Shaping of Sulfated Zirconia Catalysts by Extrusion: Understanding

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Shaping of sulfated zirconia catalysts by extrusion: understanding the role of binders Sergey Yu. Devyatkov, Aigiza Al. Zinnurova, Atte Aho, Dennis Kronlund, Jouko Peltonen, Nikolai V. Kuzichkin, Nikolay V. Lisitsyn, and Dmitry Yu. Murzin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00820 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Industrial & Engineering Chemistry Research

Shaping of sulfated zirconia catalysts by extrusion: understanding the role of binders Sergey Yu. Devyatkov1*, Aigiza Al. Zinnurova1, Atte Aho2, Dennis Kronlund3, Jouko Peltonen3, Nikolai V. Kuzichkin1, Nikolay V. Lisitsyn1, Dmitry Yu. Murzin2** 1

St. Petersburg State Institute of Technology (Technical University), Moskovski pr. 26, 190013, St. Petersburg, Russia 2

Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process

Chemistry Centre,Åbo Akademi University, Biskopsgatan 8, FIN-20500, Turku/Åbo, Finland 3

Laboratory of Physical Chemistry, Center for Functional Materials, Åbo Akademi University Porthansgatan 3, FIN-20500 Turku, Finland *

e-mail: [email protected]

**

corresponding author: e-mail: dmurzin @abo.fi

1 ACS Paragon Plus Environment


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Abstract Sulfated zirconia hydroxide shaping by extrusion is studied based on elucidating impact of zetapotential and rheological characteristics on physical and chemical properties of technical catalysts. Zeta potential measurements of sulfated zirconium hydroxide prior and after polyvinylalcohol (PVA) additions were made. Rheological curves of sulfated zirconium hydroxide suspensions with different amounts of PVA were recorded at different zeta-potential values. Addition of PVA non-linearly decreased shear yield stress. Experimental data were quantified with the Krieger-Dougherty equation. The influence of a boehmite-type binder on the zeta-potential confirmed that the surface properties of the particles to a large extent are determined by the presence of binder. The pore structure of shaped catalysts was unaffected by rheological parameters when zeta-potential is close enough to zero. At the same time deviations of the zeta potential from the zero-value afforded more uniform pore size distribution.


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1. Introduction Heterogeneous catalysis has a wide range of applications in a large number of processes in oil refining, petro- and chemical industry. To be commercially attractive, heterogeneous catalysts should ideally demonstrate, among other characteristics, high activity, appropriate selectivity to the desired products, resistance to deactivation, stability to various mechanical stresses and optimal texture properties. In industrial processes, catalysts are very seldom utilized in a powder form, and only in few cases in the form of crushed grains. A nonregular shape of catalyst grains leads to dust formation, high pressure drop and inefficient use of the active phase. The catalyst shaping methods should be focused on production of the catalyst granules with specified physical and chemical properties, such as specific surface area, optimal pore size distribution, mechanical strength of grains and active phase distribution across a granule. The above mentioned characteristics are controlled by the active component synthesis per se and shaping steps. Not surprising the main attention of the academic research groups has been focused on the synthesis of active components, as reported for example in some of the previous studies1–5, while significantly less attention has been paid to catalyst forming or scaling-up to an industrial level 6–14. The most commonly used techniques for catalysts shaping are tableting, extrusion, spray-drying and granulation

15,16

. All of these methods have their own advantages and disadvantages.

Tableting requires that the catalyst in a powder form should have enough plasticity to be shaped. An apparent shortcoming of tableting is related to a high forming pressure resulting in granules that have low pore volume and surface area. Spray drying is usually applied when spherical particles with a size range from 10 to 100 µm are desired. Granulation is performed in rotational devices and spherical particles from 1 to 20 mm with a broad size distribution are produced.



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Extrusion is the most widely used technique having an advantage of yielding high grain porosity and easy implementation. Behavior of materials during extrusion is mainly defined by rheology, which is a science of material behavior undergoing flow deformations. In case of poor extrudability, tuning the rheological properties, done mainly by catalyst manufacturers, can make extrusion possible. Particle sizes of materials involved in extrusion usually lie in the micrometer range, which implies that colloidal chemistry plays a decisive role in molding systems made of these powders. Among the most important characteristics of colloidal systems is zeta-potential, which is defined as the electric potential at the slipping plane which separates mobile fluid from fluid that remains attached to the surface. Zeta-potential being the easiest one to measure compared to intrinsic particle surface charge or potential at the Stern plane, can be used as a characteristic of colloidal systems. In fact zeta-potential has already been linked to rheology of catalyst shaping

11,17,18

,

however, up to now no clear conclusions about an optimal zeta-potential for extrusion have been made. Many studies are devoted to the rheology of ceramics

