Dr. H.N. Cheng is active in research and has made significant advances in developing green chemistry methodologies pertaining to agro-based materials and using agricultural materials and byproducts to produce eco-friendly and sustainable products. In his view, in order to successfully develop agro-based materials as a platform for new product development, concerted efforts are needed in four areas of opportunities, as shown below:
Thus, the thrust of his research has been: 1) to use natural renewable raw materials (e.g., polysaccharides, proteins, and triglycerides) as source materials for new chemistry and products; 2) to exploit biocatalysts (e.g., enzymes and micro-organisms) for chemical and polymer conversions; 3) to apply green chemistry concepts to conversion and processing; and 4) to develop green methodologies to promote the development of green products. He has made significant contributions to all these areas. His work can be grouped into 4 categories below, where # refers to his papers and P refers to his patent publications, as given in the list of his publications.
1. Agro-based Source Materials
a. Polysaccharides and Agriculture Wastes. Agricultural production generates many byproducts or waste materials, which are often disposed of or used in low-value applications. Cheng successfully converted many commodity agricultural materials into potential products of higher value. Even prior to coming to USDA, he had already published on polysaccharides and their modifications (#93, #95-97, #100, #110-111, #114, #119-120, #135). These efforts have been brought to a sharper focus since his tenure at USDA. At USDA he and his collaborators worked with cellulose and cotton byproducts, particularly cottonseed hull and cotton burr, and converted them into carboxymethyl cellulose (CMC) and carboxymethyl xylan (#144). Similar reaction was done on agricultural waste products like wheat straw, barley straw and rice hull to produce CMC and CMC acetate (#178), methyl cellulose (#165), cellulose acetate (CA) (#136), and mixed cellulose esters (#141, #145). Other polysaccharides studied included starch acetate (#143), cellulose succinate (#225), xylan derivatives (#247), and cashew gum derivatives (#254). Also investigated were polyblends involving hemicelluloses (#207, #234, #253), and the potential uses of polysaccharides in packaging films (#199, #240, #241, #249).
b. Proteins. There has been increasing interest in soy protein as a wood adhesive in the past 20 years in view of the potential hazards of formaldehyde (in formaldehyde-based adhesives) and the benefits of renewable feedstock. Since cottonseed protein is readily available in the South, Cheng and his coworkers studied cottonseed protein and found it to be superior to soy protein in wood adhesion (#168). Because cottonseed protein costs higher than soy protein, he found a number of cheap polysaccharides (starch, cellulose, and hemicellulose) that could be added to cottonseed protein at 50% level without affecting its adhesive properties (#195). He also found several families of additives that could further improve the adhesive performance of cottonseed protein (#197, 217, 226, 237). To decrease raw material cost even further, cottonseed meal was processed in different ways and found to perform well in comparison to protein isolate (#180, 183, 204, 205, 219, 244, 252). In addition to being a wood adhesive, cottonseed protein was found to enhance the dry strength of paper (#236) and non-woven cotton (#245).
c. Triglycerides. Triglyceride oils are relatively cheap, abundant, and renewable and can be used as raw materials for the syntheses of value-added products. At Hercules Cheng had already used soybean oil in pulp mill defoamers (called biodefoamers) (P18,P19); they were commercially successful and nominated for the EPA Presidential Green Chemistry Award in 2009. For quite some time, Cheng and his collaborators chemically modified triglyceride oils to extend their applications. One feature of this work that differed from others was the use of nitrogen-containing reagents in the modification reactions (#123-124, #127-129, #131-132, #143, #146, #166). For example, they explored the application of pericylic reactions (such as Diels-Alder and ene reactions) to produce modified oils (#128, #166). The process required no solvent or catalyst, and the products exhibited a self-curing behavior as a function of time, suggesting their possible use as additive in lubricants, caulking, cement and thickener applications.
2. Green Chemistry – Methodologies, Processing and Product Development
Green chemistry can be defined as the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. It is compatible with agriculture and serves as a useful tool for agricultural R&D. Cheng and his coworkers have contributed significantly to this area.
a. Biocatalysis for Biobased Materials. Biocatalysis includes the use of enzymes and living organisms to carry out reactions. To facilitate his innovative efforts, Cheng devised several strategies. One strategy was to employ enzymes to edit existing molecular structures. Another was to apply enzymes to non-natural substrates, thereby discovering new reactions. Yet another was to combine enzyme reactions with some of the tricks of organic reactions. Particularly notable were his efforts to use enzymes to modify natural polysaccharides (such as starch, pectin, guar, and cellulosics), new enzymatic methods to synthesize polyamides, and enzyme-catalyzed ring-opening graft reactions and polymerization. Examples of his contributions are given below:
- New enzymatic grafting reactions of b-keto esters and hydrophobes onto polysaccharides (P4, P5, P7, P10, #100, #162, #258),
- Novel grafting reactions on pectin (#115, P3)
- New or improved reactions of enzymes with non-substrate polymers (#117)
- Novel esters and amides (#102, #109, #118, #122).
