Catalyst preparation science and engineering

Catalyst Preparation
Science and Engineering

John Regalbuto

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Catalyst Preparation: Science and Engineering

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Improving the effectiveness of catalysts is the best way to ensure cleaner, more efficient industrial processes for a wide range of applications. Catalyst Preparation: Science and Engineering explores the optimization of catalytic materials through traditional and novel methods of catalyst preparation, characterization, and monitoring on laboratory and industrial scales.

The book presents many key principles of heterogeneous catalyst preparation and the methods used to synthesize a catalyst with a particular composition and morphology. The first chapters examine the synthesis of bulk materials including amorphous and mesoporous oxide supports, heteropolyacids, and colloidal metals. Subsequent chapters focus on the syntheses of heterogeneous nanoscale materials, including those based on metal complex–substrate interactions and those using non-interacting precursors via viscous drying. The final chapters concentrate on pretreatment, drying, and finishing effects before concluding with a prognosis on future applications involving catalyst preparation and the technological advances necessary for continued progress.

An ideal companion for scientists exploring the preparation of application-specific catalysts based on desired catalytic properties, Catalyst Preparation: Science and Engineering provides a balanced overview of important synthesis parameters to consider for good catalyst design.

Table of Contents

Synthesis of bulk materials. Synthesis of heterogeneous catalysts. Catalyst finishing.

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Catalyst Preparation

Polymerization and catalyst preparations are carried out under nitrogen using capped pressure vessels fitted with a Buna N rubber, self-sealing liner that has been extracted with benzene for three days and dried.

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Catalyst Preparation

Julian R.H. Ross , in Contemporary Catalysis , 2019

4.4.2.1 Precipitation of single cations

Before considering coprecipitation, we will discuss briefly the precipitation of individual ions. The solubility of a compound, A + B − , is a function only of the temperature and is determined by the solubility product, K_s, a thermodynamic quantity:

where a A + and a B − are the solubilities of the cation and anion, respectively. (For our purposes, B − is often the hydroxide ion, OH − ). For hydroxide compounds containing multicharged cations, the expressions for the solubility product are more complex; as an example, the solubility product for aluminum hydroxide, (Al(OH)₃), is given by

Table 4.2 shows the values of K_s for a number of metal hydroxides commonly encountered in catalyst preparation . All of the hydroxides are very insoluble and so we can conclude that if OH − ions are added to a solution containing one of the metal ions (cations) shown, there will be effectively 100% precipitation of the corresponding hydroxide. Further, we can make an estimate of the pH at which precipitation each of these hydroxide species will occur when the OH − ions are added. 24 For example, if we start with a 1 M solution of aluminum nitrate, Al(NO₃)₃, the Al 3+ starting concentration is 1.0 and so precipitation will start when the OH − concentration is such that K s = 3 × 10 − 34 = a Al 3 + ⋅ ( a OH − ) 3 . In other words, a OH − = 6.7 × 10 − 12 . In water, K w = a H + ⋅ a OH − = 10 − 14 ( i .e . , pH + pOH = 14 ) and so this OH − ion concentration corresponds to a pH of about 2.83.

Table 4.2 . Values of the solubility product (K_sp) for some common hydroxide compounds at 298K

Compound	Formula	Ksp (298K)
Aluminum hydroxide	Al(OH)3	3×10 −34
Cobalt hydroxide	Co(OH)2	5.92×10 −15
Copper (II) hydroxide	Cu(OH)2	4.8×10 −20
Iron (II) hydroxide	Fe(OH)2	4.87×10 −17
Iron (III) hydroxide	Fe(OH)3	2.79×10 −39
Magnesium hydroxide	Mg(OH)2	5.61×10 −12
Nickel hydroxide	Ni(OH)2	5.48×10 −16
Zinc hydroxide	Zn(OH)2	3×10 −17

If we now consider the case for a 1 M solution of Ni(OH)₂, we find that the pH at which precipitation will occur is about 6.37. In other words, if hydroxide ions are added to a mixture of Ni and Al ions, the aluminum species should precipitate at a much lower pH than should the nickel species. As we shall see in the following section (4.4.2) , this is not the case.

