Polyols: An Alternative Sugar Platform for Conversion of Biomass to

Chapter 22

Polyols: A n Alternative Sugar Platform for Conversion of Biomass to Fuels and Chemicals

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch022

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J. Michael Robinson , Caroline E. Burgess , Melissa A. Bently , Chris D. Brasher , Bruce O. Horne , Danny M. Lillard , José M. Macias , Laura D. Marrufo , Hari D. Mandal , Samuel C. Mills , Kevin D. O'Hara , Justin T. Pon , Annette F. Raigoza , Ernesto H. Sanchez, José S. Villarreal , and Qian Xiang 1

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Chemistry Department, The University of Texas of the Permian Basin, 4901 East University Boulevard, Odessa, T X 79762 Champion Technologies, 115 Proctor, Odessa, T X 79762 Pfizer Global Research and Discovery, St Louis, M O 63017 Chemistry Department, University of Texas at Dallas, Richardson, T X 75083 Chemistry Department, University of Notre Dame, South Bend, IN 46556 Abenogoa, 1400 Elbridge Payne Road, Suite 212, Chesterfield, M O 63017 Current address: Energy Institute, Pennsylvania State University, University Park, PA 16802 2

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Polyols provide an alternative "sugar platform" for biomass conversion to hydrocarbon fuels, alcohols, chemicals and hydrogen. Polyols are obtained directly from biomass by an "intercepted dilute acid hydrolysis and hydrogenation" ( I D A H H ) , wherein the incipient unstable aldoses are intercepted by catalytic hydrogenation to produce a solution of stable polyols with no detectable phenols. Granular catalyst is retained by a screen and insoluble lignin is simply filtered from the product slurry. Complete conversions are obtained for several biomass types within 3-6 hr at~185°C with 0.8% H PO . Minimum cost for polyols ranges from $0.12-0.16/lb. 3

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© 2006 American Chemical Society

In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction Two classical biomass processing platforms (Figure 1) are described by the US Dept of Energy's Office of Energy Efficiency and Renewable Energy as the "Sugar Platform" and the "Thermochemical Platform". These platforms generally follow lines of considerable efforts in fermentation of aldose sugars and direct pyrolysis of biomass. Neither platform has been overly successful because of the inability to obtain the desired monomeric sugars (e.g., glucose) in suitable yields and thermal transformations are nonselective and give low quality products. New technologies are emerging along chemical pathways that are neither fermentations nor pyrolyses. Rather than only seeking a myriad of slight improvements to each of these classical platforms, feedstocks for the future must embrace new alternative pathways where developments may provide more attractive economic solutions to the current inefficient biomass conversions and may afford high quality fuels.

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Sugar Platform (Fermentation) Biomass

2.

C H 0 "aldose sugar" 6

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2 CH CH OH + 2 C 0 < 67% Carbon efficiency 3

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2

Thermochemical Platform (Pyrolysis) Biomass

(gas)

oil + char + C 0 < 80% Carbon efficiency 2

Figure 1. Classical Biomass Platforms

Our research has established a clean fractionation of biomass carbohydrate polymers into monomeric polyols which serve as a new alternative "sugar" platform. Polyols are obtained from biomass by chemical means, rather than by enzymatic degradation, and are further transformed by chemical means into quality fuels and other chemicals.

Strategy to Solve the Biomass Fractionation Problem Obtaining a clean fractionation (/) of biomass polysaccharides has been a difficult task. Ultimately, utilization of biomass to produce liquid fuels economically must solve this challenge. Many government agencies (2), national labs (3), industry and academia (4,5,6,7,8) have attempted to solve this problem by almost as many methods and improvements. The reactive nature of


306 the monomeric carbohydrates generated during acidic hydrolysis unfortunately provides for continued reactions (degradation). Currently, the digestion of polysaccharides is limited by the simultaneous undesirable side reactions of aldoses. Hydrolysis conditions and rates of hydrolysis and degradation reactions (ki & k ) have to be delicately balanced in order to achieve a reasonable yield of desired sugars as shown in Figure 2, which is a pictorial statement of the problem. High glucose yields are not available even at high temperatures for very short reactions in dilute acid. In a special case, high conversion (including a secondary lower temperature digestion) was achieved by a fast flow shear factor (9). However, the shrinking bed reactor that was employed is not practical for industrial use and a clean separation from lignin was not achieved.


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k, Cellulose/ Hemicellulose

k

Monomeric Sugars (unstable)

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Decomposition Products

k »k, k »k 3

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Figure 2. Intercepted Hydrolysis Strategy

Herein, a strategy is defined which provides for trapping the reactive carbonyl carbohydrates immediately upon hydrolysis from biomass polysaccharides. The lower section of Figure 2 depicts how incipient aldoses are rapidly reacted (e.g., catalytic hydrogénation) to -100% polyols, which are much more stable to further reaction. This reductive mode defines the "intercepted dilute acid hydrolysis and hydrogénation" ( I D A H H ) strategy wherein the rate limiting step is the rate of hydrolysis. Figure 3 displays tangent lines to the dissolution curve of cellulose and the production curve of glucose calculated using Saeman's kinetics for 1% H S 0 at the relatively low temperature of 170 °C. These tangents represent the continued initial reaction rates for both curves with such an interception reaction strategy, whereupon cellulose might be completely digested and high yields of a stable derivative, sorbitol, might be produced in less than 2 hr, rather than the limited amounts of glucose that would otherwise continue to decompose under these conditions. 2