19–21

and zeolites11,12,22,23,

unfortunately not focusing on the tensile strength and texture characteristics of final materials. There are very few studies 24,25 of impact of rheological behavior of molding masses on textural properties of extrudates. There are many catalytic masses, which cannot be shaped without the addition of a binder. A binder can substantially influence the catalyst characteristics, also changing its rheological properties. Because of distinct surface chemistry each binder behaves in its own ways when mixed with an active component. Although the scope of academic investigations in catalyst preparations is predominantly based on the active component itself, several essential properties of technical catalysts result from the presence of binders to industrial levels for monoliths

30

13,26–29

. Scale-up of catalytic materials

and pelletized materials 7,8,16,31 has been reported addressing

to some extent the influence of binders.


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Modification of shaping masses from the colloidal viewpoint is clearly controlled by zetapotential, which could be modified through addition of water-soluble polymers polyelectrolytes

34–37

, dispersants

38–41

42–44

, surfactants

32,33

,

influencing thus the rheological

properties. The latter, in a simple way, can be described as dependences of viscosity and shear stress (i.e. the component of stress coplanar with a material cross section) on the shear rate (or the rate at which a progressive shearing deformation is applied to a material). Several semiempirical models exist to describe such dependences and can be found, for example, in 45. Molding pastes have non-Newtonian flow behavior and during their movement through dye channels different velocity profiles are observed

46

, as illustrated in Figure 1, determining the

tensile strength of shaped catalysts. Obviously, in case of substantial velocity profiles in the radial direction, produced grains will have macro defects, originating from improper “layering” of the material.

Figure 1. Flow deformation profile during extrusion. The present work focuses on investigation of such important catalyst material as sulfated zirconia, which is used in a number of acid-catalyzed chemical reactions alkylation of olefins practiced on industrial level

47–51

including

52–54

. In the current article sulfated zirconia

catalyst shaping process is discussed from the viewpoint of rheology. In more detail an influence of zeta-potential, polyvinylalcohol addition and alumina binder content on the mechanical strength and textural characteristics is addressed.



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2. Experimental methods 2.1.Materials Sulfated zirconia hydroxide from MEL Chemical with sulfate content 10% (sodium content not greater than 200 ppm, rare earth metals not greater than 2000 ppm) was used as the active catalyst component. Pural SB from Sasol (sodium content not greater than 5 ppm) was used as an alumina boehmite binder source. Polyvinylalcohol (PVA supplied by Sigma Aldrich, 99+% hydrolized) was used as a surface modifying agent.

2.2.Methods The measurements of zeta-potential were performed with a Malvern Zetasizer Nano ZS. For the dispersed phase (sulfated zirconia hydroxide) a refractive index of 2.123 and an absorbance of 0.1 were used. Deionized water was used as a continuous phase, for which values of 0.8872 mPa·s for the viscosity at 25 ˚C, 1.33 for the refractive index and 78.5 for the dielectric constant were used. The ζ-potential was calculated measuring electrophoretic mobility of particles using the Henry’s equation 55 Ue =

2εζ f ( Ka ) 3η

(1)

where U e is the electrophoretic mobility, ε is the dielectric constant, ζ is the zeta potential,

f(Ka) is the Henry function, and η is viscosity. The Henry function generally has a value of between 1.5 or 1.0 55. The former value, corresponding to the Smoluchowski approximation, was used in the current work. The measurements were performed in dilute suspensions (0.2 % mass.) with different amounts of polyvinyl alcohol. Before measurements suspensions were left to equilibrate for 10 minutes. Adjustment of pH was performed using 0.1 M HCl and 0.1 M NaOH. Three measurements of ζpotential at each pH point were made and the mean value was used.


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Catalytic masses were made by mixing sulfated zirconia hydroxide with a predefined amount of boehmite (aluminium oxide hydroxide γ-AlO(OH) ) in excess of deionized water with addition of predefined amounts of polyvinyl alcohol. The suspensions were stirred for one hour, and dried thereafter. The resulting powder materials were used in rheological measurements and for shaping by extrusion. Rheograms were collected using a Bohlin-CS Rheometer with a cone-plate measuring geometry (a cone with 4° inclination) attached (CP4) with a gap set at 0.05 mm. The temperature of the measuring system was maintained at 25 °C using a temperature control system. Different catalyst powders were mixed with deionized Milli-Q purified water and sodium hydroxide (0.1 M) or hydrochloric acid (0.1 M) in order to achieve the desired solid fraction volume and pH. Rheograms were recorded with a sweep up option and viscosity and shear stress were plotted vs shear rate. Catalyst shaping was performed using a hand-operated piston extrusion device with dye channels 1.5 mm. Shaping masses were prepared from combined powders obtained as described above by adding deionized water as the dispersed phase and sodium hydroxide or nitric acid as pHmodifying agents. The crushing strength was measured with a pressure registration device connected to the system of two parallel plates according to the ASTM D6175 method. Two plates were moved towards each other using a hydraulic device, recording the pressure at which catalyst extrudates were broken. A series of 20 measurements were done giving a mean catalyst crushing strength. The surface area and pore size distribution were measured with an Autosorb Q-6 instrument. Adsorption-desorption isotherms of 27 points were registered and the multi-point Brunauer– Emmett–Teller equation was used for the adsorption isotherm to calculate the specific surface area. Pore size distributions were calculated using the Barrett-Joyner-Halenda method for the desorption branch of the isotherm. 7 ACS Paragon Plus Environment