Preparation of Catalysts VII

P. Atanasova , . M. Hampden-Smith , in Studies in Surface Science and Catalysis , 1998

1 INTRODUCTION

Catalyst preparation by means of chemical vapor deposition (CVD) can be conducted by vaporizing a suitable precursor and adsorbing it on the support material. 1 Subsequently, as a result of a surface reaction with or without a co-reactant, the adsorbate is transformed to the catalytically active species. The key to controlling the metal’s dispersion is the understanding of the relationship between the precursor properties and surface reactivity. The formation of highly dispersed metal clusters can be achieved by controlling the surface concentration and reactivity of the adsorption centers and the deposition parameters such as reactivity of the CVD precursor, precursor partial pressure and deposition temperature.

Different approaches for chemical vapor deposition (decomposition) have been applied for catalysts fabrication: the precursor molecule may be decomposed to the final species at the temperature of the adsorption of the precursor or only after additional heating to a higher temperature or reaction with a gaseous co-reactant; these processes have been performed under inert, reductive, or oxidative gas atmospheres. 1 Recent results on the preparation of catalysts by CVD have shown that this approach has several advantages when compared with liquid-phase routes. 1-15 In some cases the traditional steps in catalyst preparation such as impregnation, drying, high temperature calcination and/or reduction, which critically affect catalyst performance, can be eliminated when CVD is used; the reduction and activation of the catalysts can be simultaneously carried out in the reactor before the catalytic run. 2-5

To our knowledge, when CVD has been used for applications relevant to catalysis: preparation of monometallic catalysts — Mo/Al₂O₃ 6 , Ni/Al₂O₃ 7 , 8 and zeolite-supported Pt catalysts; 3-5 9-11 preparation of multicomponent catalytic systems; 12 it has been carried out either by equilibrium adsorption or by flowing the metal precursor vapors through a heated bed containing a particular support, or in a few cases in a fluidized bed reactor. 1 A number of specific examples exist for the benefits of catalyst preparation by CVD. 1-15 . Successful application of CVD involves modification of the support properties by depositing a thin layer of oxide thus changing the surface acidity and/or modifying the pore structure and thereby influencing the catalytic properties. 10 , 13 · CVD of Pt on molecular sieves 2-4 , 8 as well as other porous supports 5 has produced excellent aromatization catalysts with much higher catalytic activity and longer life than catalysts prepared by the usual routes of ion-exchange or liquid impregnation.

There are several areas of application, such as catalytic reforming, hydrocarbon processing and automotive catalysts, where noble-metal-supported catalysts prepared by conventional liquid-phase routes were applied and their performance thoroughly analyzed. The demand for development of novel catalysts with unique characteristics such as high activity and selectivity has led to a fundamental investigations of new methods (CVD, in particular) for preparation of catalysts with uniform composition and controlled morphology. In the studies of C. Dossi and co-workers 2 , 4 , 14 , 15 on the application of CVD for catalyst preparation, the research has been focused on Pt- and Rh-based zeolite-supported catalysts. It has been demonstrated that CVD routes to the preparation of catalysts offer the possibility of formation of an extremely wide variety of catalytic structures and materials; however systematic studies are necessary to clarify the parameters of the CVD processes that lead to the desired type of product.

Among noble-metal-supported catalysts, palladium-based mono- and bi-metallic catalysts supported on Al₂O₃, SiO₂, TiO₂, ZrO₂ and CeO₂ are some of the most active catalysts for hydrogenation of unsaturated hydrocarbons 16 and CO, 17 ethylene oxidation, 18 , 19 hydrogenation of aromatic compounds 20-23 as well as for the total oxidation of methane. 24-27 The effort in the present study is focused on CVD of Pd onto high surface area silica and alumina supports, and on developing an understanding between the CVD process parameters (precursor sublimation temperature, deposition temperature, reduction temperature) and the dispersion, composition and uniformity of the deposited metal clusters onto model high-surface-area supports.

Science and Technology in Catalysis 2006

2 Experimental

Typical catalyst preparation , catalyst screening and characterization method employed in this study have been described in elsewhere [5] . Selective oxidation of p-xylene was carried over the temperature range of 450-590°C at an atmospheric pressure. The feed composition was: р-xylene/Air = 0.5/99.95 (%). Reactants and products were analyzed with an on-line GC. The products identified with GC were terephthaldehyde (TPAL), p-tolualdehyde (PTAL), CO₂, CO and benzo-tolualdehyde observed as a trace in most cases. A few unknown products were also absorbed depending on catalyst.

