Numeric solution to partial differential equation

Hi, i am trying to make a script for numeric solution to partial differential equations (rank higher than 2, that means 3). So far i have encountered problem in adding partial differences to cell or matrix. In theory, partial differential equations of rank 3 is basically 3 times derived function.

clear all
all = cell(1);
syms x y;
all(1)=x^2 + 2*y^2-22;
all(2)=diff(all(1),x);

I am getting this error:

Conversion to cell from sym is not possible.

So basically i would like to partially derive the function (by x and by y), store it to the cell and then partially derive the partially derived function and so on (depends on the rank of the partial). This would be in for cycle.

Thanks for answer.