Study On Cellulase Adsorption Desorption Behavior Onto Lignocellulosic Substrates And Pretreatments For Simultauous Nanocellulose Preparation

Cellulose accessibilities of a set of hornified lignocellulosic substrates derived by dryingthe never dried pretreated sample and a set of differently pretreated lodgepople pine substrateswere evaluated using solute exclusion and protein adsorption methods. Direct measurementsof cellulase adsorption onto cellulose surface of the set of pretreated substrates were alsocarried out using an in-situ UV-Vis spectrophotometric technique. The celluloseaccessibilties measured by the solute exclusion and a CBM-containing green fluorescentprotein (TGC) adsorption methods correlate well for both sets of samples. The substrateenzymatic digestibilities of the hornified substrates are proportional to the measured celluloseaccessibilities. Approximately over90%of substrate enzymatic digestibility was contributedby the accessible pore surfaces of the hornified substrates, suggesting that the substrateexternal surface plays a minor role contributing to cellulose accessibility and substrateenzymatic digestibilities. The cellulose accessibilities of the pretreated substrates arecorrelated well with the amounts of cellulase adsorbed. The substrate enzymaticdigestibilities directly are correlated with the amounts of adsorbed cellulase.Quantitative kinetic modeling and in-situ and temporally resolved measurements ofadsorption, desorption, and re-adsorption of a commercial endoglucanase in lignocellulosicsuspensions was conducted. The study defined a cellulase adsorption and desorptioncompetition parameter, a pseudo rate of binding and desorption, binding and desorptioncapacity, as well as cellulase binding reversibility and recyclability. The results indicate thatboth substrate chemical and physical structures play important roles in cellulase binding anddesorption. Binding of a commercial cellulase onto a cellulosic substrate was reversible.Bindings to two different lignocellulosic substrates were almost irreversible. While lignin andits structure positively affect binding capacity to substrate, they negatively affect cellulaserecyclability. Collapsing of substrate pores reduce cellulose accessibility and cellulase bindingcapacity and increase reversibility and recyclability. Increasing temperature and pH increasecellulase desorption and increase binding reversibility and capacity. This study lays thefoundation for developing effective cellulase recycling strategies.Integrating preparation of biofuel and cellulose nanofibrils as a co-product was carriedout using softwood pulp. One key barrier to converting woody biomass to biofuel through thesugar platform is the enzymatic cellulose saccharification because of the strong recalcitranceof the crystalline cellulose. Cellulase cocktails were optimized for integrated preparation ofsugar and cellulose nanofibrils. Enzymatic hydrolysis of substrates was quenched as long as all fast sugar was degraded. Cellulosic substrate left with high crystallinity was separatedfrom hydrolysate and utilized as staring material for cellulose nanofibrils production. Highpress microfluidizer was used to liberate cellulose nanofibrils from cellulosic residual. Therelationships between enzymatic hydrolysis conditions, cellulosic residual yield, andproperties of cellulose nanofibrils, including degree of polymerization, crystallinity andmechanical performance were investigated.The potential of simultaneously recovering cellulosic solid residues and producingcellulose nanocrystals by strong sulfuric acid hydrolysis to achieve near zero cellulose losswas demonstrated in the3rd chapter. A set of slightly milder acid hydrolysis conditions thanthat considered as “optimal” were used to significantly minimize the degradation of celluloseinto soluble sugars that cannot be economically recovered, but resulted in cellulosic solidresidues that is easily recoverable through conventional centrifuge. It was found that thewindow for simultaneous recoveries of cellulosic solid residues and cellulose nanocrystals instrong acid hydrolysis was extremely narrow. The cellulose nanocrystals yield may not bereduced compared with that obtained under the “optimal condition”. The maximal totalcellulosic solid yield can be improved to approximately84%using a bleach eucalyptus pulpwith cellulose content of90%. The resultant cellulosic solid residues contain sulfonate groupsthat facilitate subsequent mechanical nano-fibrillation to nanowhiskers, a potential high valuenanocellulosic material for a variety of applications. The resultant cellulose nanofibrils filmsexhibit excellent optical and mechanical properties.Cellulose nanofibrils from a bleached eucalyptus pulp were isolated using a commercialstone grinder. SEM and TEM images were used to reveal morphological development ofcellulose nanofibrils at micro and nano scales, respectively. Two major structures wereidentified:(1) highly kinked, naturally helical, and untwisted fibrils that serve as backbones ofcellulose nanofibrils networks,(2) entangled, less distinctively kinked (or curled) and twistednanofibril “meniscus” structure. These two major structures forme different types of cellulosenanofibrils structures such as “trees”,“net”,“flower”, single fibril, etc. Prolonged fibrillationcan break the nanofibrils into nanowhiskers from the untwisted fibrils with high crystallinity.Energy input for mechanical fibrillation is on the order of20-30kWh/kg. The graduatereduction in network size of cellulose nanofibrils with time is only in a statistical sense whichmay be used to fractionate cellulose nanofibrils.The potential utility of wood nanocellulose as a building block of a variety of highlyfunctional and high value materials and products warrants a complete understanding of itsmorphological properties. The last chapter is intended to provide a relatively complete morphological picture of several kinds of wood nanocellulose including cellulose nanocrystalsand cellulose nanofibrils produced using most commonly used processes. TEM imaging aswell as AFM analysis were applied to provide visual examinations of several nanocellulosesamples. It was found that cellulose nanocrystals are mainly rodlike nano-whiskers asexpected and cellulose nanofibrils are fibril network particles. Commercial particle sizinginstruments were evaluated for quick particle sizing of these samples. The results suggest thata centrifuge and a Brownian motion based instruments show good repeatability in particlesizing. Both instruments are well suited for semi-quantitative characterizing non networkparticles even with large aspect ratios such as cellulose nanocrystals, but require further studybefore using for characterizing network particles such as cellulose nanofibrils.