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Distribution of the acid sites by their strength was determined by FTIR spectroscopy of adsorbed pyridine using a Shimadzu IRTracer-100 spectrometer with an attached vacuum gas cell. The catalyst samples were crushed in an agate mortar and pressed into self-supported pellets of ca. 35 mg/cm² thickness. A single catalyst pellet was placed into a gas cell, first outgassed, thereafter heated up to 450 °C and maintained at this temperature for 30 minutes. Then, the cell was cooled down to room temperature and the spectrum of the material was recorded. Pyridine was dosed by purging helium gas through a bubble saturator filled with pyridine. Step-programmed desorption of pyridine was made at 150, 250 and 350 °C with cooling to room temperature and recording spectra in between. The molar extinction coefficients for Lewis and Brønsted acid sites were taken from the literature 56. Concentration of strong acid sites is related to non-desorbed pyridine after treatment at 350 °C, concentration of medium acid sites reflects pyridine desorbed between 250-350 °C and the weak acid sites concentration corresponds to the amount of pyridine desorbed between 150-250 °C.

3. Results and discussion 3.1. Zeta-potential of sulfated zirconium hydroxide Zeta potential measurements of sulfated zirconium hydroxide prior to and after PVA additions are presented in Figure 2. As can be seen, PVA acts as a weak surface modifying agent. It is considered

57

that suspensions should have a zeta-potential larger than 20 mV or below -20 mV

to be stable with time. Additional amounts of PVA decreased zeta-potential of particles at lower pH and elevated it at higher pH, thus decreasing their stability. In suspensions containing PVA at low pH, changes in the zeta potential compared with neat sulfated zirconium hydroxide are rather low, which could indicate adsorption of polymer at the external boundary of the diffusion layer. Contrary at high pH adsorption of PVA occurs on the surface of particles counterbalancing


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their positive charge on the surface and subsequently increasing the diffusional layer thickness and zeta potential 58. Initial addition of PVA led to an increase of the isoelectric point (IEP), while a further increase had a marginal impact on the IEP, which can be probably explained by complete coverage of the surface of sulfated zirconium hydroxide with the polymer. It was demonstrated that a complete coverage of nano-sized zirconia by the polymer can be reached at ca. 1 mass % of the latter 37.

25

a)

b)

Without PVA 0.375 PVA 0.5 PVA 1 PVA 1.6 PVA

20 15

IEP 8,5

10

IEP (pH)

Zeta-potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 0 4 -5

5

6

7

8

8,0

9 7,5

pH

-10 -15 7,0 0,0

0,5

1,0

1,5

PVA (% mass)

Figure 2. a) Zeta potential measurements of sulfated zirconium hydroxide with different amounts of added PVA; b) The isoelectric point of sulfated zirconium hydroxide with PVA suspensions as a function of pH.

3.2. Rheology of sulfated zirconium hydroxide Rheological data curves of the sulfated zirconium hydroxide suspensions (50% mass in distilled water) with different amounts of PVA were recorded at different zeta-potentials (Figure 3).



30

shear stress (Pa)

shear stress (Pa)

60

ξ = 48.7 mV ξ = 14.3 mV ξ = 1.6 mV ξ = -11.4 mV ξ = -14.9 mV

b) 35

40

20

25

20

15

c)

shear stress (Pa)

ξ = 9.2 mV ξ = 3.0 mV ξ = -3.6 mV ξ = -10.1 mV ξ = -25.4 mV

a)

20


10

10

5 0 500

1000

1500

2000

0

1000

0.09 0.08 0.07 0.06 0.05 0.04

0.12

3000

0

1000

0.08

2000

3000

shear rate (s-1)

ξ = 48.7 mV ξ = 14.3 mV ξ = 1.6 mV ξ = -11.4 mV ξ = -14.9 mV

e) 0.10

visc (Pa*s)


d)

0.10

2000

shear rate (s-1)

shear rate (s-1)

0.06


f)

0.07

0.06

visc (Pa*s)

0

visc (Pa*s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.05

0.04

0.03

0.04 0.02

0.03 0.02

0.02

0.01

0.01 0.00 0

500

1000

1500

-1

shear rate (s )

2000

0.00 0

1000

2000

3000

-1

shear rate (s )

0

1000

2000

3000

-1

shear rate (s )

Figure 3. Rheological curves for sulfated zirconium hydroxide suspensions (50% mass): a, b and c – dependence of shear stress on shear rate for suspensions with 0, 0.1 and 0.2 % PVA for different zeta-potentials; d, e and f – dependence of viscosity on shear rate for suspensions with 0, 0.1 and 0.2 % PVA for different zeta-potentials.

