New and Future Developments in Catalysis (eBook)

Catalytic Biomass Conversion

Steven L Suib (Herausgeber)

eBook Download: PDF | EPUB

2013 | 1. Auflage
412 Seiten
Elsevier Reference Monographs (Verlag)
978-0-444-53879-6 (ISBN)

Infos

New and Future Developments in Catalysis is a package of books that compile the latest ideas concerning alternate and renewable energy sources and the role that catalysis plays in converting new renewable feedstock into biofuels and biochemicals. Both homogeneous and heterogeneous catalysts and catalytic processes will be discussed in a unified and comprehensive approach. There will be extensive cross-referencing within all volumes. This volume covers all the biomass sources and gives detailed and in-depth coverage of all current chemical/catalytic conversion processes of biomass into liquid hydrocarbons to be further used as a feedstock for the production of not only biofuels but a large array of chemicals. - Offers an in-depth coverage of all catalytic topics of current interest and outlines the future challenges and research areas - A clear and visual description of all parameters and conditions enables the reader to draw conclusions for a particular case - Outline the catalytic processes applicable to energy generation and design of green processes

Textauszug

Chapter 1

Metal Catalysts for the Conversion of Biomass to Chemicals

Pierre Gallezot,     Institut de recherches sur la catalyse et l’environnement, Université de Lyon/CNRS, 2 avenue Albert Einstein, 69626, Villeurbanne Cedex, France,     [e-mail]

Acknowledgment

European COST action CM0903 (UBIOCHEM) is acknowledged for support.

1.1 Introduction

Extensive literature surveys on biomass conversion to chemicals were recently published [1–9] revealing the rapid development of new catalytic systems and reaction media adapted to the structure of biomolecules. The present chapter deals with the design and performance of metal catalysts employed for the conversion of platform molecules obtained from carbohydrates, triglycerides, and terpenes into chemicals that are either already synthesized from fossil resources or consisting of new bioproducts with no synthetic counterpart. Bifunctional metal catalysts converting biopolymers such as starch, cellulose, and hemicellulose into a mixture of chemicals that could be employed for the manufacture of high tonnage end-products such as paper additives, paints, resins, foams, surfactants, lubricants, and plasticizers will also be considered [7,10]. Because the literature on catalytic biomass conversion is presently bursting and because a complete survey of catalytic systems was not possible in the framework of this chapter, focus will be laid on selected examples of biomass conversion catalyzed by metals, particularly hydrogenation, hydrogenolysis, and oxidation reactions.

1.2 Hydrogenation Catalysts

1.2.1 Catalysts for the Hydrogenation of Carbohydrates and Derivatives

1.2.1.1 Hydrogenation of Glucose

More than 800,000 ton/y of sorbitol are produced industrially by catalytic hydrogenation of D-glucose, a cheap and abundant feedstock obtained from starch-containing crops such as maize, wheat, and potatoes. Sorbitol is used as additives in many industrial products, particularly in the food, cosmetic, and paper industries, and as building block for the synthesis of various fine chemicals including vitamin C (Figure 1.1). Highly active and stable metal catalysts are required for the industrial hydrogenation of glucose. Because selectivities higher than 99.5% to sorbitol at total glucose conversion are required for a number of applications epimerization of sorbitol to mannitol and Cannizarro reaction to gluconic acid should be avoided. The requirements for a long-term stability toward metal leaching and sintering were well documented, but the leaching of supporting materials in highly chelating reaction media was often overlooked in the literature. Also, the deactivation of metal catalysts by impurities in glucose feedstock, or formed by side reactions, and the procedures of catalyst regeneration were seldom studied. Although sorbitol is a high tonnage commodity product, hydrogenation reactions are still mainly carried out discontinuously in stirred tank reactors at 373–453 K and 5–15 MPa of H2 pressure in the presence of suspended catalyst powders. The design of catalyst formulation to replace the prevailing batchwise production by continuous processes is not well documented.

Figure 1.1 Glucose hydrogenation to sorbitol.

