Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 21

A Review of Recent Literature

By James J. Spivey, Kerry M. Dooley

The Royal Society of Chemistry

Copyright © 2009 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-249-4

Contents

Preface James J. Spivey and Kerry M. Dooley, 7,
Heterogeneous catalysis for production of value-added chemicals from biomass Kresten Egeblad, Jeppe Rass-Hansen, Charlotte C. Marsden, Esben Taarning and Claus Hviid Christensen, 13,
Catalytic and photocatalytic removal of pollutants from aqueous sources J. A. Anderson and M. Fernández-García, 51,
Nano-architecture and reactivity of titania catalytic materials. Part 2. Bidimensional nanostructured films Gabriele Centi and Siglinda Perathoner, 82,
Recent advances in heterogeneous catalysis enabled by first-principles methods Ye Xu, 131,
Ionic liquids as catalysts, solvents and conversion agents Amit C. Gujar and Mark G. White, 154,
Measurement techniques in catalysis for mechanism development: kinetic, transient and in situ methods Nora M. McLaughlin and Marco J. Castaldi, 191,

CHAPTER 1

Heterogeneous catalysis for production of value-added chemicals from biomass

Kresten Egeblad, Jeppe Rass-Hansen, Charlotte C. Marsden, Esben Taarning and Claus Hviid Christensen

DOI: 10.1039/b712664f

1. Introduction

Almost everything around us is in some way a product of controlled chemical processes. That is either chemical processes conducted in Nature or chemical processes conducted in the chemical industry. In the most developed parts of the World, it is in fact products from the chemical industry that completely dominate our everyday lives. These products range from fuels and fertilizers to plastics and pharmaceuticals. To make these products widely available, a huge amount of resources have been invested during the last century to develop the chemical industry to its current level where it is the largest industry worldwide, a cornerstone of contemporary society, and also a platform for further global economic growth. It can be argued that the enormous success of the chemical industry can be attributed to the almost unlimited availability of inexpensive fossil resources, and to a continuously increasing number of catalysts and catalytic processes that make it possible to efficiently transform the fossil resources into all the required compounds and materials. Accordingly, more than 95% of the fuels and chemicals produced worldwide are derived from fossil resources, and more than 60% of the processes and 90% of the products in chemical industry somehow rely on catalysis. It has been estimated that 20–30% of the production in the industrialized world is directly dependent on catalytic technology. Therefore, it is not surprising that we are continuously expanding our already vast empirical knowledge about catalysis to further improve the efficiency of existing catalysts and processes, to discover entirely new ways of valorizing available resources, and to lower the environmental impact of human activities. Due to the overwhelming importance of fossil resources during the 20th century, most catalysis research efforts have, so far, concerned the conversion of these resources into value-added fuels and chemicals. There are, however, indications that the era of easy access to inexpensive fossil resources, especially crude oil, is coming to an end. The resources are certainly limited and the demand from everywhere in the world is growing rapidly. At the same time, it is becoming increasingly clear that the emission of CO2 that follows the use of fossil resources is threatening the climate of the Earth. Together this makes the development of a chemical industry based on renewable resources one of the most important challenges of the 21th century.

This challenge has two different facets. One is the discovery and development of methods to use renewable resources to supply suitable energy carriers, in sufficient quantities at acceptable costs, and with minimal impact on the environment. The other is the discovery and development of new ways to provide all the chemicals needed to sustain a modern society. Whereas there are several possible energy scenarios that do not involve carbon-containing energy currencies, it is in fact impossible to envisage how it should be possible to provide the required chemicals and materials without relying extensively on carbon-containing compounds. Thus, to develop a chemical industry that does not depend on fossil resources, there are only two alternative carbon sources and that is CO2 and biomass. Since transformation of CO2 into useful chemicals always requires a significant energy input and since it is usually only available in minute concentrations, it appears attractive to instead utilize biomass as the dominant feedstock for chemical industry. In this way, it is possible to harvest the energy input from the Sun that is stored by photosynthesis in the C–C, C–H, C–O, and O–H bonds of the biomass. Clearly, a shift from fossil resources to renewable resources as the preferred feedstock in chemical industry is a formidable challenge. However, it is worth pointing out that during the early part of the 20th century, before fossil resources became widely available, biomass was the preferred feedstock for the emerging chemical industry, and today, biomass still finds use as a feedstock for a range of very important chemicals. Interestingly, these processes often rely mostly on the availability of biological catalysts whereas the processes for conversion of hydrocarbons use mostly heterogeneous catalysts. However, to explore the full potential of biomass as a feedstock in chemical industry, it appears necessary to integrate processes that rely on biological catalysts with processes that use heterogeneous or homogeneous catalysts to develop new, cost-competitive and environmentally friendly technologies. Here, we will survey the possibilities for producing value-added chemicals from biomass using heterogeneous catalytic processes.