The primary experimental data was fitted to the Herschel-Bulkley model

59

, which has more

adjustable parameters resulting in more accurate fitting of experimental data compared to also conventionally used Ostwald- de-Waele and Bingham models 60:

τ = τ 0 + k ⋅γ n η=

(2)

τ0 + k ⋅ | γ |n −1 |γ |

(3)

where τ , τ 0 and γ are shear stress, shear yield stress and shear rate; k is consistency constant and n is flow index. This model has been applied to predict rheological properties of different concentrated suspensions, such as ceramics pastes 19,28,61, clays 62,63, cements 64 and catalysts pastes 65,66. Results of the fitting of the data measured here to eq. 2 and 3 are summarized in Table 1.The shear yield stress was fitted to the following equation 18: 10 ACS Paragon Plus Environment

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τ 0 = τ 0max − k0 ⋅τ 0max ⋅ ζ 2

(4)

where τ 0max is the maximal shear yield stress at zeta-potential equal to 0; ζ is zeta-potential; k0 is a system specific constant. The results are shown in Figure 4.

0 PVA 0.1 PVA 0.2 PVA

10

8

Shear-yield stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

2

0 -60

-40

-20

0

20

40

60

Zeta-potential (mV)

Figure 4. – Dependence of shear yield stress on PVA content at different zeta-potentials. As can be seen from Figure 4 an increase of PVA content leads to a decrease of the yield stress, as a result from water-polymer interactions thus leading to more prominent slipping of particles in the suspension.

Table 1 - Rheological parameters for suspensions of sulfated zirconia hydroxide (50% mass) τ₀, Pa PVA content, % Zeta-potential, mV k n 0

0.1

9.17

3.90

0.050

0.87

3.01

9.26

0.056

0.83

-3.59

8.19

0.053

0.90

-10.01

8.50

0.043

0.93

-24.98

5.00

0.005

1.09

48.57

3.17

0.002

1.14



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2

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14.31

7.00

0.011

0.97

1.59

7.99

0.063

0.81

-11.43

7.80

0.010

0.93

-14.94

7.30

0.010

0.90

48.66

2.11

0.001

1.14

14.05

2.89

0.005

1.09

11.96

3.57

0.049

0.80

0.06

3.41

0.050

0.78

-9.94

3.10

0.007

1.00

-20.07

3.00

0.003

1.03

The obtained values have the following meaning: n – a degree of non-Newtonian character of a suspension, k – a consistency index with dimensions Pa·sn; when shear yield stress equals to 0 and n equals to 1, k becomes viscosity. When zeta-potential is driven to 0, n deviates from 1 and the suspension loses its Newtonian character. The rise in shear yield stress makes the material more solid-like, while elevation of the consistency index results in a more viscous material. It follows from experimental data that addition of PVA non-linearly decreased the shear yield stress. As a consequence, retaining of the shape by extruded green-bodies is becoming more problematic. Further investigation of the behavior of sulfated zirconium hydroxide suspensions with different PVA content was done by viscosity measurements for different amounts of the solid fraction at a 1900 s⁻¹ shear rate and at a zeta-potential of ca. 9mV. The Krieger-Dougherty equation

67

used to fit the experimental data:

ηr =

ηs  f  = 1 −  ηl  f cr 

− K ⋅ f cr

(5)


was

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where ηr is the relative viscosity, ηs is the suspension viscosity, ηl is the viscosity of liquid media, f and fcr are the solid fraction and the maximum solid load fraction respectively, K is the hydrodynamic factor. The latter parameter for spherical monodisperse particles should be 2.5, and deviations from this value are related to shape changes of the particles. Results of the fitting are shown in Figure 5 and Table 2. Table 2 - Fitting of experimental data on relative viscosity vs solid content with the KriegerDaugherty equation (eq. 5) PVA content. % K fcr 0

2.44

0.521

0.1

1.45

0.514

0.2

1.36

0.511

The hydrodynamic factor for sulfated zirconium hydroxide without PVA is close to the theoretical value (2.5) for suspensions of monodisperse particles. However, the value of the maximum volume fraction is much lower that the theoretically predicted 0.74 for the maximum close packing and somewhat lower than the values ranging from 0.64 to 0.68 reported in literature

68–70

. Even if the latter values were reported for polymer melts, theoretically these

values should be always the same for every suspension of hard spheres. Such discrepancy in the values of the maximum volume fraction could be explained by the disorders which are introduced during the flow at high shear rates and by the presence of van-der-Waals forces which are more prominent than inter-particle and hydrodynamic forces

71

. In suspensions containing

PVA a decrease of the hydrodynamic factor can be linked to particles’ de-agglomeration and formation of polymer-stabilized aggregates. However, it should be noted that such low value of the hydrodynamic factor has not been reported in the literature for systems similar to the one reported in the current work.