Alcohol Fuel Cells

16.2.2 Colloidal Method

This technique of catalyst preparation was firstly used by Bonneman [15] and later by others [11,15,21] , wherein a colloid precursor is first synthesized under an inert atmosphere from anhydrous metallic salts in an organic solvent in the presence of a suitable surfactant, followed by the chemical reduction of the mixture. The colloid method is considered as better and more suitable as compared to the impregnation method, particularly for the synthesis of polymetallic system of perfect composition [15] . Several research papers on the colloidal method are available in the literature [15,21,27–29] .

A Review of Preparation Methods for Supported Metal Catalysts

Bahareh A.T. Mehrabadi , . John R. Regalbuto , in Advances in Catalysis , 2017

2 Literature Study of Synthesis Efficacy

Despite the large number of publications and patents about catalyst preparation , the field of catalyst preparation method can be still considered too much an empirical art and not sufficiently a science, as different methods show different particle sizes and wide ranges of size distributions. This is borne out by a comprehensive review of the literature to examine the efficacy of the most important techniques for catalyst preparation which have been used or developed over the recent past. To this end, the common catalyst metal, platinum, on the most common supports, alumina (Al ₂O₃), silica (SiO₂), titania (TiO₂), and carbon (C) have been selected for the review to limit the number of papers surveyed to a tractable number (about 1500). The preparation methods and particle sizes, where available, were culled from these papers. For the purpose of this review, metal particle size will be considered the chief metric of synthesis efficacy.

Since the number of papers surveyed is far too many to list in the references, we have placed all papers that were analyzed only for method and particle size in Supplementary Information, grouped according to the support.

2.1 Method Surveys

Methods of catalyst preparation are very diverse, and each catalyst may be produced via different routes. The different preparation methods for the synthesis of supported Pt catalysts are summarized in Fig. 5 for Pt/SiO₂ [74 references], Pt/Al₂O₃ [212 references], Pt/TiO₂ [376 references], and Pt/C [853 references] catalysts as reported in the literature from 2014 to 2017. Impregnation is separated into simple impregnation with a single metal, and co- and sequential impregnation for multiple metals. While not often employed, SEA and CEDI are included in the figure as they will be contrasted with the other methods in the subsequent sections of this chapter. Other methods include sol gel, microemulsion, reverse microemulsion, wet chemistry, ion exchange, chemical vapor deposition, electroless deposition (ED), electrodeposition, physical deposition, photodeposition, atomic layer deposition, dendrimer-encapsulated metal, aerosol spray pyrolysis, low-temperature electrostatic self-assembly method, cool sputtering, electron beam evaporation, simple direct adsorption, extractive-pyrolytic method, fluoride-induced self-transformation approach, evaporative-crystallization deposition, arc plasma deposition, liquid-phase synthesis, supercritical fluid reactive deposition, sol immobilization, and extractive-pyrolytic method. These results show that the various modes of impregnation are the most prevalent methods for preparation of Pt on SiO₂, Al₂O₃, and TiO₂ and made up 49%, 72%, and 22% of the preparations reported in 2014 and 27%, 76%, and 9% in 2017. For Pt on carbon, in addition to the impregnation method, colloidal and reductive deposition methods are used more frequently; these stem from the electrochemistry literature and arise from the need for relatively high metal loadings. Also of note is that alumina employs the highest fraction of impregnation preparations. This is likely because of the effectiveness of DI with alumina. Alumina is often impregnated with chloroplatinic acid (CPA, H₂PtCl₆). When dissolved in solution, CPA gives anionic Pt hexachloride, [PtCl₆] 2 − , and two protons, which can charge the alumina surface and, in doing so, create electrostatic attraction between the anionic metal precursor and the protonated, positively charged alumina surface (21) .

Fig. 5 . The methods of preparation for Pt catalysts reported in the literature from 2014 to 2017 for (A) silica, (B) alumina, (C) titania, and (D) carbon.