Most of the current industrial production of sorbitol is performed in stirred tank reactors loaded with Raney-type nickel catalysts (sponge nickel, skeletal nickel) promoted by various transition metals. Nickel catalysts present the advantage of a relatively low price and because of their high density they are easily separated from the liquid phase by sedimentation possibly accelerated by magnetic methods. Raney-type nickel catalysts are often prepared from Ni–Al–M alloys where M stands for transition metals such as Mo, W, or Cr, added to nickel-aluminum melt at a concentration of 0.5–5 mol%. The alloy is then attacked with alkali solutions to remove part of the aluminum and yield highly porous, tri- or polymetallic catalysts. Alternatively, metal promoters could be added to skeletal nickel by various methods of surface deposition. The presence of metal promoters favors the stability of the porous framework and accelerates reaction rates [11–13]. Glucose hydrogenation was studied in a well-stirred, high pressure batch reactor on Mo-, Cr-, and Fe-promoted Raney-type nickel catalysts prepared by soda attack on Ni–Al–M alloys [12]. Sn-promoted catalysts were obtained by controlled surface reaction of Sn(Bu)4 on the hydrogen-covered surface of a Raney Nickel obtained from a Ni2Al3 alloy. The promoted catalysts were up to seven times more active provided that metal promoters were homogeneously distributed with an optimum concentration. The rate enhancement was attributed to the polarization of CO bonds of the aldehyde form of glucose by electropositive metal promoters acting as Lewis acid sites. Iron- and tin-promoted catalysts deactivated very rapidly because the promoters were leached away from the surface. In contrast, the aging of molybdenum and chromium-promoted catalysts was attributed to the poisoning of the active sites by organic species. The major cause of deactivation of commercial Raney-type nickel catalysts was the presence of gluconic acid formed by the Cannizarro reaction poisoning catalytic sites and favoring nickel leaching [14], but after many recycles under industrial operation the loss of active surface area due to metal sintering was also a cause of deactivation [12]. Several attempts have been made to use supported nickel catalysts as substitutes for Raney-type nickel. Because of the high nickel loadings required (typically >40 wt.%) to obtain a sufficient activity in industrial operating conditions, the specific surface area of nickel was usually quite low. The kinetics of hydrogenation of 40 wt.% glucose solution was studied in a trickle-bed reactor in the presence of an industrial catalyst based on extrudates of kieselguhr-supported nickel catalysts containing 48.4 wt.% nickel [15]; the catalyst activity was low ( at 403 K, 8 MPa) and decreased with time because of the progressive leaching of nickel and support in the reaction medium. Ni–B/SiO2 amorphous catalyst prepared by reduction with KBH4 aqueous solutions exhibited a higher activity (TOF: 0.024 s−1) than commercial Raney-type catalysts (TOF: 0.013 s−1) [16]. Ni/SiO2 catalysts prepared by various methods deactivated by metal leaching, metal sintering, and support degradation [17]. Ni/SiO2 catalysts prepared by impregnation with nickel ethylenediamine complexes did not leach significantly after 5 h on stream, but they were slightly less active than commercial catalysts and less selective to sorbitol [18].

Because nickel catalysts are prone to leaching and sintering and because their activities are comparatively low, the present trend is to develop industrial processes based on supported ruthenium catalysts. Comparison of the specific activities measured on nickel and ruthenium catalysts under the same reaction conditions showed that ruthenium was 20–50 times more active than nickel per mass of metal. Specific reaction rates measured over various ruthenium catalysts are given in Table 1.1. Rates measured in trickle-bed reactor were lower compared to stirred tank reactors because of mass transfer limitation between the solid, liquid, and gas phases. Carbons of various origins have been widely used as supporting material for ruthenium because of their resistance to leaching, and because they adsorb organic impurities present in feedstocks thus preventing to some extent the poisoning of ruthenium surfaces. The hydrogenation activities of Ru/C catalysts in slurry reactors were proportional to the ruthenium surface area and independent of the preparation method [13,19]. Activated carbon cloths (ACC) present significant advantages with respect to conventional activated carbons such as efficient mass transfer from the liquid phase, no necessity of decantation or filtration, and high flexibility to fit into any reactor geometry [20,21]. ACC were prepared from woven rayon cloths carbonized at 1200 °C under nitrogen and activated at 900 °C under CO2; 0.9 wt.% Ru/ACC catalysts were very active () and selective to sorbitol (99.5% at 99.7% conversion) and could be easily recycled. The catalytic performances were even better with 10 wt.% Pt/ACC catalysts (Table 1.1). In most studies the selectivity to sorbitol was higher than 98% at total glucose conversion, but the selectivity decreased as the time of contact of catalyst with sorbitol solution increased because sorbitol was subject to further conversion to mannitol. Thus, the selectivity decreased as the time of contact with a Ru/C catalyst loaded in a trickle-bed reactor was increased beyond 100% conversion [19]; however, using a Pt–Ru/C bimetallic catalyst containing 1.6 wt.% of ruthenium and 0.2 wt.% of platinum it was possible to maintain a selectivity higher than 99% even after a long contact time. After long time on stream in the trickle-bed reactor, a Ru/Al2O3 catalyst deactivated because of structural modification of alumina and of ruthenium poisoning by sulfur compounds, gluconic acid, and deposition of iron atoms leaching from the reactor walls [22]. The loss of conversion from 99.9% to 98% experienced by a Ru/Al2O3 catalyst after 1080 h on...

Produkt-Details

• Erscheint lt. Verlag: 17.7.2013
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie ► Medizinische Fachgebiete ► Arbeits- / Sozial- / Umweltmedizin
• Themenwelt: Studium ► Querschnittsbereiche ► Prävention / Gesundheitsförderung
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Themenwelt: Naturwissenschaften ► Chemie ► Technische Chemie
• Themenwelt: Technik ► Umwelttechnik / Biotechnologie
• ISBN-10: 0-444-53879-8 / 0444538798
• ISBN-13: 978-0-444-53879-6 / 9780444538796