0 PVA 0.1 PVA 0.2 PVA

65 60 55 50

Relative viscosity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

45 40 35 30 25 20 15 10 5 0 0,35

0,40

0,45

0,50

Solid fraction (%)

Figure 5. Krieger-Dougherty model (eq. 5) fitting for suspensions of sulfated zirconium hydroxide.

3.3.Shaping of sulfated zirconium hydroxide It is well known that extrusion of technical catalysts requires presence of binders. Thus not surprisingly preliminary attempts of the authors of the current paper to shape catalysts in the absence of binders using only sulfated zirconium hydroxide even with varying PVA content, zeta-potential or the solid fraction amount in a broad range were unsuccessful. During extrusion, water was forced out from the shaping mass, leaving the solid residue behind. Thus, utilization of binders was deemed necessary to obtain stable extrudates.

3.4.Zeta-potential of sulfated zirconium hydroxide with an alumina-type binder To achieve the required plasticity and moldability, alumina of boehmite type was introduced into the composition of processing powders. The influence of boehmite addition on zeta potential is illustrated in Figure 6. As can be seen the addition of boehmite shifts the zeta-potential titration curves to higher pH and even a comparatively small amount of binder (i.e. 30%) did not lead to the original zeta-potential curve of sulfated zirconium hydroxide. It means that the surface properties of the particles in the shaping mass to a large extent are determined by the binder. 14 ACS Paragon Plus Environment

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pure sulfated Zr sulfated Zr and 0.3 Al sulfated Zr and 0.5 Al sulfated Zr and 0.7 Al pure Al

50

Zeta-potential (mV)

40 30 20 10 0 2

3

4

5

6

7

-10

8

9

10

11

12

pH

-20

Figure 6. Zeta-potential titration curves of neat sulfated zirconium hydroxide and its mixtures with different amounts of a boehmite alumina binder.

There are no data available in the literature for zeta-potentials of sulfated zirconium hydroxide and boehmite mixtures. However, there is a good agreement between the existing data on the neat pure boehmite 18,39 with the results of the present study.

3.5.Rheology of sulfated zirconium hydroxide and boehmite mixture suspensions Rheological curves of sulfated zirconium hydroxide and boehmite suspensions are plotted in Figure 7. The curves were obtained at zeta-potential ca. 9 mV.

140

0.10 0.09

Viscosity (Pa*s)

120

Without binder 0.3 binder 0.5 binder 0.7 binder

0.11

100 80 60 40

0.08 0.07 0.06 0.05 0.04 0.03

20

Experimental points Fitted

Shear yield stress (Pa)

Without binder 0.3 binder 0.5 binder 0.7 binder

160

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

5

4

0.02 0 0

500

1000

1500

Shear rate (s-1)

2000

0

500

1000

1500

2000

Shear rate (s-1)

0.0

0.2

0.4

Binder content

Figure 7. Rheological curves for suspensions with different binder content.


0.6

0.8


Data in Figure 7 clear show that addition of the binder increases the shear stress and apparent viscosity at the same shear rates in comparison with sulfated zirconium hydroxide without the binder. The influence of the additional binder on the suspension’s shear yield stress can be described in the following way 17: 



2

τ y mix =  ∑ xvi ⋅ τ yi  

(6)



i

where τ ymix is the shear yield stress of the mixed suspension, xvi is the fraction of the i-th component and τ yi is the shear yield stress of individual components. Measurements of the neat boehmite suspension shear yield stress were not performed, instead the corresponding value was obtained by fitting the experimental data to eq. 6. The calculations are presented in the far right graph of Figure 7. Rheological curves of aqueous suspensions (50% mass in distilled water) of sulfated zirconium hydroxide and boehmite at a ratio 0.7/0.3 were recorded with addition of PVA (Figure 8). Additional amounts of PVA decreased the shear yield stress with the results being in line with the data obtained previously for sulfated zirconium hydroxide. PVA as an additive used in catalyst forming is known to lower the shear yield stress and make the flow profile through round channels closer to the Poiseuille-type 72. 0.3 Al and 0 PVA 0.3 Al and 0,2 PVA 0.3 Al and 0,375 PVA