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Figure 3. Calculated Cellulose Hydrolysis Profile in 1% H S0 2

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at 170 °C

In fact, this preparation of inexpensive polyols is the first step in the proposed new platform for a bio-refinery (Figure 4) that is capable of producing several types of products from these intermediate polyols ranging from hydrocarbons to alcohols and hydrogen. In the massive literature of biomass hydrolysis efforts, almost all have sought to produce glucose for fermentation. Few have attempted to intercept the hydrolysis at the aldose stage before other undesirable degradation reactions occur. Choosing to use an interception strategy provides a distinct advantage. For example, if simultaneous hydrogénation is employed during the polysaccharide hydrolysis, each incipient aldehyde group is reduced irreversibly. If the speed of the reduction is greater than the hydrolysis, then aldose concentration is kept very small. Consequently, raw biomass is converted to a solution of C / C polyols, predominantly sorbitol and xylitol, which reflects the hemicellulose and cellulose content of the biomass resource utilized. Lignin is not appreciably affected and can be simply filtered from solution. A remarkably facile biomass fractionation results. Several factors must be considered for this strategy to deliver an inexpensive polyol mixture and ultimately, inexpensive final products. These include: 1) single step reaction, 2) aqueous solvent system, 3) total carbohydrate perspective, 4) the stability of polyol products to reaction conditions, and 5) a 6

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308 highly active and stable catalyst. Acidic conditions are the best choice for hydrolysis because both the acetal group forming the carbohydrate polymer and the ring opening of the cyclic hemiacetal group to the reducible acyclic aldose form are both acid catalyzed.

Biomass +

H /H 0 & H Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch022

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Lignin

C & C Polyols 5

Hydrogen

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Hyrdocarbons

Phenols

Alcohols

Figure 4. Biomass Refinery via Polyols Platform.

Indeed, a simultaneous acid hydrolysis/hydrogenation was found in the Soviet literature (10,11,12,13). One report briefly discussed the use of dilute H 3 P O 4 (0.7%) with a Ru/C catalyst at an initial hydrogen pressure of 440-735 psi and at 155-160 °C for 1 hr to generate sorbitol from pine sawdust. However, the available (English) papers were brief, particularly with respect to raw biomass conversion. Therefore, our efforts to develop an "intercepted dilute acid hydrolysis and hydrogénation" ( I D A H H ) process for biomass fractionation began by re-examining these conditions for a variety of biomass resources (14).

Results and Discussion

Biomass Resource Survey for I D A H H Table I shows the biomass resource characterization by C and C aldose as well as lignin content. FTIR analyses of our biomass samples agreed closely with these literature values. However, Table 1 simplifies the complex mix of carbohydrates that differ considerably between resources. In addition to mainly glucose and xylose, the hemicellulose fractions contain aldoses such as m annose and galactose, but also have small amounts of acid forms such as glucuronic 5

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309 acid. Nevertheless, these diverse resources are all hydrogenated to mixed polyols (sorbitol, mannitol, xylitiol, etc.) which contain small amounts of polyhydroxy acids, such as gluconic acid. While the stereochemical differences between the starting sugars (e. g., mannose and glucose) are retained in the polyol products, they have no impact in later chemical reactions where they are reduced into fuels with no stereochemistry. Since -35 mesh oak sawdust was available locally, it was selected to study the initial variables (temperature, time, biomass load, intrinsic acidity, and acid concentration) needed for I D A H H . Seven raw biomass resources as well as C standards, cellulose and starch, were surveyed. These feedstocks have 59.073.2% holocellulose and include a hardwood, softwood, perennial energy crop, and an agricultural waste. Switchgrass and corn stover have been deemed as the "best quantity" biomass resources with the advantage of fast growth, cyclical harvest, and ready availability. However, it is important to note that such "crops" have a significant protein content which may contribute to catalyst poisoning in our reactions.


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Table I. Major Components of Selected Biomass Resources %C

%C

Ce/Cs

%C +C

%Lignin

%Total

Aspen

57.6

15.6

3.7

73.2

15.5

88.7

Cedar

61.4

8.1

7.6

69.5

32.5

102

Pine

58.1

11.2

5.2

69.3

29.4

98.7

Oak

46.9

20.4

2.3

67.3

23.2

90.5

Switchgrass

38.5

27.4

1.4

65.9

17.6

83.5

Baggasse

41.0

23.0

1.8

64.0

25.2

89.2

Corn Fiber

36.9

22.1

1.7

59.0

16.7

75.7

Biomass

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Woods

Crops

SOURCE: Adapted from Reference 14. Copyright 2004 Elsevier.

Most biomass resources gave essentially complete conversion of their carbohydrates to polyols within 3-5 hr at 165-190 °C with 5-15% (w/v) biomass loading (Table II). A negative Benedict's test (Cu) for reducing sugar (35% liquid phenols.


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Conclusion Conversion of biomass carbohydrate polymers, hemicellulose and cellulose, to polyols can be complete (>99%) with 15% load at

Polyols: An Alternative Sugar Platform for Conversion of Biomass to

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