was most effective catalyst.

Main reaction undergoing in the process in presence of ZSM-5.

Methanol Propylene

0.45 M Na2CO3 solution was used as alkaline media for four different catalysts

differing in Si/Al ratio denoted as S1-S6. From fig 9 it is clear that as Si/Al increased theconversion also increased. Also, from fig. 10 selectivity of S5 is displayed. Black

triangle at topmost position represents propylene, throughout process it remains constant.

4.4 Conversion of n-Octene [30]

Fig. 9. Conv. of MeOH TOS Fig. 10. Selectivity of HC over S5 TOS


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Amount alkenes in the fuel have

become a big question for environment

so their conversion to aromatics has

become inevitable; also that octane no.

will be retained [28, 29]. Present text

describes us how n-octene is converted

in presence of ZSM-5 at different

temperatures.

4.5 Mesoporosity development in ZSM-5 [31]

Due to magnificent acidic properties ZSM-5 has attracted many industrial processes.

But, these acidic properties are of no use if reactants could not access them. Present text

will relate dependence of mesoporosity developed on couple of parameters, temperature

of bath and treatment time. Here alkali, concentration is 0.2M NaOH kept constant

throughout the treatment.

Initially let us study effect of temperature. Following results were obtained when time

was kept constant for 30 min and temperature in the range of 308-358 K at 10 K interval.

Table 4.5.1.1 Influence of temperature on hierarchal properties.

T (K) SBET a

(m2/g)

Vtotal b

(cm3/g)

Vmicro c

(cm3/g)

Smicro c

(m2/g)

Smeso c

(m2/g)

Untreated 430 0.26 0.17 390 40

308 440 0.3 0.16 385 55

318 455 0.33 0.16 385 70328 520 0.41 0.15 360 160

338 550 0.53 0.13 325 225

348 520 0.58 0.13 320 200358 495 0.68 0.13 315 180

Fig. 11. Product Selectivity of HC T


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Now, focus on time on mesoporosity development. In this temperature is kept at 338 K

for 0.2 M NaOH for 4 ranges of time.

Table 4.5.1.2 Influence of alkali bath time on hierarchal properties.

T (K) SBET a

(m2/g)

Vtotal b

(cm3/g)

Vmicro c

(cm3/g)

Smicro c

(m2/g)

Smeso c

(m2/g)

Untreated 430 0.26 0.17 390 40

15 505 0.42 0.14 325 18030 550 0.53 0.13 325 225

60 515 0.58 0.12 300 215

120 510 0.59 0.13 310 200a BET method.

b Volume adsorbed at p/p0 = 0.99.

c t-plot method.

Grey lines represent optimum mesoporous values and corresponding temperature and

time associated with it. So thus form this we can infer that controlled time and

temperature selection leads to optimum mesoporosity values.

Fig. 12. Smeso %,( full triangles) Vmicro(hollow

triangles) T (K)

Fig. 13. Smeso %,( full triangles) Vmicro(holl

triangles) t (min)


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Different parameters were changed to give best selectivity values and ZSM-12 at

optimum parameters was found to give best selectivity value of 70% ,which was recorded

highest among all the zeolites observed up to now.

4.8. Improved heavy oil conversion and diffusional properties [56].

Diffusional properties can be improved directly by introducing mesopores in the

catalyst, which also results in better catalytic results like improved conversion,

selectivity, increased TOS etc.

Properties of heavy oil feed stocks are tabled.

Table 4.8.1 Properties of heavy oil feedstock

Items Daqing heavy oil

Density (g/cm3) (293 K) 0.913

Viscosity (Pas) (323 K) 0.213

Carbon residue (wt %) 4.3

Mn (g/mol) 577

H (wt %) 12.87

C (wt %) 86.77

H/C 1.78

Saturated carbons 59.2

Fig 15 Catalyst activity of ZSM12 (left) and MCM-22 (right)


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Aromatics 29.1

Resins and asphaltenes 11.7

Mn: average molecular weight.

Various treatments with alkali solution can also be represented as follows.

Table 4.8.2 Treatment parameters

Sample Si/Al Treatment Temp Conc. of NaOH

molar ratio time (min) water bath (K) solution (mol/L)

OZ5 30

AZ5-1 29 120 333 0.25

AZ5-2 24 300 353 0.20

AZ5-3 22 300 343 0.25

Also the results obtained after treatment can be summarized in below table.