2. Setting a new scene

2.1 Biomass for production of fuels and chemicals

Currently, there exists a strong focus on the manufacture of transportation fuels from biomass. Clearly, this can be attributed to a desire to relinquish our dependence on fossil fuels, in particular crude oil, and also to significantly lower the emission of greenhouse gasses to minimize global warming. In some regions of the world, it seems that production of bio-ethanol is indeed already cost-competitive with gasoline and this demonstrates the potential of biomass as a renewable raw material. However, it is also clear that widespread use of biomass as a raw material for biofuel production remains controversial from both an economical and an ecological perspective. These issues must, of course, be resolved soon in a fully transparent way to identify sustainable paths forward. However, it is undisputable that we will eventually need alternatives to the fossil resources for producing chemicals and materials. It can be argued that if the amount of biomass available is too limited to substitute fossil resources in all its applications and if sufficiently efficient methods for transforming biomass into value-added chemical can be developed, this will represent the optimal use of biomass. There are two reasons for this. First of all, most chemicals, even most of the simple petrochemical building blocks, are significantly more valuable than transportation fuels. This can be illustrated in a semi-quantitative way by comparing the value chains in a chemical industry based on fossil and renewable resources, respectively. In this context, it is instructive to compare the cost of renewable resources to fossil resources over time. It is noteworthy that today, the cost of glucose is comparable to the cost of crude oil (on a mass-to-mass basis). Secondly, it is clear that by use of renewable resources as a feedstock for the chemical industry, significantly higher reductions in the emissions of green-house gases can be achieved than what is possible by production of biofuels. This can be attributed to the fact that production of many large-scale commodity chemicals from fossil resources is associated with a substantial co-production of CO2 as expressed e.g., by the C-factor (kg CO2 produced by kg of desirable product). This can often be attributed to the high temperature required to transform hydrocarbons. To illustrate this, the C-factor for industrial production of hydrogen from natural gas is about 9 and for ethylene from naphtha it is 0.65. If hydrogen or ethylene was produced efficiently from biomass, the C-factor would approximately express the amount of CO2 emission that would be saved compared to what would be possible by production of biofuels instead. Since ethylene alone is currently produced in an annual amount close to 100 mill. tonnes, it is obvious that this would have a substantial impact on the total emission of green-house gases.

2.2 Biomass in chemical industry

There are many ways in which biomass can be envisaged to become an increasingly important feedstock for the chemical industry, and this has already been the topic of numerous studies. The most comprehensive study was published recently by Corma et al. and it contains a very detailed review of possible routes to produce chemicals from biomass.

In Fig. 1, we illustrate schematically how selected commodity chemicals could be produced using abundant bio-resources, i.e., carbohydrates (starch, cellulose, hemi-cellulose, sucrose), lipids and oils (rapeseed oil, soy oil, etc.), and lignin as the sole raw materials. From these bio-resources, it is possible to directly obtain all the compounds classified in Fig. 1 as primary renewable building blocks (of which only selected examples are given) with only one purification step. For example, ethanol can be obtained by fermentation of sucrose, glucose by hydrolysis of starch, glycerol by transesterification of triglycerides (or by fermentation of glucose), xylose by hydrolysis of hemi-cellulose, fructose by hydrolysis of sucrose (and by isomerization of glucose), and finally synthesis gas can be obtained directly by gasification of most bio-resources or by steam-reforming of the other primary renewable building blocks. From the primary renewable building blocks a wide range of possible commodity chemicals can be produced in a single step, and again examples of selected transformations are shown in Fig. 1. For instance, acetic acid can be produced by fermentation of glucose or by selective oxidation of ethanol. Lactic acid is available by fermentation of glucose, and 5-hydroxymethyl furfural can be obtained by dehydration of fructose. These compounds can again be starting materials for other desirable products and so forth. Some of the commodity chemicals shown are already produced on a large scale from fossil resources, e.g., ethylene, acetic acid, acrolein and butadiene. Others are envisaged to become important large-scale commodity chemicals in the future when biomass gradually becomes a more important feedstock. The different commodity chemicals are labeled to categorize them according to their number of carbon atoms. It is seen that a wide range of C1 to C6 compounds can be made available by quite simple means. Moreover, the chemical transformations in Fig. 1 are labeled with different arrows to illustrate specific ways to convert one building block into another. As it is apparent, the reactions all require a suitable catalyst, and this can be either a biological catalyst or a heterogeneous/homogeneous catalyst. Most of the primary renewable building blocks are produced today from bio-resources using mainly biocatalytic processes, and similarly several of the proposed commodity chemicals can also be produced from the primary renewable building blocks using biological catalysts. On the other hand, it is also clear that a very substantial number of the desirable transformations rely on the availability of suitable heterogeneous or homogeneous catalysts. Thus, it appears likely that a chemical industry based on renewable resources as the dominant feedstock will feature biological and chemical processes intimately integrated to efficiently produce all the desired chemicals and materials.

