Thermodynamic and topological structure of methanol conversion process on zeolite molecular sieve.

Is the world's largest producer of chemicals in China, is also a shortage of oil resources, how to make use of our limited fossil fuel resources in the production of chemicals is an important strategic problem.
It is well known that China has the energy pattern of "rich coal poor oil and low gas", which determines the clean utilization of coal resources in the development of China's energy industry.
In 2016, China's foreign oil dependency has more than 64%, combined with the complex international political environment, unstable factors such as the Middle East oil exports, China's oil security and energy security cannot be guaranteed, long-term dependence on imported oil development model is unsustainable.
From the perspective of energy security, the coal as reserves in our country, one of the biggest sources of energy, should be to undertake the important task of protecting national chemical industry base of raw materials, especially the development of production through coal chemical triene platform compounds and the relevant technology of the three benzene and industrial equipment.

In fact, the production of organic chemicals based on coal is another revolution in the world chemical industry in recent years.
This not only makes coal in nearly a decade after the scale of the methanol to olefins in China quickly reached nearly ten million tons a year level, also makes our country coal to aromatic hydrocarbons such as ethylene glycol, coal has also reached the industrial device or industrial test stage.
Nobel prize-winning chemist George Andrew euler has proposed the concept of "methanol economy" : using methanol as a platform compound to produce other compounds and as energy supplies.
Methanol to hydrocarbon (methanol - to - hydrocarbon, MTH) quickly become the study hotspot and focus of the chemical industry development in our country.

Modern new coal chemical industry mainly has two main directions.
A route is represented by feto, which synthesizes gas as the intermediate and directly synthesizes the coal oil process of the target product.
The other is methanol synthesis as the precursor, and methanol as the core of the coal-based chemical.
Among them, coal is first experienced by gasification transformation and other processes to generate syngas, then synthesize methanol, and then methanol synthesis of olefins, aromatics and other products.
The process of methanol production is the core of the current industrial route because of the mature methanol synthesis and the easy separation of methanol and syngas.
In this step, the most critical is the methanol conversion process on the molecular sieve, and the selectivity and energy consumption of methanol conversion process directly affect the economy of coal chemical route.
In syngas direct to olefin and aromatic hydrocarbons in recent years also has made great progress in basic research, but how to solve the syngas efficiently converted to olefins, aromatics, solving engineering problems such as separation of the products from the syngas remains to be further research.

In our country, the methanol olefin industry is in the ascendant.
As of January 2017, a total of 13 methanol olefin devices have been put into production in China, and the production capacity is about 9 million tons of olefin/year.
At the same time, there are about ten sets of olefins in methanol.
It is estimated that the production capacity of methanol will reach 15 million tons of olefin/year by 2020.
With oil at just $40 a barrel, the methanol olefin device remains profitable, reflecting the viability of the technology.
But the technology is technically there are two big problems, one is the catalyst deactivation is fast, generally need holes shape-selective zeolites for olefin, due to the hydrocarbon pool mechanism and catalyst deactivation, raw material with 2% ~ 5% to coke, how from the molecular topology structure, hydrocarbon pool deactivation mechanism and molecular sieve in-depth understanding the process of the Angle of discrete transfer is a very important problem.
The second is that the methanol alkene process is accompanied by the generation of hundreds of different products, and the generated alkenes are sometimes restricted by thermodynamic equilibrium.
How to understand this phenomenon from the perspective of molecular sieve structure, inactivation and process thermodynamics is very important for deep understanding of this problem.

In addition, the methanol aromatics process is also starting.
With the cooperation of tsinghua university and huadian group, it completed the verification of 30,000 tons of methanol/year industrial test equipment in 2013 and became the first industrial test device in China.
At present, there are more than 100,000 tons of aromatics/year installations in China, which will be completed and put into production.