2.2 Particle Size Ranges

One of the most widely used metrics of the efficiency of catalyst preparation is metal “dispersion,” defined as the ratio of exposed metal surface atoms to the total number of metal atoms. Dispersion has a reciprocal relation with particle size: in 1 nm particles, virtually 100% of the metal atoms are exposed, at 2 nm, about 50% are exposed, at 3 nm, dispersion is about 33%, and at 5 nm, dispersion is about 20% (22) . It is of interest to compare the particle size and size distributions which were obtained by different catalyst preparation methods to see which methods can obtain the smallest particle size and narrowest size distribution. To this purpose, we have summarized the average Pt particle sizes, averaged over all papers for a given year and support, which obtained by each method over the last 4 years. Fig. 6 A–D shows the average Pt particle sizes and size ranges for Pt/SiO₂, Pt/Al₂O₃, Pt/TiO₂, and Pt/C catalysts. The Pt particle size standard deviations obtained by different methods, again averaged for all papers for a given year and support, are also shown in Fig. 6 A–D as error bars. The standard deviations for most years and most supports are larger than the particle sizes for most methods, with the exception of SEA and CEDI.

Fig. 6 . The average particle size and size distribution calculated for Pt catalysts for different preparation methods reported in the literature from 2014 to 2017, for (A) silica, (B) alumina, (C) titania, and (D) carbon.

The histograms show that the preparation of Pt on different supports by impregnation, reductive deposition, and colloidal methods gives larger particle size with larger size distributions. As an example, the Pt particle size obtained by the impregnation method in different references has been reported widely different (i.e., as small as 1 nm and as big as 20 nm). In contrast, the limited number of SEA papers used for Fig. 6 suggests that tight size distributions are achieved by the SEA method. The average particle size of the catalyst that was prepared by SEA is 1.5 nm for carbon, 1.8 nm for silica, and 2.9 nm for alumina. These are much smaller particles than those prepared by DI, 10.3, 10, and 10 nm, respectively. This reveals that the development of supported catalysts by the SEA method is a promising new approach that, in principle, allows for much better control of the active metal phase during catalyst synthesis. Among the most exciting prospects are the ability to tightly control the particle size distribution of supported metal catalysts, with averages in the 1–3 nm or even subnanometer range.

The histograms also show that the standard deviations from the average particle size for the SEA method are much smaller in comparison to other methods. The large standard deviations in the other methods suggest that these methods do not have great control over particle size and size distributions and have resulted in widely different and sometimes contradicting results.

Poly(vinyl ethers)

a Polymerization procedure

Polymerization and catalyst preparations are carried out under nitrogen using capped pressure vessels fitted with a Buna N rubber, self-sealing liner that has been extracted with benzene for three days and dried. Hypodermic equipment is used for evacuations and nitrogen reagent addition. Polymerizations are run with 10 gm of monomer. In general, nonvolatile components such as diluent are charged into the pressure bottle, the free space swept out with nitrogen, and the bottle capped using a self-sealing liner. Air is further removed by evacuating the system through a 20-gauge needle with an oil pump for 1 min (for a 250 ml vessel), the system is nitrogen-pressured to 15 psi, evacuated again for 1 min, and then either repressured with nitrogen to 15 psi or charged with a volatile monomer, such as vinyl methyl ether. Other low-boiling ingredients are then injected. Next, an organoaluminum compound, referred to as activator, is added and the pressure bottle is placed on a rotating rack in a 30°C water bath for about 1 2 to 1 hr. At this time the second catalyst component, usually the transition metal component, is added to initiate the polymerization.

Adsorption and its Applications in Industry and Environmental Protection

6 CONCLUSIONS

Adsorption control in catalyst preparation can be achieved from both liquid and gas phase once the necessary conditions for the strong interaction between precursor and support have been created. This review has focused on the atomic layer epitaxy (ALE) method where the gas-solid reactions of precursors are directed to the strong interaction of covalent bond formation. In ALE, surface saturation is systematically utilized, providing the means for precise control of metal density and rendering the method truly adsorption controlled.

The simultaneous presence of covalent bond formation and surface saturation has been shown to provide good homogeneity of metal concentration throughout the particles, excellent reproducibility of the process as evaluated on both macroscopic and atomic scale, and feasible scale-up. The nanotechnological approach, which is dominating materials science today, is demonstrated in the ALE method, which can produce atomically controlled structures even on high surface areas. The build-up of structures in nano-scale, not only with one component but with multicomponents, is already a reality.