40 35

0.3 Al and 0 PVA 0.3 Al and 0,2 PVA 0.3 Al and 0,375 PVA

0,04

30

Viscosity (Pa*s)

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

25 20 15

0,03

0,02

10 0,01 5 0

500

1000

1500

2000

0

500

Shear rate (s-1)

1000

1500

Shear rate (s-1)


2000

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Figure 8. Rheological curves of suspensions with 0.7/0.3 sulfated zirconium hydroxide to boehmite ratio and different PVA amounts.

a

b

c

d

e

Figure 9. SEM images of sulfated zirconium hydroxide and boehmite particles in the ratio 0.7/0.3 at different magnification (a-d) and dynamic light scattering data (e).



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SEM images of sulfated zirconium hydroxide and boehmite mixtures in the ratio 0.7/0.3 presented in Figure 9 show that the processing powder is represented mainly by near-spherical shape particles with the size range approximately 1-10 µm, confirmed by dynamic light scattering data (Figure 9e). It makes rheological behavior of such suspensions quite smooth compared to materials with non-regularly shaped particles. Influence of an additional binder can be also described through fitting to the Krieger-Dougherty equation (eq. 5). Relative viscosities at the shear rate equal to 1900 s⁻¹ and different solid fractions were measured at initial pH (approx. 4) with the fitting results being shown in Table 3. Table 3 - Fitting of experimental data for mixed suspensions of sulfated zirconium hydroxide and boehmite to the Krieger-Daugherty equation (eq. 5) Zr to Al ratio

K

fcr

100/0

2.44

0.521

70/30

2.54

0.536

50/50

2.77

0.540

30/70

2.89

0.543

An increase in the maximum solid fraction is moderate and was still (similar as for neat sulfated zirconium hydroxide) low compared to the theoretically predicted values 68–70. This deviation can be a consequence of disturbances in the particles’ order caused by measurements conducted at a high shear rate. An increase in K was more prominent and can be explained by the formation of particle agglomerates (flocs) of different sizes according to the following equation 73:

K=

2.5 f pc

(7)

where f pc is the quotient between the volume fraction of particles in the suspension f , and the volume fraction of clusters fc (so-called secondary structures).


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Thus an increase in the ratio between sulfated zirconium hydroxide and boehmite leads to an increase of aggregated particles. It can be suggested that sulfated zirconium hydroxide and boehmite in fact form a combined flock. Such hypothesis is in line with non-linear changes of zeta potential titration curves when the ratio between the binder and sulfated zirconium hydroxide was varied. When the latter ratio increased, the zeta potential increased which should lead to a decrease in the number of flocculated particles, however, most probably some secondary aggregated structures with a larger zeta-potential are formed.

3.6.Shaping of sulfated zirconium hydroxide with boehmite binder The catalysts shown in Figure 10 were extruded at different pH, which was gradually adjusted by adding sodium hydroxide (0.1 M) from 4.1 (Figure 10, a) to 7.2 (Figure 10, f). An increased amount of defects accumulated in the green bodies corresponded to changes in the rheological properties and surface characteristics of the particles in the pastes. Thus, high shear yield stress (closer to IEP) is not favorable due to disturbances of the flow pattern through the forming channel. Designation of catalysts is given in the caption to Figure 10. The catalysts samples SZ-7 and SZ-8 (not shown in Figure 10) were shaped with addition of concentrated nitric acid (10M) in, respectively, 5 and 10% vol. from the shaping mass. Extruded bodies did not contain any macro-defects. The catalyst samples PSZ-1 and PSZ-2 were shaped analogously to SZ-7, with the only difference that additional PVA was introduced in amounts of 0.1 and 0.2 %. The dried green bodies of the two last samples were much more mechanically stable, which is a result of PVA solidification. A choice of calcination temperature and atmosphere is of high importance and influences the state of functional groups on the surface

74,75

. Preliminary experiments were done in the current

work to evaluate catalytic activity in a model reaction of hexane isomerization using shaped 19 ACS Paragon Plus Environment


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Page 20 of 37

samples calcined at different temperature. It was found, that a maximum activity of the synthesized catalysts was reached when they are calcined at 650 and 700 °C, while calcination at 600 °C gave too low conversion and selectivity. We have thus chosen 700°C as the calcination temperature as it gives pores with a somewhat larger size being beneficial for enhancing mass transfer. In the current work all samples after shaping were dried overnight in at room temperature and calcined at 700 °C for 3 hours. a)

b)

c)

d)

e)

f)

