Solvent consumption in non-catalytic alcohol solvolysis of biorefinery lignin

Abstract

a Lignin solvolysis in supercritical alcohols provides a method for producing a deoxygenated liquid bio-oil. Solvent consumption is however inevitable and due to the high cost of alcohols, relative to a bio-oil product, it can hinder commercial viability. In order to investigate the reactions of solvent consumption we studied solvolysis of biorefinery lignin in several primary alcohols. Lignin solvolysis in methanol, ethanol, 1-propanol and 1-butanol performed similarly with respect to bio-oil composition; however, methanol gave much lower bio-oil yield. Solvent consumption increases with reaction temperature for all alcohols and from 10 wt% at 300°C to 35 wt% at 400°C when using ethanol. The mechanism for solvent consumption was found mainly to take place through three different reactions: Direct decomposition to gas through decarbonylation, formation of light condensation products and incorporation of the alcohol into the bio-oil through covalent bonding. Incorporation of the alchohol into the depolymerised oil product by covalent bonding may be a desirable effect which contributes to increased oil yield, inhibition of repolymerisation, reduced oxygen content and elimination of acidity. IntroductionSolvolysis of lignin rich biomass is of growing interest as a process for producing liquid fuels and chemicals. 1, 2 An important issue in such a process is consumption of the solvent, since the cost of solvent may exceed the value of the depolymerised lignin product. Undesired solvent consumption can therefore hinder commercial viability.The mechanisms of solvent consumption in biomass solvolysis reactions are not understood in detail. Alcohols thermally decompose to gasses at elevated temperatures starting already at the supercritical temperature. 3 Even in reactions where the solvent is recognised as being a reactant, and acting as a hydrogen donor, solvent consumption is often an overlooked parameter. 1,2 Reactions in which alcohol solvent is consumed due to reaction with lignin is seen in some studies where the yield of solids and liquid depolymerised lignin species exceed the amount of initially added lignin 4, 5 hence transformation of solvent to biooil must occur. Direct alkylation of depolymerised lignin by ethanol can be described by a mechanism by Zhao et al. 6 , where alcohol radicals facilitate cleavage and resulting in its incorporation in the depolymerised lignin molecule. The alcohol most likely both facilitates lignin polymer cleavage and inhibition of repolymerisation by reaction with reactive radical species formed at elevated temperatures. Barnard investigated the thermal decomposition of ethanol at 576 °C -624 °C and observed significant gas yields and in particular also a brown polymer. 7 Higher alcohol synthesis as described by Guerbet 8, 9 may be responsible for formation of heavier alcohol polymerisation products that will be found in the bio-oil fraction after lignin solvolysis.We previously conducted a parametric study of bio-refinery lignin solvolysis by ethanol assessing the effect of re...

Mild thermolytic solvolysis of technical lignins in polar organic solvents to a crude lignin oil

“…The same group later demonstrated that solvent decomposition occurs in three primary ways: (i) decomposition of the alcohol to gases through decarbonylation, (ii) the formation of light condensation products through condensation and dehydration reactions, and (iii) formation of ethers or esters through alcohol condensation with carboxyl and hydroxyl groups present in lignin. 26 Solvent consumption, together with char fouling, can be controlled to some extent by lowering the reaction temperature and reaction time. Choi et al…”

“…al. 25,26 A second similarity relates to the primary objective, which is in all cases to (partially) deoxygenate and crack the lignin, thereby necessarily requiring severe (i.e., supercritical) process conditions.…”

Scaling-Up Catalytic Depolymerisation of Lignin: Performance Criteria for Industrial Operation

“…While interesting from a scientific stance, such diluted feeds would require an unrealistically high capital expenditure (CAPEX), given that CAPEX scales quite well with total volume/mass flow through the plant. Moreover, were such ratios to be applied commercially, even seemingly insignificant solvent losses of a mere few percents at lab scale, arising from, for example, flashing or chemical conversion towards less useful compounds [19], would surely be considered unacceptably high.…”

Ozone mediated depolymerization and solvolysis of technical lignins under ambient conditions in ethanol

“…Lignin-solvent reactions may also play a role on inhibiting competitive repolymerization pathways. 48 For most cases, increasing the reaction time from 20 to 60 minutes led to an increase of the lower M w fractions due to the subsequent oxidation of the lignin fragments solubilized in ethanol, which is known to happen at extended ozonation times. 54,55,59 The amounts of these fractions were greatly increased at the longer reaction times of 120 minutes (KL and ball-milled KL), likely accompanied by carbon loss due to CO 2 formation from over-oxidation.…”

“…Overall, the use of catalysts adds extra complexity to lignin processing due to separation and reusability difficulties, fast deactivation, poisoning, high costs and irreversible morphologic changes in the case of heterogeneous catalysts. 44 Non-catalytic routes for the upgrading of technical lignins include pyrolysis [45][46][47] and solvolysis, [48][49][50][51][52] which despite the promising results, also need high temperatures to efficiently break down the structure and minimize repolymerization pathways. In addition, low monomer yields and high carbon losses to the gas, solid and aqueous phases are common.…”