New Developments and Application in Chemical Reaction Engineering

Shinya Hodoshima , . Yasukazu Saito , in Studies in Surface Science and Catalysis , 2006

2.1 Catalyst preparation

Prior to the catalyst preparation , a base-pretreatment toward activated carbon (KOH-activation, BET specific surface area: 3100 m 2 / g, average pore size: 2.0 nm, Kansai Netsukagaku Co. Ltd.) by immersing an aqueous solution of NaOH (pH 12,14) for 24 h was carried out in order to promote the anion exchange between the ligand chloride of impregnated metal precursers (K₂PtC1₄) and the aqueous hydroxide ion inside the micropores of activated carbon [1-5] . A carbon-supported platinum catalyst (Pt / C, 5 wt-metal%) was prepared by an impregnation method [1-5] . Carbon-supported platinum-tungsten (Pt-W / C, 5 wt-Pt%, mixed molar ratio of Pt / W: 5) [1-3] and platinum-rhenium (Pt-Re / C, 5 wt-Pt%, mixed molar ratio of Pt / Re: 4) [3,5] composite catalysts were prepared by a dry-migradon method [5] .

Scientific Bases for the Preparation of Heterogeneous Catalysts

Nicola Pernicone , . Francesco Pinna , in Studies in Surface Science and Catalysis , 2010

4 Conclusions

Mechanical strength is a property of utmost importance for the industrial use of heterogeneous catalysts. Abrasion resistance and radial crush strength (for pellets) and attrition resistance (for powders) should be routinely measured for quality control of industrial catalysts before reactor loading or better before catalyst purchasing. Such measurements can be conveniently performed using the respective ASTM standard methods, whose possible improvements are suggested.

Many variables of the catalyst preparation procedure can influence the mechanical properties of the final catalyst. It is remarked that this R-D step must be performed during the scale-up of catalyst production at the pilot scale. It is shown that the mechanical strength of the following catalysts can be improved in the following ways:

Ammonia (oxide-promoted magnetite)	Prerounding
PTA (Pd on active carbon)	Support prerounding
Styrene (Fe-K-Ca-Ce-Mo oxides)	Optimizing calcination temperature
Formaldehyde (Fe-Mo oxides)	Optimizing calcination temperature
Methanol (Cu-Zn-Al oxides)	Dryness before tableting
Fluid bed aluminas	Fluorination

Finally, it is stressed that the mechanical properties of the fresh catalyst are completely useless when the catalyst has to be activated in the industrial reactor [15] . The activation conditions must of course be optimized to get a high mechanical strength of the working catalyst, which should be tested after activation and, if necessary, suitable passivation.

Current Catalytic Processes with Hybrid Materials and Composites for Heterogeneous Catalysis

Kajornsak Faungnawakij , Kongkiat Suriye , in New and Future Developments in Catalysis , 2013

4.3.1.4 Capable to Develop the Novel Superior Catalyst by Conventional Catalyst Manufacturing Technologies

Although, many new catalyst preparation procedures and technologies have been developed and announced during the past decade, only very few are found in commercial catalyst production [40] . This is attributed to most of them requiring a huge investment for building up a new commercial catalyst manufacturing system resulting in the difficulty to compete with the existing conventional systems. New catalyst preparation technologies, usually, are associated with additional capital investment. The room for such an investment gets smaller and smaller primarily due to low catalyst prices caused by the increased buying power of the catalyst users from new mega-producers [40] . Based on Sud-Chemies’ point of view, the downstream unit operations of the catalyst manufacturing such as the separation, mixing, shaping, and calcination have not changed during the past decade since only minor technical improvements of the equipment but no step change have been observed [40] . This is a good opportunity for hybrid catalysts because they could significantly improve the catalytic properties and/or create new catalytic properties by just physically mixing two (or more) active catalysts without the major investment of new catalyst manufacturing processes. The commercial hybrid adsorbent for purifying olefin streams has been commercialized by UOP [41] . This is hybridization between high selectivity and high adsorption capacity of molecular sieve materials and low reactivity and low heat of adsorption of activated alumina [42] . This hybrid adsorbent is classified to be a functionally hybridized adsorbent exhibiting higher adsorption capacity and stability with lower heat of adsorption. The stability upon multiple regenerations is also increased [41] . This is an example showing that the novel highly efficient catalyst and adsorbent could be developed and manufactured, without the major investment of new catalyst manufacturing processes, by using the hybridization approach. Therefore, this is a promising way to shorten research and development time for commercial processes.

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