Application Note - Magnetic Resonance

Qualitative and Quantitative Analysis of Pyrolysis Liquids from Industrial Hemp Can be Performed by NMR Spectroscopy

The Energy Information Administration predicts that the global energy demand will increase by nearly 50% by 2050. And with the drive to reduce the use of non-renewable energy sources, such as coal, oil, and natural gas, to minimize climate change, there is a high demand for renewable energy sources.

Industrial Hemp is Used in the Production of Renewable Energy by Pyrolysis

One renewable energy process is the pyrolysis of biomass – the generation of energy from burning organic matter such as wood and plants in the absence of oxygen. To produce the maximum amount of energy, plants used for biomass should ideally be fast growing and have a short rotation period. The industrial hemp plant Cannabis sativa. L. meets these requirements and is one of the fastest growing markets of biomass since the 1990s. Furthermore, the plant has a low concentration of the psychoactive compound delta9-tetrahydrocannabinol (d9-THC), so the intoxication effects of the cannabis plant when burnt do not occur. 

There are several different types of pyrolysis; slow pyrolysis, fast pyrolysis and gasification, and each method is characterized by its temperature, heat rate and residence time. Slow pyrolysis produces biochar and pyrolysis liquids as by-products and to increase the environmental sustainability of this process, applications of these compounds need to be identified. Biochar has been found to be effective as a soil ameliorant, but the use of pyrolysis liquids, which are rich in organic compounds, is more uncertain. 

Pyrolysis Liquid By-products Require Analytical Characterization 

In order to identify applications of pyrolysis liquids, it is first necessary to identify the chemical composition of these by-products. With industrial hemp the leaves, hurds (a by-product from the production of hemp fibers and seeds), and roots all undergo thermal degradation by slow pyrolysis. The distillates produced from this process can be placed in two categories: water-based fractions and bio-oil fractions. 

Several different techniques have been used to analyze the chemical composition of both types of pyrolysis liquids, including Fourier-transform infrared spectroscopy (FTIR), gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR) spectroscopy. However, limited information has been obtained regarding the concentration of the different compounds in the distillate. 

Recently, a group of researchers from the University of Eastern Finland, Finland, performed a comprehensive qualitative and quantitative investigation into the chemical composition of both the water-based and bio-oil fractions of pyrolysis liquids using NMR spectroscopy. 

NMR Spectroscopy Can be Used for Qualitative and Quantitative Analysis

In this study, the thermal degradation by slow pyrolysis of the leaves, hurds, and roots of industrial hemp was performed separately to identify the main chemical components from each area of the hemp plant. For each process, the two pyrolysis liquid fractions were separated using two different condensation temperatures: 70 °C for the water-based fraction and 130 °C for the bio-oil fraction. 1H NMR and 13C NMR measurements were then taken using a Bruker Avance III HD 600 MHz spectrometer and structural elucidation was carried out with 2D NMR techniques. Using the Cubic Spline Baseline Correlation routine in the Bruker Topspin software the concentration of the different compounds was then determined. 

From these data, more than 115 compounds were identified in the pyrolysis liquid. The bio-oil fractions from all areas of the hemp plant were mainly composed of fatty acids, phenols, levoglucosan, triterpenes, and cannabidiol. Interestingly, different areas of the hemp plant had different chemical compositions; the leaves were found to contain the highest concentration of aromatic compounds (12.4 mM) in comparison to the hurds and roots (6.9 mM and 8.6 mM, respectively). 

In the water-based fractions, the hurds contained the highest concentration of water-soluble compounds in comparison to the leaves and roots. Three compounds were found in the highest concentrations in all three areas of the hemp plant – acetic acid (50–241 g L-1), methanol (2–30 g L-1), and propanoic acid (5–20 g L-1). However, the hurds were found to contain the greatest variety of compounds, as a ketone and some furans were only identified in this part of the hemp plant.

One proposed application of pyrolysis liquids is for use as biopesticides, as there is a goal to decrease the use of synthetic pesticides to minimize the risks to the environment and human health. However, limited progress has been made to move these potential pesticides to commercial application due to the minimal amount of information on the chemicals' concentration profiles in these distillates. Therefore, these findings provide important information to aid in the development of pyrolysis liquids as biopesticides.

NMR is the key technology for analysing unknown and complex plant matrices such as Cannabis. Without having the need for chemical separation prior to analysis and also being inherently quantitative without any need for compound specific reference substances this technology uniquely features the comprehensive decomposition of any mixture. With more than 60 years of groundbreaking innovation in the area of NMR Bruker still pushes the boundaries of what is possible.