4.8.3 Hierarchal properties of catalysts.

Sample Daver SBET Smicro Smeso V p V micro Vmeso

(nm) (m2/g ) m2/g m3/g m3/g

OZ5 2.2 380 358 22 0.212 0.171 0.042

AZ5-1 2.4 383 347 36 0.231 0.167 0.064

AZ5-2 3.5 414 283 131 0.357 0.144 0.213

AZ5-3 4.0 427 252 175 0.431 0.130 0.300

Cumene molecule was taken as

probe molecule for studying

diffusional properties of different

catalysts Fig 16 helps us to compare

performance of original and treatedcatalyst.

The initial adsorption rate of cumene

on OZ5 was 48% of the maximum

adsorption amount was after 32 min.

Then, the adsorption rate declined, indicating that with the increase in the amount of

Fig 16 Adsorption amount time (min)


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cumene adsorbed, it becomes more difficult for cumene to diffuse into the micropore of

OZ5 zeolite channels. However, it was different for AZ5-3. It is clear that there were

many steps when cumene adsorbed on/into AZ5-3. At the beginning the cumene

molecules adsorbed and diffused quickly on the outside surface of AZ5-3 with a high

adsorption rate, this phenomenon was similar to that of OZ5. The cumene molecules also

diffused easily into the mesopores and/or super-micropore of AZ5-3 at a high adsorption

rate, and the amount of adsorption became higher than that on OZ5 because of the

existence of mesopores. When sufficient cumene molecules were adsorbed on the

mesopore and/or super-micropore of AZ5-3, the interaction between molecules became

dominant. Consequently, a phase transition could occur, which means, the adsorption of

cumene molecules changed from monolayer adsorption to multilayer adsorption or

condensation. At this time, due to the balance adsorption, the rapid decrease of

adsorption rate is observed. After that, the cumene molecules commenced to diffuse into

micropores via rearrangement. In this step the diffusion rate was low due to the limitation

of pore size and interaction between molecules and zeolite walls. The increased

adsorption amount can also be observed with smaller slope comparing to the very

beginning. Cumene molecules could not enter the intersection of two different size pores

and/or between straight channel and zig-zag channel until re-arrangement occurred. The

re-arrangement of molecules resulted in an increase of adsorption amount and rate.


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Table 4.8.4 Product distribution of cumene cracking

Catalyst OZ5 catalyst AZ5-3 catalyst

Yields of gas products (wt%)

Methane 0.30 0.38

Ethane 0.31 0.40

Ethene 1.01 1.04

Propene 9.75 23.52

Butene 0.58 0.88

Yields of liquid products (wt%)

Benzene 16.12 36.58

Toluene 0.18 0.23

Ethylbenzene 0.32 0.50

Styrene 0.40 0.46

Methyl-styrene 1.19 1.39

Propylbenzene 0.21 0.62

Cumene 69.29 33.65

Yields of solid products (wt%)

Coke 0.34 0.36

Cumene conversion (%) 30.71 66.35

Cumene cracking generally occurs in Bronsted acid sites, so more Bronsted acid sites

will result into more catalytic activity. But, on the contrary Pyridine-IR revealed that

there was subsequent decrease in acid sites after alkali treatment but still catalyst

performance was enhanced. This can be justified as follows, treated catalyst leads to

lower adsorption activation energy also the diffusivity of catalyst is better than parent.


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derived from Hg intrusion curve.

Actually in this case alkali treatment hardly contribute to micropore change, so

enhanced diffusion in neo-Pentane can be credited to the fact that alkaline treated species

of catalyst are successful in creating improved accessibility inside the pores as well as

shorter diffusion path length due to selective extraction of silica from zeolite framework.

4.10 Diffusivity investigation in ZSM-12 [58]

In case of ZSM-12 if comparison is done with ZSM-5 for pore window opening, slight

increase in window size for former helps to use molecules slightly larger than window

opening of latter for investigating diffusional properties and reaction kinetics [59].

Very wide arrays of parameters were used to find governing parameter for mesopore

formation which will however decide diffusional properties. These can be tabled out.

Fig 17 neo-pentane uptake time (min)


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Table 4.10.1 Influence of alkali concentration and temperature on hierarchal properties

t=30 min, Si/Al = 58

CNaOH T dSBET a dVmicrob dVmeso c

M C m2/g m cm3/g m cm3/g m

0.05 95 0 0 0

0.1 65 0 0 0

0.1 95 +12 -0.01 +0.05

0.2 35 0 0 0

0.2 65 +22 -0.02 +0.27

0.2 95 +19 -0.04 +0.31

0.4 35 +16 -0.03 +0.29

0.4 65 +17 -0.05 +0.61

Table 4.10.2 Influence of time on hierarchal properties Si/Al =58, 0.2 M NaOH at 65 C

