Experimental and computational modeling of molecular products from the thermal degradation of tobacco, marijuana and khat

Abstract

The economic burden caused by death due to lung cancer and other respiratory diseases is attributed in part to tobacco use – presently linked to approximately 8 million deaths per year worldwide. Notably, 80% of these deaths occur in low and middle income countries making tobacco epidemic a major global public health concern. Biomass pyrolysis remains an important chemical process in the thermochemical conversion of biomass materials to produce high density reaction products such as bio-oil, and molecular products that are hazardous and of interest to public health and the environment. This work investigated the compounds that are released from the pyrolysis of tobacco, marijuana and khat, and their ternary mixtures; explored the elemental composition of thermal char; and performed geometry optimization of hazardous molecular products released in the entire pyrolysis temperature range of 200-700 oC, at increments of 50 oC. Research license, NACOSTI/21/11/11440, was obtained prior to sampling and analysis. A quartz tubular reactor was designed for the pyrolysis of the biomass components at 1 atm. in nitrogen at a residence time of 2 s – under conditions representative of cigarette smoking and municipal waste incineration characteristics. The pyrolysate was characterized using a Gas chromatograph hyphenated to a mass selective detector. Geometry optimization and reactivity coefficients of selected molecular products were performed under the density functional theory formalism at B3LYP level of theory coupled to 6-31++G (d,p) basis set using Gaussian ‘9 computational suite of programs. Elemental composition of thermal char was done using an elemental analyzer; thermal analysis explored using thermogravimetric analysis and differential scanning calorimetry (DSC). It was noted that the thermal degradation of khat is accompanied by the evolution of phenolic compounds such as p-cresol, catechol, hydroquinone and other substituted methoxy phenols at temperatures between 300 °C and 500 °C. Marijuana pyrolysis led to the evolution of molecular compounds such as phytol, phytocannabinoids, cannabidiol (CBD), tetrahydrocannabinol (THC), ∆-9- tetrahydrocannabivarin, cannabigerol, and 2,6-dimethylhydroquinone. Pyrolysis of marijuana-khat mixture showed a decrease in the concentration of phytocannabinoids such as cannabichromene, cannabicoumaronone, ∆-9-tetrahydrocannabivarin, cannabigerol, and cannabinol, while the equimassic mixture of tobacco and khat pyrolysis yielded nicotine, nicotyrine and cotinine at concentrations lower by 4.34%, 5.63% and 1.60%, respectively. TGA results revealed that the thermal decomposition of khat, marijuana and tobacco biomass is accompanied by weight loss of 4.52%, 81.37% and 5.22% in the first, second and third stages, respectively in the temperature range of 200 °C to 500 °C. DSC results showed that most of the khat- tobacco biomass pyrolysis occurred in the exothermic zone with peak temperatures of 350 °C and 477 °C, and final temperature of 682 °C. Elemental analysis of khat thermal char at 400 °C found C and O content at 37.08% and 43.73%, respectively. The computational details revealed the optimization step number for THC, CBD, cathinone and cathine to be 68, 40, 64 and 22 at energies of 968.73, 968.41, 231.52 and 480.65 hartrees correspondingly and found THC as the most unstable compound. This study has demonstrated that molecular toxins are produced in higher concentrations at pyrolysis temperatures of 350 °C to 550 °C with a possible reduction of their release on co-pyrolysis of equimassic mixtures of tobacco, khat and marijuana. The study recommends heightened research on the use of khat-tobacco mixtures in cigarettes to reduce the evolution of toxic molecular products.