Figure 10. Extruded sulfated zirconium hydroxide catalysts with the alumina binder (30 wt%) and 0.3wt% PVA obtained at different pH – for a at 4.1 and gradually increasing for b-f up to 7.2 3.7. Crushing strength and textural characteristics Results on crushing strength measurements, specific surface area and average pore diameter are shown in Table 4. Standard error for crushing strength measurement is not higher than 0.3. Table 4 - Characteristics of the catalysts shaped at different pH


Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SZ-1

SZ-2

SZ-3

SZ-4

SZ-5

SZ-6

SZ-7

SZ-8

PSZ-1

PSZ-2

Crushing strength. N/mm²

4.3

4.5

4.5

4.0

3.8

3.7

7.6

10.1

9.5

10.3

Surface. m²/g

193

183

186

185

204

197

201

190

194

196

BET-C

73.18

62.97

72.19

75.71

76.00

81.62

36.57

47.92

74.21

73.52

Average pore diameter. nm

6.1

5.8

5.9

5.8

5.1

5.5

5.1

6.2

5.5

5.7

Total pore volume. cm³/g

0.29

0.26

0.28

0.27

0.26

0.27

0.23

0.29

0.28

0.30

For samples SZ-1 to SZ6, rheological characteristics of the shaping masses did not influence textural characteristics having an impact only on the crushing strength. Since the decrease in crushing strength is not so prominent, it can be assumed that the strength of individual contacts between the particles did not change, and only rough geometry of extrudates determines a decrease in mechanical stability. For samples SZ-7 and SZ-8 it can be assumed, that lowering pH favors smooth rheology resulting in defect-free catalyst extrudates. A decrease of pH elevated the zeta-potential, which can lead to deagglomeration of particles and an increase in the number of individual contacts between alumina and zirconia species. The last suggestions can be deduced from the pore size distribution measurements of the shaped catalysts (Figures 11). Catalysts SZ-1 – SZ-6 are characterized by bimodal distribution not present in samples SZ-7 and SZ-8. The catalyst samples PSZ-1 and PSZ-2 did not differ from SZ-7 either in crushing strength or textural characteristics. Thus, neither mechanical stability deteriorated nor additional porosity was generated after PVA during calcination.



0,10

3

3

Volume (cm /nm/g)

0,10

SZ-1 SZ-7 SZ-8

0,15

Volume (cm /nm/g)

SZ-1 SZ-2 SZ-3 SZ-4 SZ-5 SZ-6

0,15

0,05

0,00

3

4

5

6

7

8

9 10 11 12 1314151617181920

Pore diam (nm)

3

4

5

6

7

8

9 10 11 12 1314151617181920

Pore diam (nm)

200

SZ-1 SZ-7 SZ-8

SZ-1 SZ-2 SZ-3 SZ-4 SZ-5 SZ-6

150

Volume (cm3/g)

150

Volume (cm3/g)

0,05

0,00

200

100

100

50

50

0,0

0,5

1,0

0,0

0,5

Relative press.

1,0

Relative press.

Figure 11. Pore size distribution of extruded catalysts (two upper graphs) and N₂ adsorptiondesorption isotherms (two bottom graphs).

0,15

SZ-1

SZ-7

SZ-8 0,15

0,10

0,05

0,10

Volume (cc/nm/g)

Volume (cc/nm/g)

0,15

Volume (cc/nm/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

0,05

0,00

0,00

5

10

Pore diam (nm)

15

20

0,10

0,05

0,00

5

10

15

Pore diam (nm)

20

5

10

Pore diam (nm)

Figure 12. Deconvolution of pore size distribution into two peaks for SZ-1, SZ-7 and SZ-8.


15

20

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Deconvolution of pore size distribution (shown in Figure 12) for catalysts SZ-1, SZ-7 and SZ-8 into two peaks was done using Gaussian distribution (eq. 8): − 4 ln 2( x − xc ) 2

y = ybase +

w2

Ae w

(8)

π 4 ln 2

where ybase is a base line, xc is a peak center, is an area.

w

is a full width at half maximum (FWHM), A

The statistical data for the fitting are presented in Table 5.

Table 5. Results on fitting of pore size distribution curves on Fig. 12

Peak 1

Peak 2

Center Area FWHM Center Area FWHM R-Square

Value 3.670 0.041 0.373 3.929 0.229 4.642 0.992

SZ-1 Standard error Value 0 3.670 0.002 0.026 0.016 0.358 0.193 3.670 0.025 0.179 0.402 2.555 0.985

SZ-7 Standard error Value 0 3.670 0.004 0.018 0.036 0.317 0.117 4.036 0.021 0.169 0.287 1.627 0.993