- New enzymatic synthesis of polyamides (P8, P16)
- Enzymatic galactosylation reactions (#102, #103, #162)
- Reactions with galactose oxidase (P2, P9, P15, #96, #111)
b. Green Processing. Among his many contributions, he devised novel approaches for the production of haloamine biocides (P20, P22) and an alternative process for bleaching via singlet oxygen (P23). He contributed to the use of microbes to clean up hazardous byproducts in industrial products (P6, P17), to bleach paper pulp (P11-P14), and to promote milder and greener reaction pathways (#162), including product design and elimination of organic solvents in selected reactions (#134, #139). He also collaborated with others on the use of microwave to accelerate reactions (#163, 176, 188, 201) or extractions (#133, #157). Encouraging results were found using natural deep eutectic solvents (NADES) for the extractions of polar ingredients in foods (#246, 252, 257). Efforts were also made to minimize the use of acids in the esterification of cellulose (#130, #136, #141, #145).
c. Other Conversions and Reactions. He was interested in reactions on polymers and biobased materials even before coming to USDA (e.g., #5, #10, #82, #87, #88, #89, #93, #95). At USDA he and his collaborators carried out various reactions involving biopolymers. Thus, they made polyurethanes from cyclodextrin (#176), starch (#188), xylan (#201), sucrose (#242), and sorbitol (#262). They derivatized zein protein with a hydrophobic moiety (#155). They used cationic means to polymerize vernonia oil (#177) and epoxidized triglycerides (#189, #229), and further derivatized the latter through Diels-Alder reaction (#190). A related study was iron-initiated polymerization of fatty acids (#256).
d. Leadership. He has co-organized 7 symposia on Green Polymer Chemistry at ACS national meetings (2000, 2003, 2006, 2009, 2012, 2014, 2017). He has also edited 9 ACS symposium books on this topic.
3. Polymer Theory and Analysis
Since many agricultural raw materials are polymers and structurally complex, detailed characterization is needed to provide in-depth and mechanistic understanding. Cheng has also contributed notably to this area.
a. NMR and Polymer Analysis. He developed many new techniques and methods to facilitate the use of NMR to analyze complex polymers and biopolymers, including shift prediction methods (#18, #22, #30, #55, #76), computer-assisted approaches towards spectral analysis (reviewed in #101, #77, #46), multicomponent (mixture) analysis (e.g., #90, #75, #70, #48), compositional heterogeneity (e.g., #112, #91, 220), analysis of high conversion polymers (#92), fractionation-NMR (e.g., #54, #72, #75), SEC-NMR (#107). He was among the first to publish on two-dimensional (2D) NMR for polymers, the first ever to use HETCOR for polymers (1984, #23), the first to use 2D NMR on polyolefins (#23, #24, #33, #34, #51), and the first to combine 2D NMR with lanthanide shift reagents (#39). He used NMR successfully at USDA for many agro-based materials, e.g., triglyceride derivatives (#124, 127, 129, 132, 143, 146, 166, 189, 190); carbohydrate derivatives (#135. 136, 141, 144, 163, 165, 178); carbohydrate polyurethanes (#176, 188, 201); cardanol derivatives (#198, 200); modified zein (#155); poly(lactic acid) (#154, 160, 170); phytates (#164); and biochar (#137, #191, #214).
b. Polymerization Statistics and Modeling. Many probability models had previously been known in copolymerization, but as he dealt with complex biopolymers and industrial problems, he realized the inadequacy of some of these models and developed a large number of new polymerization models. These include: second-order Price model (#41, #74), tetrapolymerization model (#57, #101, #78, #80), perturbed Markovian and enantiomorphic-site models (#64, #71, #91, #92, #94, #98, #105, #107, #112, #220), consecutive two-component models (#68, #69), concurrent multi-component models (#40, #48, #54, #70, #72, #75, #94, #98, #107, #112, #220), kinetic models (#86, #113), and others (#87-89).
c. Leadership. Cheng has co-organized international NMR polymer symposia at regular intervals: 1994 (FACSS), 1997, 2001, 2003, 2008, 2013, 2018, 2020 (all ACS), and 2005, 2010, 2015 (Pacifichem). He has co-edited three symposium books on the NMR of polymers (1994, 2002, 2011).