2.3 Heterogeneous catalysis and biomass

Often, it appears that the possible role of heterogeneous catalysis in this scenario is not receiving sufficient attention in comparison with that of the biocatalytic methods. Therefore, in the present chapter we will highlight some of the existing possibilities for converting bio-resources, primary renewable building blocks, and commodity chemicals derived from these into value-added chemicals. We will focus on production of chemicals that can prove useful on a larger scale since they will contribute most to the valorization of significant quantities of biomass, and thereby contribute most to relinquishing the dependence on fossil fuels and to lowering the emission of green-house gases. Hopefully, this will be useful as a starting point for others to discover and develop new reactions and catalysts that can become useful in the efforts to make biomass a more useful resource for chemical industry. Our emphasis here is the catalytic reactions and the corresponding catalysts. Therefore, we have organized the literature covered in separate chapters according to five important reaction types, specifically, C–C bond breaking, hydrolysis, dehydration, oxidation, and hydrogenation. We envisage that these reaction types will be the most important for producing value-added chemicals from biomass since they can be conducted on large scale and they do not involve expensive reagents that will make them prohibitively expensive for industrial applications. Clearly, other reactions will also be important but several of those will be analogues to current methods in chemical industry. In each chapter, the presentation is organized hierarchically to first discuss the catalytic conversion of compounds that are most closely related to the bio-resources (carbohydrates, lipids and oils, and lignin) and then successively those derived from these renewable raw materials.

3. Catalytic C–C bond breaking

3.1 Introduction

This section concerns catalytic processes that transform chemicals from renewables by C–C bond breaking. Among these are thermochemical processes, such as pyrolysis and also gasification, catalytic reactions, such as catalytic cracking and different reforming reactions, and decarbonylation and decarboxylation reactions. Many of these reactions occur simultaneously, particularly in the thermochemical processes. Another technically important class of C–C bond breaking reactions is the fermentation processes, however, they will not be considered in this section since they do not involve heterogeneous catalysis.

3.2 C–C Bond breaking reactions involving bio-resources

3.2.1 Crude biomass. Next to combustion, gasification is probably the easiest and most primitive method for degradation of biomass. In the simplest form, gasification involves heating of biomass (or any other carbonaceous material) to temperatures around 800–900 °C, in an atmosphere with only little oxygen, until it thermally decomposes into smaller fragments. This partial oxidation process obviously requires a significant energy input and is not particularly selective; on the other hand, it is reasonably flexible since essentially all types of biomass can be gasified. Gasification, in particular of coal, has been known for long and was previously used to produce town gas. However, the gas resulting from gasification has a relatively low heating value of only 10–50% of that of natural gas, and this was a major reason for replacing town gas with natural gas. During World War II, biomass gasification advanced in Europe, but it was not until the oil crisis in the 1970s that new developments in the area truly took place. Today, the main purpose of biomass gasification is to produce synthesis gas, with a H2:CO ratio close to two, which is suitable for methanol synthesis or Fischer-Tropsch fuels.

There exist many different types of gasification furnaces but they generally work by having several different cracking and reforming zones. These zones are typically a pyrolysis zone, an oxidation zone and a reduction zone. Biomass is broken down either by pyrolysis (without oxygen) or by partial oxidation (with oxygen or air as oxidant) to a mixture of CO, CO2, H2O, H2, CH4, other light hydrocarbons, some tar, char and ash, as well as some nitrogen and sulfur containing gasses such as HCN, NH3, HCl, H2S etc. The hydrocarbons and the char are further partially oxidized to mainly CO and H2O (1–4) and steam reformed (5–6) or dry reformed (7–9) to CO and H2. The heat from the exothermic oxidation reactions is used to supply the heat for the endothermic cracking reactions. Finally, the H2:CO ratio can be adjusted by the water gas shift reaction (10).

CH4 + 1/2O2 = CO + 2H2 (1)

H2 + 1/2O2 = H2O (2)

CnHm + (n/2 + m/4)O2 = nCO + (m/2)H2O (3)

C + 1/2O2 = CO (4)

CnHm + nH2O = nCO + (n + m/2)H2 (5)

C + H2O = CO + H2 (6)

CnHm + nCO2 = 2nCO + (m/2)H2 (7)

C + CO2 = 2CO (8)

CH4 + CO2 = 2CO + 2H2 (9)

CO + H2O = CO2 + H2 (10)

The major challenge in gasification is to avoid the formation of tars, which have a tendency to clog filters and condense in end-pipelines. Tars are considered as the condensable fraction of the organic gasification products, and consist mainly of different aromatic hydrocarbons with benzene as the main species. For removal of tars three types of catalysts have been widely investigated; alkali metal salts, alkaline earth metal oxides and supported metallic oxides.

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