SZ-8 Standard error 0 0.002 0.030 0.024 0.006 0.052

In the data fitting procedure because of bimodal nature of pore size distribution two peaks were used for deconvolution applying the ordinary least squares method in order to minimize the differences of the areas beneath the overall curve and the two fitted ones. Such procedure helped to assign the pore structures formed by sulfated zirconia and alumina species. It is assumed that sulfated zirconia is responsible for formation of a narrow peak at ca. 3.7 nm while the second peak is due to alumina. Addition of nitric acid (samples SZ-7 and SZ-8) leads to deagglomeration of alumina particles into smaller ones and formation of a narrower pore structure of alumina. It is also noticeable, that in the samples with additional nitric acid, the pore 23 ACS Paragon Plus Environment


volume generated by sulfated zirconia oxide is diminished, what can be explained by interactions of alumina with sulfated zirconia and formation of joint secondary structures. An overview of acidity measurements is presented in Figure 13 and Table 6. Only catalysts SZ1, SZ-7 and SZ-8 were investigated as the most different from each other. Additional nitric acid slightly changed Brønsted acidity, increasing concentration of the strong acid sites. A change in the Lewis acidity is more prominent with addition of nitric acid giving an increase in the total amount of Lewis acid sites. Such information can be useful in manufacturing technical catalysts with different acidity. It was shown, for example, that for alkylation of olefins the preferable catalyst is the one with the minimum Lewis acidity

76

, while for isomerization of paraffins the

Lewis acidity plays a decisive role 77. 150

SZ-1 SZ-7 SZ-8

SZ-1 SZ-7 SZ-8

60 55

Lewis acid sites (µmole/g)

Bronsted acid sites (µmole/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

100

50

0

50 45 40 35 30 25 20

Weak

Medium

Strong

Weak

Acid sites

Medium

Strong

Acid sites

Figure 13 – The distribution of acid sites in catalysts SZ-1, SZ-7 and SZ-8.

Table 6. Acidity data from pyridine adsorption by FTIR. The units are in µmol/g and mmol/m² in brackets.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acid site type

SZ-1

SZ-7

SZ-8

Weak Brønsted acid sites

6.0 (1.16)

0 (0)

0 (0)

Medium Brønsted acid sites

6.9 (1.33)

9.5 (1.91)

3.3 (0.63)

Strong Brønsted acid sites

110.0 (21.27)

123.0 (24.97)

134.0 (25.46)

Weak Lewis acid sites

58.5 (11.29)

49.1 (9.97)

49.5 (9.41)

Medium Lewis acid sites

32.6 (6.29)

40.4 (8.20)

50.9 (9.67)

Strong Lewis acid sites

23.5 (4.54)

26.1 (5.30)

35.6 (6.76)

Total Brønsted acid sites

122.9 (23.76)

132.5 (26.88)

137.3 (26.09)

Total Lewis acid sites

114.6 (22.12)

115.6 (23.47)

136.0 (25.84)



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Page 26 of 37

4. Conclusions. In this work shaping of sulfated zirconia hydroxide was considered and some parameters addressing catalysts’ physical and mechanical properties were discussed. Results obtained for the crushing strength showed that the mechanical properties are determined by rheological parameters of shaping masses only from the viewpoint of the grain defects, which are accumulated during extrusion due to presence of the moving material “layering” effects. Mechanical stability of extrudates is improved in case of regular (without defects) geometry of shaped bodies and when applying nitric acid as a peptizing agent. A subsequent decrease of pH elevated the zeta potential values and as a result the particles disintegrate increasing the number of contacts between zirconium and aluminium hydroxides. Changing zeta-potential far from zero leads to a decrease of the yield stress making the shaping mass behave more like a Newtonian-type. In some extreme cases this might lead to green bodies not capable to retain their shape, which should be taken into account while selecting the optimum zeta-potential. Results obtained for the pore structure indicate that the latter is unaffected by rheological parameters when zeta-potential is close enough to zero. Significant deviations of the zeta potential from the zero-value afforded more uniform pore size distribution. Because the presence of nitric acid changed the acidic properties of the final catalysts, in particular the Lewis acidity, a compromise between a desire to have high mechanical characteristics and specific catalyst acidity should be made. Suspensions containing sulfated zirconia hydroxide and PVA were characterized in this work demonstrating that while adding polymer to the suspension has no influence on the pore size distribution, specific surface area and crushing strength of the final catalysts, and minor impact on the isoelectric point position, there is a clear influence on the shear yield stress allowing to use PVA as an extrusion aid without significant limitations. 26 ACS Paragon Plus Environment

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Acknowledgement The work was performed as part of the state contract awarded on the basis of a grant of the Government of the Russian Federation for support of scientific research conducted under the supervision of leading scientists at Russian institutions of higher education, research institutions of State Academies of Sciences and state research centres of the Russian Federation on March 19, 2014, no. 14.Z50.31.0013. The authors are grateful to Prof. E. A. Vlasov for valuable discussions.



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