The mechanism of methanol to olefin process research is an important problem troubled catalytic researchers for a long time, the core problem is how the first carbon - carbon bonds formation and why there will be a lot of ethylene generated at low temperatures, at present more consistent view is that dual cycle mechanism, namely generation olefin by carbon olefins reaction cycle, the formation of positive ions system produced a large number of olefins.
Subsequently, a large number of olefin aromatization forms aromatic rings, which are made from methanol and aromatic methylation, side chain isomerization and broken chain into alkenes. This process is called hydrocarbon pool mechanism or carbon catalytic mechanism.
Methanol conversion of Gibbs free energy is low, but that is not affected by the thermodynamic equilibrium dynamics control, such as the production of olefin of, between olefin conversion are often affected by the thermodynamic equilibrium, at the same time in the presence of molecular sieves, channel selection type role makes this problem is very complex, type selection and dynamic problems have a lot of research, and consider with molecular sieve choose the type of thermodynamics and kinetics are no literature reports.
Molecular sieve is a very regular three-dimensional space network structure, and the inactivation process under the hydrocarbon pool mechanism is mainly due to the growth and blockage of its cross-hole or caged aromatic ring.
This blockage is a discrete 0-1 process, like the concept of qi in go.
Channel activity in a number of too much, caused a certain probability of blocking the activity of "gas" center the deactivation and the product or raw material in the deactivation of the area, causing active center deactivation quickly, and can make the olefin selectivity has fallen dramatically.
This problem is largely related to the topological structure of molecular sieve, which is the most famous problem in the discrete phase transition.
There is no phase change in the one-dimensional system.
Of two-dimensional Ising found Onsaga phase transition and give the analytic solution under the condition of limited, Yang proposed by electron spin up and down the magnetic problem can be described by mathematical 0 and 1, and to solve the two-dimensional Ising model of spontaneous magnetization coefficient, the analysis provides a good foundation for the construction of Ising phase change.
However, the problem of three - dimensional phase transition is still unsolved and can only be obtained by computer simulation.

Under such background, firstly, the structure of the catalyst of methanol to olefins, olefins conversion between thermodynamics and balance the restriction mechanism, in-depth analysis of the process of methanol to olefins distribution regularity of products.
Further caused by reaction of hydrocarbon pool mechanism model of discrete inactivation and Ising phase change problem, analyzes the deactivation problem will appear this kind of reaction of catalytic active center, instead of reaction selectivity decline and the reasons for the rise in carbon deposit, and using the concept of multi-level structure of the catalyst to solve this kind of catalyst design problems, in order to have a deeper understanding of the process.

Conclusion and prospect

Holes reaction-diffusion - choose the type of molecular sieve deactivation problem is one of the core problems in recent years, coal chemical, applied the method of discrete and topology, discrete response can be more in-depth understanding of the molecular sieve - inactivation and thermodynamics.
The traditional molecular sieve catalytic theory holds that the type of molecular sieve and the size of the pore size and the size of the cage can be determined by "sifting" the molecules to determine the product distribution.
The implicit assumption of the thermodynamic model is that the model contains molecules that can be diffused, can be generated, and calculated by thermodynamic models.
The influence of molecular sieve structure is also implied.
At this point, the model of this paper enriches the cognition of traditional molecular sieve catalytic theory from another Angle.

Based on the analysis of carbon catalysis in the process of molecular sieves, the distribution mechanism of thermodynamic products in the distribution of olefin products was discovered, and the distribution of the product was established.
Based on the topological structure of molecular sieve, this paper modeled the discrete inactivation of molecular sieve with a new perspective of graph theory, and analyzed the problem of the loss of life phase change, which was consistent with the experiment.

From the dynamics research to the thermodynamic analysis and the attention to the molecular sieve topological structure, the research on the methanol transformation on the molecular sieve is gradually deepening.
For the future, it is necessary to further development of the molecular sieve discrete analysis, multi-scale research: the micro scale, combination of dynamics and thermodynamics, a deep understanding of each reaction of the specific molecular sieve in a cage;
On the mesoscopic scale, using network and graph theory to combine macroscopic performance and microscopic reaction;
On the macro level, the molecular sieve is designed and modified to achieve better reaction performance.