Effect of alkyl chain length on the thermophysical properties of pyridinium carboxylates

Effect of alkyl chain length on the thermophysical

properties of pyridinium carboxylates

Tazien Rashid 1,Chong Fai Kait2,Thanabalan Murugesan 1,*

【摘 要】A B S T R A C T In the presentstudy,new series of pyridinium carboxylate protic ionic liquids(PILs)were synthesized by pairing pyridinium cation with carboxylate anion from C1–C3 forming pyridinium

formate([C5H6N+][HCOO?]),pyridinium

pyridinium physical

acetate([C5H6N+][CH3COO?])and

propionate([C5H6N+][CH3CH2COO?])respectively.The

properties namely,density,viscosity,surface tension(298.15–343.15)K,and refractive index(293.15–323.15)K were measured.Thermal properties namely,glass

transition

temperature,molar

were

heat also

capacity,and

thermaldecomposition temperatures determined.The

thermalexpansivity was calculated using the experimental density data.The effect of increasing the alkyl chain length on the thermophysical properties of the pyridinium carboxylate PILs has been evaluated.As

expected

the

physical

properties

i.e.density,viscosity,surface tension and refractive index of the investigated pyridinium carboxylates decreased with increasing temperature.In general pyridinium carboxylate PILs possessed low viscosity,high thermal stability and excellent hydrogen bonding capability,and these properties lead them to outperform conventional

solvents employed for lignin dissolution. 【期刊名称】中国化学工程学报（英文版） 【年(卷),期】2017(025)009 【总页数】7

【关键词】Keywords:Protic ionic liquids Pyridine Carboxylic acids Anion Physicochemical properties

1.Introduction

Effective lignocellulosic biomass exploitation has significantly enhanced over the past few years.Lignin is the second most abundant biorenewable resource in nature.Lignin is rich in aromatic groups and is a good source of potential products such as;phenol,carbon fiber,aromatic stock chemicals,and polymers.However,due to its complex molecular structure formed by inter-and intra-molecular hydrogen bonding,it remains difficult to dissolve it in typical organic solvents.This con fines the broader application of lignin due to which itis primarily burnt as a low grade fuel[1,2].Due to its complex structure,it is difficult to develop a general technique to depolymerize lignin into desired aromatic feedstock and compounds.Furthermore,none of these challenges can be overcome unless efficient solvents can be synthesized/developed for effective dissolution/extraction of lignin[3]. Recently,ionic liquids(ILs)have gained much consideration as an auxiliary solvent for lignocellulosic biomass due to their distinctive features and

extensive properties[4].However,large scale industrial applications of ILs are still limited due to high production costs and complex synthesis routes with sophisticated purification steps required for their production[5,6].Recently,protic ionic liquids(PILs)have appeared as attractive replacements for ILs due to their numerous bene fits over the conventional ILs[3,7–9].The main advantage of PILs is that they can be synthesized in a one-step reaction and no further purification steps are involved[10].Furthermore,they possess low viscosity,high thermal stability,enhanced hydrogen-bonding capability,less corrosive,and have high capacity for the dissolution of lignin[3].

Due to these advantages,PILs are considered for industrial applications in the recent few years,such as desulphurization of fuel[11],CO2 capture[12],biomass processing[3,7]and in many acid–base-catalyzed organic reactions such as Knoevenagelcondensation[13].However,very little information is available on the elementary physical properties of this family of solvents,which are vital for the designing and scale up of any commercial process.In this work,pyridinium carboxylate PILs with three

different

alkyl

chains

i.e.([C5H6N+][HCOO?]),([C5H6N+][CH3COO?]),([C5H6N+][CH3CH2COO?])were

synthesized.Physical

properties

namely

density,viscosity,refractive index and surface tension and thermal properties

namely

glass

transition

temperature

and

thermal