Time dSBET a dVmicrob dVmeso c

min m2/g cm3/g m cm3/g m

30 +22 -0.02 +0.27

90 +16 -0.03 +0.34

150 +14 -0.03 +0.35

Table 4.10.3 Influence of Si/Al on hierarchal properties T=65C, t=30min

Sample CNaOH dSBET a dVmicrob dVmeso c

M m2/g cm3/g m cm3/g m

ZSM-12-31 0.1 0 0 0

ZSM-12-45 0.1 0 0 0

ZSM-12-58 0.1 0 0 0

ZSM-12-80 0.1 +19 -0.02 +0.16

ZSM-12-140 0.1 +17 -0.02 +0.18

ZSM-12-500 0.1 +24 -0.03 +0.24

ZSM-12-31 0.2 +20 -0.02 +0.17

ZSM-12-45 0.2 +24 -0.02 +0.19


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which can be considered approximately cut for gasoline, showed increase with increase

in severity in alkali concentration.

4.11.1.3 Investigations on HTI (Hydrogen Transfer Index)

C4 HTI is defined as combined yields of iso- and n- butanes over C4 alkanes and

alkenes and comprises of Hydrogen transfer activity of catalyst [62]. From Fig. 20 0.05

Fig 19 Conversion TOS(hr) at 370 C and WHSV 8 g/g hr

Fig 20 C4 HTI Conversion at 370 C and WHSV

8 g/g hr


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B) Drawbacks

4.12 Alkylation over Zeolite Beta [64]

Zeolite Beta was treated with 0.2M NaOH at wide array of treatment conditions and

their hierarchal properties can be studied for corresponding conditions. But an attention is

made while performing desilication in case of BEA zeolites is that they should be

performed under milder conditions than in MFI and MOR due to fact that framework of

fromer is less stable.

Table 4.12.1 Hierarchal properties of zeolite BEA after treatment.

Sample NaOH T t Si/Ala SBET b Smeso c Vpore Vmicro c

K min m2/g cm3/gm

B 35 615 60 0.31 0.22

B-at-1 0.2 318 10 30 680 125 0.37 0.21

B-at-2 0.2 318 60 - 675 305 0.46 0.12

B-at-3 0.2 298 30 - 650 70 0.32 0.22

B-at-4 0.2 308 30 - 595 80 0.32 0.19

B-at-5 0.2 318 30 23 695 250 0.43 0.15

B-at-6 0.2 328 30 - 705 300 0.44 0.13B-at-7 0.2 338 30 20 705 370 0.52 0.10

a ICP-OES of solid material.

b BET method.

c t-plot method.

Catalytic performance is studied with alkylation of Benzene and is compared with

desilicated ZSM-5 to compare the results. From Fig 21 it is clear that as the Beta zeolite

was treated with severe conditions the more apparent and undesired results were

obtained. These apparent results can be directly related to loss of active sites in catalysts.

Aluminum species are capable of shielding Silicon atoms in the framework, in case of

BEA type zeolites aluminum is less stable in framework implies Aluminum provides less

resistance for silicon extraction, so it was mentioned earlier that milder conditions were


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used for desilication process. So, low stability of Aluminum is responsible for decline in

catalyst activity with severity in conditions.

Fig 21 Catalytic performances of Zeolite Beta a) and ZSM-5 b)

a)b)


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5 Conclusion

Various conclusions can be made while performing selective removal of silica on

zeolites. These can be summarized.

Desilication does not affect acidic properties of catalyst, as in case of dealumination.

Also, as Aluminum density increases they are more likely to shield Si atoms, so from this

we can infer that as desilication progresses resistance for silicon extraction increases.

Also stability of Aluminum in zeolites framework is important parameter, as it is

deciding factor for conserving Bronsted Acidic sites. So catalyst activity is dependent on

acidic sites. Stable sites results in enhanced activity, while on the contrary less stable sites

lead to loss of activity and consequently apparent and undesired results are obtained.

Parameters which affect desilication are mainly temperature of bath, time of bath, alkali

concentration and Si/Al ratio. Desilication means selective removal of silicon from

zeolite framework. So as silicon is removed, the formation of mesopores results. These

formations of mesopores are likely to increase selectivity by discriminating undesired

molecules due to their geometrical properties. It also discourages coking up to some

extent and thus increases catalyst life. Formation of mesopores also in turn enhances

diffusivity in the treated samples. Desilication also plays important role in Industrial

cracking and fuel production. Because of shortened diffusion path length desired

progressive cracking is avoided because of less residence time and thus giving desired

fraction of hydrocarbons. Due to this advantage desilication has found many commercial

processes.


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[1] Herman van Bekkum, P. A. Jacobs, E. M. Flanigen, J. C. Jansen, Introduction to

zeolite science and practice, second ed., Elsevier, 2001

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